document3

169

• Understand therelationship ofearthquakes to faulting

• Understand how themagnitude of anearthquake isdetermined

• Know the types ofearthquake waves, theirproperties, and howstrong ground motion isproduced

Earthquakes6

• Understand how seismicrisk is estimated

• Know the major effectsof earthquakes

• Understand thecomponents of theearthquake cycle

• Understand the methodsthat could potentiallypredict earthquakes

• Understand theprocesses of earthquakehazard reduction andhow people adjust toand perceive the hazard

The study of earthquakes is an exciting field with significant social consequences, includingpotential catastrophic loss of life; damage or loss of homes, large buildings, and infrastructure,

such as roads, train tracks, airports, dams, and power plants; disruption of people’s lives; andloss of income. In this chapter, we focus on the following learning objectives:

Learning Objectives

CHAPTER 6 Earthquakes170

quakes in recent years that havetaken thousands of lives have a com-mon dominator: They usually causetremendous loss of life because thebuildings people were in collapsed. InHaiti, homes, schools, hospitals, andindustrial buildings were subject tocollapse during strong shaking be-cause they were not constructed withearthquakes in mind or because therequired construction codes were ig-nored—presumably to save money.In short, most deaths were human-caused.3 We have experience withstrong shaking in California (North-ridge in 1994) and, more recently, inChile (February 27, 2010), where amagnitude 8.8 earthquake, about500 times as strong as the Haitiearthquake a month earlier, killedabout 800 people—an astronomi-cally lower number than in Haiti.4

Where buildings are designed towithstand shaking and are builtproperly, as are many in Chile andCalifornia, deaths from collapsedbuildings are many times lower than in areas where construction is substandard.

similar earthquake in Californiaprobably would have caused no orvery few deaths. The Italian eventwas a catastrophe because the buildings were not constructed tosustain moderate seismic shaking.1

It is a classic case of people notheeding warnings about futureearthquakes and continuing to buildsubstandard structures. The Italianearthquake was a human-causedcatastrophe that could have andshould have been avoided. Californiaand other places that design andconstruct buildings resistant to col-lapse during earthquake shaking may suffer damage from an earth-quake, but human loss of life is mini-mized. Until older buildings areretrofitted to withstand seismicshaking and new buildings are heldto a higher engineering standard,earthquakes will continue to takemany more lives in places that faceearthquake hazard.

On January 12, 2010, a majorearthquake (magnitude 7.0) struckHaiti and killed about 240,000 peo-ple (an appalling loss of life).2 Earth-

Catastrophic earthquakesare devastating events

that can destroy largecities and take thou-sands of lives in amatter of seconds.Many millions ofpeople live in haz-

ardous areas withsubstandard pro-tection against

earthquakes.As human

populationcontinuesto increasein vulnera-ble areas,

the risk willcontinue toincrease.

A strongearthquake(magnitude

6.3) struck themidlevel town

of L’Aqila in 2009,and many of the buildings

collapsed, killing about 300 people. A

Italian Earthquake of 2009 and Haiti Earthquake of 2010

6.2 Earthquake MagnitudeWhen a news release is issued about an earthquake, itgenerally gives information about where the earth-quake started, known as the epicenter. The epicenteris the location on the surface of Earth above the focus,which is the point at depth where the rocks rupturedto produce the earthquake (Figure 6.1, page 172). Thenews also reports moment magnitude, which is ameasure of the energy released by the earthquake.The moment magnitude is based, in part, upon im-portant physical characteristics, including the seismicmoment, the area that ruptured along a fault planeduring an earthquake, the amount of movement or

6.1 Introduction toEarthquakes

There are approximately 1 million earthquakes ayear that can be felt by people somewhere onEarth. However, only a small percentage of thesecan be felt very far from their source. Earthquakescan be compared with one another by the energythey release, known as their magnitude, or by theirintensity of shaking, referred to as ground motion,and the resulting impact on people and society.Table 6.1 lists selected major earthquakes thathave struck the United States since the early nine-teenth century.

Case History

Earthquake Magnitude 171

fault slip during an earthquake, and the rigidity (i.e.,shear modulus) of the rocks. The seismic moment isestimated by examining the records from seismo-graphs (i.e., instruments that record earthquake mo-tion), determining the amount and length of rupture,and estimating the shear modulus (from laboratorytesting of rocks). The moment magnitude (Mw) isthen determined by Mw � 2/3 log M0 � 10.7, whereM0 is the seismic moment.

Before the use of moment magnitude, Richtermagnitude, named after the famous seismologistCharles Richter, was used to describe the energy re-leased by an earthquake. Richter magnitude is basedupon the amplitude, or size, of the largest seismic waveproduced during an earthquake. A seismograph is aninstrument that records earthquake displacements;

seismographs produce seismographic records, orseismograms. The amplitude recorded is converted toa magnitude on a logarithmic scale; that is, each inte-ger increase in Richter magnitude represents a ten-fold increase in amplitude. For example, a Richtermagnitude 7 earthquake produces a displacement onthe seismogram 10 times larger than does a magni-tude 6. Although the Richter magnitude remains thebest-known earthquake scale to many people, earth-quake scientists, known as seismologists, do not com-monly use it. For large, damaging earthquakes, theRichter magnitude is approximately equal to themoment magnitude, which is more commonly usedtoday. In this book, we will simply refer to the size ofan earthquake as its magnitude, M, without desig-nating a Richter or moment magnitude.

TABLE 6.1 Selected Major Earthquakes in the United States

Year Location Damage (millions of dollars) Number of Deaths

1811–1812 New Madrid, Missouri Unknown Unknown

1886 Charleston, South Carolina 23 60

1906 San Francisco, California 524 700

1925 Santa Barbara, California 8 13

1933 Long Beach, California 40 115

1940 Imperial Valley, California 6 9

1952 Kern County, California 60 14

1959 Hebgen Lake, Montana (damage to timber and roads)

11 28

1964 Alaska and U.S. West Coast (includes tsunami damage from earthquake near Anchorage)

500 131

1965 Puget Sound, Washington 13 7

1971 San Fernando, California 553 65

1983 Coalinga, California 31 0

1983 Central Idaho 15 2

1987 Whittier, California 358 8

1989 Loma Prieta (San Francisco), California 5,000 62

1992 Landers, California 271 1

1994 Northridge, California 40,000 57

2001 Seattle, Washington 2,000 1

2002 South-central Alaska (sparsely populated area) 0


The magnitude and frequency of earthquakesworldwide are shown in Table 6.2. An event of mag-nitude 8 (M 8) or above is considered a great earth-quake, capable of causing widespread catastrophicdamage. In any given year, there is a good chancethat one M 8 event will occur somewhere in theworld. A M 7 event is a major earthquake, capable ofcausing widespread and serious damage. Magnitude6 signifies a strong earthquake that can cause consid-erable damage, depending upon factors such as loca-tion, surface materials, and quality of construction.Ground motion can be recorded as the displacementor actual separation of rocks produced by an earth-quake. Relationships between change in magnitudeand change in displacement and energy are shown inTable 6.3. This table illustrates that the differencebetween a M 6 and a M 7 earthquake is considerable.A M 7 earthquake releases about 32 times more

Surface rupture(forms fault scarp,amount of slipon fault)

Epicenter(point on surfaceabove focus)

Focus(when ruptureon fault plane started)

Fault slip

Fault planeSurface of Earth

Spreading areaof rupture on faultplane

FIGURE 6.1 Basic earthquake features Block diagram showing fault plane, amount of dis-

placement, rupture area, focus, and epicenter. Rupture starts at the focus and propagates up, down,

and laterally. During a major to giant earthquake, slip may be 2 to 20 m along a fault length of 100 km

or more. Rupture area may be 1,000 km2 or more.

energy than a M 6 earthquake, and the amount ofdisplacement, or ground motion, is 10 times greater.If we compare a M 5 with a M 7 earthquake, the dif-ferences are much greater. The energy released isabout 1,000 times greater. The topic of earthquakemagnitude is introduced here to help compare theseverity of earthquakes. We will return to a more de-tailed discussion of this topic later in this chapterwhen earthquake processes are discussed.

Earthquake CatastrophesCatastrophic, or great, earthquakes are devastatingevents that can destroy large cities and take thousandsof lives in a matter of seconds. A sixteenth-centuryearthquake in China reportedly claimed 850,000 lives.More recently, a 1923 earthquake near Tokyo killed143,000 people, and a 1976 earthquake in China killed

Earthquake Intensity 173

several hundred thousand. In 1985, an earthquakeoriginating beneath the Pacific Ocean off Mexico(M 8.1) caused 10,000 deaths in Mexico City, severalhundred kilometers from the source. Exactly 1 yearafter the Northridge earthquake, the January 17,1995, Kobe, Japan, earthquake (M 7.2) killed morethan 5,000 and injured 27,000 people, while destroy-ing 100,000 buildings and causing over $100 billion inproperty damages (Figure 6.2a). The January 26,2001, Gujarat, India, earthquake (M 7.7) killed as

many as 30,000 people, injured 166,000, damaged ordestroyed about 1 million homes, and left 600,000people homeless. An earthquake on October 8, 2005,of M 7.6, struck northern Pakistan. Although theepicenter was in Pakistan, extensive damage also oc-curred in Kashmir and India (Figure 6.2b). Morethan 80,000 people were killed and more than30,000 buildings collapsed. Entire villages weredestroyed, some buried by landslides triggered bythe violent shaking. One of the largest continentalearthquakes (M 7.9) to strike a highly populatedregion in the past 100 years occurred in the Sichuanprovince of China in 2008, killing about 87,500 peo-ple. Some villages were completely buried by land-slides, and more than 5 million buildings collapsed(Figure 6.2c).5–7 Finally the 2010 earthquake in Haitikilled about 240,000 people (see case history openingthis chapter).

6.3 Earthquake IntensityA qualitative way of comparing earthquakes is to usethe Modified Mercalli Scale, which describes 12 divi-sions of intensity, based on observations concerningthe severity of shaking during an earthquake (Table6.4, page 175). Intensity reflects how people per-ceived the shaking and how structures responded tothe shaking. Whereas a particular earthquake hasonly one magnitude, different levels of intensity maybe assigned to the same earthquake at different loca-tions, depending on proximity to the epicenter andlocal geologic conditions. Figure 6.3 (page 175) is amap showing the spatial variability of intensity forthe 1971 San Fernando earthquake (M 6.6). Suchmaps, produced from questionnaires sent to resi-dents in the epicentral region after an earthquake, area valuable, although crude, index of ground shaking.

One of the major challenges during a damagingearthquake is to quickly determine where the damageis most severe. An approach now being used is knownas a shake map that shows the extent of potentialdamaging shaking following an earthquake. Data fora shake map are recorded from a dense network ofhigh-quality seismograph stations. When seismicdata are received at seismographic stations, the areaswith the severest shaking are known within a minuteor so after the shaking has ceased. This informationis critical for directing an effective emergency

TABLE 6.2 Worldwide Magnitude andFrequency of Earthquakes,by Descriptor Classification

Average Annual

Descriptor Magnitude Number of Events

Great 8 and higher 1

Major 7–7.9 18

Strong 6–6.9 120

Moderate 5–5.9 800

Light 4–4.9 6,200 (estimated)

Minor 3–3.9 49,000 (estimated)

Very minor �3.0 Magnitude 2–3,about 1,000 per day

Magnitude 1–2,about 8,000 per day

U.S. Geological Survey. 2000. Earthquakes, Facts and Statistics.http://neic.usgs.gov. Accessed 1/3/00.

TABLE 6.3 Relationships BetweenMagnitude, Displacement, andEnergy of Earthquakes

Magnitude Ground Displacement Energy

Change Change1 Change

1 10 times About 32 times

0.5 3.2 times About 5.5 times

0.3 2 times About 3 times

0.1 1.3 times About 1.4 times

1 Displacement, vertical or horizontal, that is recorded on a standardseismograph.

U.S. Geological Survey. 2000. Earthquakes, Facts and Statistics. http://neic.usgs.gov. Accessed 1/3/00.

http://neic.usgs.gov




(a)

(b)

(c)

FIGURE 6.2 Earthquake damage(a) This elevated road collapsed as a result

of intense seismic shaking associated with

the 1995 Kobe, Japan, earthquake. (Haruyoshi

Yamaguchi/Sygma/Corbis) (b) Collapsed

buildings, Balakot, Pakistan, from a M 7.6

earthquake in 2005. (B.K. Bangash/AP Photo)

(c) Collapsed buildings from a 2008 M 7.9

earthquake in China. (Greg Baker/AP Photo)

Earthquake Intensity 175

response to those areas. The maps in Figure 6.4 showthe shake map for the 1994 Northridge, California,earthquake (M 6.7), and the 2001 M 6.8 Seattle,Washington, earthquake (M 6.8). Notice that themagnitudes of the two earthquakes are very similar,but the intensity of shaking was greater for theNorthridge earthquake. The technology to produceand distribute shake maps in the minutes followingan earthquake was made available in 2002.8 The costof seismographs is small relative to damages from

TABLE 6.4 Modified Mercalli Intensity Scale (abridged)

Intensity Effects

I Felt by very few people.

II Felt by only a few persons at rest, especially on upper floors of buildings. Delicately suspended objects may swing.

III Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as anearthquake. Standing motor cars may rock slightly. Vibration feels like the passing of a truck.

IV During the day, felt indoors by many, outdoors by few. At night, some awakened. Dishes, windows, doors disturbed;walls make cracking sound; sensation like heavy truck striking building; standing motor cars rock noticeably.

V Felt by nearly everyone; many awakened. Some dishes, windows, and so on broken; a few instances of crackedplaster; unstable objects overturned; disturbances of trees, poles, and other tall objects sometimes noticed.Pendulum clocks may stop.

VI Felt by all; many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster ordamaged chimneys. Damage is slight.

VII Everybody runs outdoors. Damage negligible in buildings of good design and construction; slight to moderate inwell-built ordinary structures; considerable in poorly built or badly designed structures; some chimneys broken.Noticed by persons driving cars.

VIII Damage slight in specially designed structures; considerable in ordinary substantial buildings, with partial collapse;great in poorly built structures; panel walls thrown out of frame structures; fall of chimneys, factory stacks, columns,monuments, walls; heavy furniture overturned; sand and mud ejected in small amounts; changes in well water;disturbs persons driving cars.

IX Damage considerable in specially designed structures; well-designed frame structures thrown out of plumb; greatin substantial buildings, with partial collapse. Buildings are shifted off foundations. Ground cracked conspicuously.Underground pipes are broken.

X Some well-built wooden structures destroyed; most masonry and frame structures with foundations destroyed;ground badly cracked. Rails bent. Landslides considerable from riverbanks and steep slopes. Shifted sand andmud. Water is splashed over banks.

XI Few if any (masonry) structures remain standing. Bridges are destroyed. Broad fissures are formed in ground.Underground pipelines are taken out of service. Earth slumps and land slips on soft ground occurs. Train rails are bent.

XII Damage is total. Waves are seen on ground surfaces. Lines of sight and level distorted. Objects are thrown upwardinto the air.

From Wood and Neuman, 1931, by U.S. Geological Survey, 1974, Earthquake Information Bulletin 6(5):28.

Limits of felt area

VVIII–XI

I–IV

VIIVI

Nevada Utah

Arizona

MEXICO

California

PACIFICOCEAN

0

100 200 Kilometers0

100 200 Miles

FIGURE 6.3 Intensity of shaking Modified Mercalli

intensity map for the 1971 San Fernando Valley, California,

earthquake (M 6.6), determined after the earthquake. (U.S. Ge-

ological Survey, 1974, Earthquake Information Bulletin 6[5])


not felt

Mt. Rainier

Seattle

Tacoma

Everett

Bremerton

Olympia

Enumclaw

Port Angeles

Aberdeen

Centralia

Kirkland

North Bend

Poulsbo

Snohomish

Duvall

Monroe

Granite Falls

Maple Valley

CarnationFall City

Eatonville

Morton

Skykomish

Satsop

48°N

47°30'N

47°N

123°W 122°30'W 122°W 121°W121°30'W 120°30'W123°30'W

Strait of Juan de Fuca

0

10 20 Kilometers0

10 20 Miles

Instrumentalintensity

Shaking not felt

none none none very light light

weak light moderate

moderate moderate/heavy heavy very heavy

strong very strong severe violent extreme

Damage

I II-III IV V VI VII VIII IX X+


Shaking

Damage

weak light moderate strong severe violent extremevery strong

I II-III IV V VI VIII IX X+VII

none none none very light light moderate/heavy heavy very heavymoderate

405

710

210

605

101101

14

1

1

5

5

5

10Los Angeles

Newhall

Palmdale

Pasadena

Inglewood

Hawthorne

Culver City

Torrance

Lakewood

Simi Valley

Thousand Oaks

Fillmore

San Fernando

Northridge

SantaPaula

Downey

Burbank

Westwood

Whittier

Malibu

Camarillo

Castaic

118°30'119°

34°

P A C I F I CO C E A N

0

5 10 Kilometers0

5 10 Miles

(a)

(b)

FIGURE 6.4 Real-timeintensity of shaking(a) Shake map for the 1994

Northridge, California,

earthquake (M 6.7), deter-

mined after the earthquake

occurred. (U.S. Geological

Survey, and courtesy of

David Wald) (b) The 2001

Seattle, Washington, earth-

quake (M 6.8). (Pacific North-

west Seismograph Network,

University of Washington)

Plate Boundary Earthquakes 177

earthquake shaking, and the arrival of emergencypersonnel is critical in the first minutes and hours fol-lowing an earthquake, if people in collapsed buildingsare to be rescued. The shake map is also very useful inhelping locate areas where gas lines and other utilitiesare likely to be damaged. Clearly, the use of this tech-nology, especially in urban areas vulnerable to earth-quakes, is a very desirable component of our pre-paredness for earthquakes.

6.4 Plate Boundary EarthquakesCalifornia, which straddles two lithospheric platesthat are moving past one another, experiences fre-quent damaging earthquakes. The 1989 Loma Prietaearthquake (M 7.1) on the San Andreas fault systemsouth of San Francisco killed 62 people and caused$5 billion in property damage. Neither the Loma Pri-eta earthquake nor the Northridge earthquake (M6.7) was considered a great earthquake. It has beenestimated that a great earthquake, occurring today

in a densely populated part of southern California,could inflict $100 billion in damage and kill severalthousand people. Thus, the Northridge quake, as ter-rible as it was, was not the anticipated “big one.”Given that earthquakes have the proven potentialfor producing a catastrophe, earthquake research isprimarily dedicated to understanding earthquakeprocesses. The more we know about the probable lo-cation, magnitude, and effects of an earthquake, thebetter we can estimate the damage that is likely tooccur and make the necessary plans for minimizingloss of life and property.

Interplate earthquakes are those between two plates,initiated near plate boundaries and producing nearlycontinuous linear or curvilinear zones in which mostseismic activity takes place (Figure 6.5). Most largeU.S. earthquakes are interplate earthquakes in theWest, particularly near the North American and Pa-cific plate boundaries (Table 6.1). However, large dam-aging intraplate earthquakes, located within a singleplate, can occur far from plate boundaries.

PACIFICATLANTIC

OCEAN

INDIANOCEAN

OCEAN

Equator

180° 160°160°

60°

60°

20°

140°140° 120°120°80°60°40°

0

250 500 Kilometers0

250 500 Miles

FIGURE 6.5 Earthquakes at plate boundaries Map of global seismicity (1963–1988), delineating plate boundaries (earthquake

belts). Red dots represent individual earthquakes. For the locations and names of Earth’s tectonic plates, refer to Figure 2.3. (Courtesy of

National Earthquake Information Center)


Charleston. Effects of the earthquake were reportedat distances exceeding 1,000 km (620 mi).

Intraplate earthquakes in the eastern UnitedStates are generally more damaging and felt over a

Jonesboro

New Madrid

Memphis

0

20 40 Kilometers0

20 40 Miles

Mis

siss

ippi

R

iver

KY

TENNESSEE

MISSOURI

ARKANSAS

FIGURE 6.6b Earthquake-prone region The New Madrid

seismic zone is the most earthquake-prone region in the United

States east of the Rocky Mountains. Locations of recorded minor

earthquakes since 1974 are shown as crosses. (U.S. Geological Survey)

6.5 IntraplateEarthquakes

Intraplate earthquakes with M 7.5+ thatoccurred in the winter of 1811–1812 inthe central Mississippi Valley nearly destroyed the town of New Madrid, Missouri, and they killed an unknownnumber of people. These earthquakesrang church bells in Boston! Seismic shak-ing produced intense surface deformationover a wide area from Memphis, Ten-nessee, north to the confluence of theMississippi and Ohio Rivers. During theearthquakes, forests were flattened; frac-tures in the ground opened so wide thatpeople had to cut down trees to crossthem; and land sank several meters insome areas, causing flooding. It was reported thatthe Mississippi River actually reversed its flow dur-ing shaking. The earthquakes occurred along a seis-mically active structure known as the New Madridseismic zone, which underlies the geologic structureknown as the Mississippi River Embayment (Figure6.6). The embayment is a downwarped area ofEarth’s crust, where the lithosphere is relativelyweak. The recurrence interval, or time betweenevents, for major earthquakes in the embayment isestimated to be about 500 years.9,10 Even in this“stable” interior of the North American plate, thepossibility of future damage demands that theearthquake hazard in the area be considered whenfacilities, such as power plants and dams, are beingdesigned and built. It is believed that the NewMadrid seismic zone is a young—perhaps less than10,000 years old—zone of deformation. The rate ofuplift is sufficient to produce significant topo-graphic relief over a period of several hundred thou-sand years. The fact that the topography of the up-lifted region in the Mississippi River floodplain areais very minor supports the hypothesis that this is avery young fault system or one that has been re-cently reactivated. The New Madrid seismic zone iscapable of producing great earthquakes and is theobject of intensive research.

Another large damaging intraplate earthquake (M 7.5) occurred on August 31, 1886, near Charleston,South Carolina. The earthquake killed about 60 peopleand damaged or destroyed most buildings in

New Madridseismic zoneNew Madridseismic zone**

FIGURE 6.6a New Madrid seismic zone The location of this zone is

thousands of kilometers from the nearest plate boundary (red lines).

Earthquake Processes 179

much larger area than are similar-magnitude earth-quakes in California. The reason is that the rocks inthe eastern United States are generally stronger andless fractured and can more efficiently transmitearthquake waves than can rocks in the west.

6.6 Earthquake ProcessesOur discussion of global tectonics established thatEarth is a dynamic, evolving system. Earthquakesare a natural consequence of the processes that formthe ocean basins, continents, and mountain rangesof the world. Most earthquakes occur along theboundaries of lithospheric plates (Figure 6.5).

FaultingThe process of fault rupture, or faulting, can becompared to sliding two rough boards past one an-other. Friction along the boundary between theboards, analogous to a fault plane, may temporarilyslow their motion, but rough edges break off, andmotion occurs at various places along the plane. Forexample, lithospheric plates that are moving pastone another are slowed by friction along theirboundaries. As a result, rocks along the boundaryundergo strain, or deformation, resulting fromstress produced by the movement. When stress onthe rocks exceeds their strength, the rocks rupture,forming a fault and producing an earthquake. Afault is a fracture or fracture system along whichrocks have been displaced; that is, one side of thefracture or fracture system has moved relative tothe other side. The long-term rate of movement isknown as the slip rate and is often recorded as mil-limeters per year (mm/yr) or meters per 1,000years (m/ky). During a major to great earthquake,displacements of several meters may suddenlyoccur along a fault. When a rupture begins, it startsat the focus and then grows or propagates up,down, and laterally along the fault plane during anearthquake (see Figure 6.1). The sudden rupture ofthe rocks produces shock waves called earthquakewaves, or seismic waves, that shake the ground. Inother words, the pent-up energy of the strainedrocks is released in the form of an earthquake.Faults are, therefore, seismic sources, and identify-ing them is the first step in evaluating the risk of anearthquake, or seismic risk, in a given area.

Fault TypesThe major types of faults, based on the direction, orsense of the relative displacement, are shown onFigure 6.7. A strike-slip fault is a fault in which thesides of the fault are displaced horizontally; a strike-slip fault is called right-lateral if the right-hand sidemoves toward you as you sight, or look along, thefault line, and left-lateral if the left-hand side movestoward you. A fault with vertical displacement is re-ferred to as a dip-slip fault. A dip-slip fault may be areverse fault or a normal fault, depending on thegeometry of the displacement. Geologists use inter-esting terminology to distinguish reverse and nor-mal faults. Notice in Figure 6.7 that there are twoblocks separated by the fault plane. One way to re-member the terminology for the two blocks is toimagine you are walking up the fault plane, likewalking up a hill. The block you would be standingon is called the foot-wall, and the other block iscalled the hanging-wall. If the fault displacement issuch that the hanging-wall moves up relative to thefoot-wall, the fault is called a reverse fault. When thefault plane of a reverse fault has an angle of lessthan 45 degrees, it is called a thrust fault. If thehanging-wall moves down relative to the foot-wall,the fault is called a normal fault. Reverse and thrustfault displacement are associated with crustal short-ening, whereas normal fault displacement is associ-ated with crustal extension.

The faults shown in Figure 6.7 generally producesurface displacement or rupture. However, thereare also buried faults, usually associated with foldedrocks. Displacement and rupture of buried faultsdo not propagate to the surface, even in largeearthquakes, as was the case with the Northridgeearthquake.

The relationship of a buried reverse fault torock folding is shown in Figure 6.8 (page 181).Shortening of a sequence of sedimentary rockshas produced folds, called anticlines and synclines.Anticlines are arch-shaped folds; synclines arebowl-shaped folds. In this illustration, anticlinesform ridges, and synclines form basins at the sur-face of the ground. Notice that, in the cores of twoof the anticlines on the right, buried faulting has oc-curred. Faulting during earthquakes causes anticli-nal mountains to be uplifted, whereas subsidence,or sinking of the ground surface, may occur in syn-clinal valleys.


Right-lateralstrike-slip faultStress: shear

(horizontal displacement)

Reverse faultStress: compression

(pushing together, shortening)

Normal faultStress: tensile

(pulling apart, extending)

Offset stream

Fault plane

(a) Right-lateral strike-slip fault

(b) Reverse (thrust) fault

Fault plane

(c) Normal faultFault plane

Displacement on fault

Stress

FIGURE 6.7 Faulting changes the land Types of fault movement and effects on the landscape, based on the sense of motion

relative to the fault.


Until recently, it was thought that most activefaults could be mapped because their most recentearthquake would cause surface rupture. Discoveringthat some faults are buried and that rupture doesnot always reach the surface has made it more diffi-cult to evaluate the earthquake hazard in someareas. (See Case History: Northridge, 1994.)

Fault Zones and Fault SegmentsFaults almost never occur as a single rupture.Rather, they form fault zones, which are a group ofrelated faults roughly parallel to each other in mapview. They often partially overlap or form braidedpatterns. Fault zones vary in width, ranging from ameter or so to several kilometers.

Most long faults or fault zones, such as the SanAndreas fault zone, are segmented, with each seg-ment having an individual history and style of move-ment. An earthquake segment is defined as thoseparts of a fault zone that have ruptured as a unit dur-ing historic and prehistoric earthquakes. Ruptureduring an earthquake generally stops at the bound-aries between two segments; however, major to greatearthquakes may involve several segments of thefault. Rupture length from large earthquakes is mea-sured in tens of kilometers. It is the earthquake segmentthat is often useful in estimating the magnitude of afuture earthquake.

When the earthquake history of a fault zone isunknown, the zone may be divided into segments,based on differences in morphology or geometry.However, from the point of view of earthquake-hazard evaluation, it is preferable to define faultsegments according to their history, which includes

both historic seismic activity and paleoseismicity(prehistoric seismic activity). Paleoseismology is thestudy of prehistoric earthquakes from the geologicenvironment. The geologic processes that governfault segmentation and generation of earthquakeson individual segments are the subject of activeresearch.

Active FaultsMost geologists consider a particular fault to be anactive fault if it can be demonstrated to have movedduring the past 10,000 years, the Holocene epoch.The Quaternary period, spanning approximatelythe past 2.6 million years, is the most recent periodof geologic time, and most of our landscape hasbeen produced during that time (see Table 1.1).Fault displacement that has occurred during thePleistocene epoch of the Quaternary period, ap-proximately 2.6 million to 10,000 years ago, butnot in the Holocene, is classified as potentially active(Table 6.5).

Faults that have not moved during the past 2.6million years are generally classified as inactive.However, we emphasize that it is often difficult toprove the activity of a fault in the absence of easilymeasured phenomena, such as historical earth-quakes. Demonstrating that a fault is active mayrequire determining the past earthquake history, orpaleoseismicity, on the basis of the geologic record.This determination involves identifying faultedEarth materials and determining when the mostrecent displacement occurred.

For seismic zoning, the state of California de-fines an active fault as a fault that has moved in the

Shortening

Buried faults

Shortening

Anticline Syncline

Anticlinalmountain

Synclinal valley

Ridge producedby resistantrock layer

FIGURE 6.8 Buried faults Block di-

agram illustrating several types of com-

mon folds and buried reverse faults with

possible surface expressions, such as an-

ticlinal mountains and synclinal valleys.

(Modified after Lutgens, F., and Tarbuck, E.

1992. Essentials of Geology, 4th ed. New

York: Macmillan)

CHAPTER 6 Earthquakes182 CHAPTER 6 Earthquakes

last 10,000 years. However, other agencies havemore conservative definitions for fault activity. Forexample, the U.S. Nuclear Regulatory Commission,when considering seismic safety for nuclear powerplants, defines a fault as capable if it has moved atleast once in the past 50,000 years or more thanonce in the last 500,000 years. These criteria pro-vide a greater safety factor, reflecting increasedconcern about the risk inherent in the siting of nu-clear power plants.

Fault activity involves two important and interre-lated concepts: slip-rates on faults and recurrenceintervals (repeat times) of earthquakes. Slip rate ona fault is defined as the ratio of slip (displacement)to the time interval over which that slip occurred.For example, if a fault has displacement of 1 m dur-ing a time interval of 1,000 years, the slip rate is 1 mm per year.

The average recurrence interval of earthquakeson a particular fault may be determined by threemethods:

1. Paleoseismic data: This method involves aver-aging the time intervals between earthquakesrecorded in the geologic record.

2. Slip rate: This method involves assuming agiven displacement per event and dividingthat number by the slip rate. For example, if the average displacement per event were 1 m and the slip rate 2 mm per year, then the average recurrence interval would be 500 years.

3. Seismicity: This method involves using histori-cal earthquakes and averaging the time inter-vals between events.

Defining slip rate and recurrence interval is easy,and the calculations seem straightforward. How-ever, the concepts of slip rate and recurrence intervalare far from simple. Fault slip rates and recurrenceintervals tend to change over time, casting suspi-cion on average rates. For example, it is not uncom-mon for earthquake events to cluster in time andthen be separated by long periods of low activity.Thus, both slip rate and recurrence interval willvary according to the time interval for which datais available.

Methods of Estimating Fault Activity. Variousmethods are used to estimate fault activity, whichincludes both historic and prehistoric earthquakes.Paleoseismicity can be estimated by investigatinglandforms produced or displaced by faulting. Fea-tures such as offset streams, sag ponds, linearridges, and fault scarps (i.e., steep slopes or cliffsproduced by fault movement) may indicate recentfaulting. However, care must be exercised in inter-preting these landforms because some of them mayhave a complex origin.

The study of soils can also be useful in estimatingthe activity of a fault. For example, soils on oppositesides of a fault may be of similar age or quite dissim-ilar age, thus establishing a relative age for the dis-placement.

TABLE 6.5 Terminology Related to Recovery of Fault ActivityGEOLOGIC AGE

Years

Before

Present

Fault

ActivityEra Period Epoch

Cen

ozoi

c

HistoricHolocene 200 Active

Quaternary 10,000

Pleistocene Potentially active

2,600,000Tertiary Pre-Pleistocene

65,000,000Pre-Cenozoic time Inactive

4,500,000,000Age of Earth

After California State Mining and Geology Board Classification, 1973.


Case HistoryNorthridge, 1994

The rupture of rocks that pro-duced the Northridge earthquakewas initiated on a steep buried fault,at a depth of approximately 18 km(11 mi). The rupture quickly propa-gated upward (northward and west-ward) but did not reach the surface,stopping at a depth of several kilo-meters. At the same time, the rup-ture progressed laterally, in a mostlywestward direction, 20 km (12.5mi). The geometry of the faultmovement is shown in Figure 6.B.The movement produced uplift and

folding of part of the Santa SusanaMountains, a few kilometers northof Northridge.11

The Northridge earthquake terrifiedpeople, especially children. The shak-ing, which lasted about 15 seconds,was intense; people were thrown outof bed, objects flew across rooms,chimneys tumbled, walls cracked, andEarth groaned and roared with eachpassing earthquake wave. When theshaking stopped, people had littletime to recover before strong after-shocks started.

The 1994 Northridge earthquakethat struck the Los Angeles area onJanuary 17 was a painful wake-up callto southern Californians. The earth-quake killed 57 people and causedabout $40 billion in property dam-age. Several sections of freeways wereheavily damaged, as were parkingstructures and more than 3,000buildings (Figure 6.A). The North-ridge earthquake is one in a series ofmoderate-sized earthquakes thathave recently occurred in southernCalifornia.

(a)

(b)

FIGURE 6.A Earthquake inLos Angeles urban regionDamage from the 1994 North-

ridge, California, earthquake.

(a) A parking structure. (R. Forrest

Hopson) (b) Damage to the Kaiser

Permanente Building.

(A. G. Sylvester)

KEY

Reverse fault, teethon upthrown side

Strike-slip fault:arrows show relativemotion

Folding caused byNorthridge earthquake

Northridge

Newhall

Los Angeles

San Gab

rie l Mountains Santa SusanaMountains.

Santa Monica Mtns.

18km

Focus

North

San Fernando Valley

PacificOcean

Northridgerupture

FIGURE 6.B Details of an earthquake Block diagram

showing the fault that produced the 1994 Northridge earth-

quake. During the earthquake, the Santa Susana Mountains

were folded, uplifted 38 cm (15 in.), and moved 21 centimeters

(8.2 in.) to the northwest. (Courtesy of Pat Williams, Lawrence Liv-

ermore Laboratory)


Prehistoric earthquake activity sometimes can bedated radiometrically, providing a numerical date ifsuitable materials can be found. For example, radio-carbon dates from faulted sediments have been usedto calculate slip rates and estimate recurrence inter-vals of large earthquakes.

Tectonic CreepSome active faults exhibit tectonic creep, that is,gradual displacement that is not accompanied byfelt earthquakes. The process can slowly damageroads, sidewalks, building foundations, and otherstructures. Tectonic creep has damaged culvertsunder the football stadium of the University of Cali-fornia at Berkeley, and periodic repairs have beennecessary as the cracks have developed. Movementof approximately 3.2 cm (1.3 in.) was measured in aperiod of 11 years. More rapid rates of tectoniccreep have been recorded on the Calaveras faultzone, a segment of the San Andreas fault, near Hol-lister, California. At one location, a winery situatedon the fault is slowly being pulled apart at about1 cm (0.4 in.) per year (Figure 6.9). Damages result-ing from tectonic creep generally occur along nar-row fault zones that are subject to slow, continuousdisplacement.

Slow EarthquakesSlow earthquakes are similar to other earth-quakes, in that they are produced by fault rupture.However, with a slow earthquake, the rupture,rather than being nearly instantaneous, can lastfrom days to months. The moment magnitude ofslow earthquakes can be in the range of 6 to 7 be-cause a large area of rupture is often involved, al-though the amount of slip is generally small (e.g.,a centimeter or so). Slow earthquakes are a newlyrecognized fundamental Earth process. They arerecognized through analysis of continuous geo-detic measurement or GPS, similar to the devicesthat are used in automobiles to identify location.These instruments can differentiate horizontalmovement in the millimeter range and have beenused to observe surface displacements from slow earthquakes. When slow earthquakes occurfrequently, such as every year, their total contribu-tion to changing the surface of the Earth to

produce mountains over geologic time may besignificant.12

6.7 Earthquake ShakingThree important factors determine the shaking youwill experience during an earthquake: (1) earthquakemagnitude, (2) your distance from the epicenter, and(3) local soil and rock conditions. If an earthquake isof moderate magnitude (M 5 to 5.9) or larger, thenstrong motion or shaking may be expected. It is thestrong motion from earthquakes that cracks theground and makes Earth “rock and roll,” causingdamage to buildings and other structures.

Types of Seismic WavesSome of the seismic waves generated by fault rup-ture travel within Earth, and others travel along thesurface. The two types of seismic waves that travelwith a velocity of several kilometers per secondthrough rocks are primary (P) and secondary (S)waves (see Figure 6.10a,b, page 186).

P waves, also called compressional waves, are fasterthan S waves and can travel through solid, liquid,and gaseous materials (see Figure 6.10a). The veloc-ity of P waves through liquids is much slower. Inter-estingly, when P waves are propagated into theatmosphere, they are detectable to the human ear.This fact explains the observation that people some-times hear an earthquake before they feel the shak-ing caused by the arrival of the slower surfacewaves.13

S waves, also called shear waves, can travel onlythrough solid materials. Their speed throughrocks, such as granite, is approximately one-halfthat of P waves. S waves produce an up-and-downor side-to-side motion at right angles to the direc-tion of wave propagation, similar to the motionproduced in a clothesline by pulling it down andletting go (see Figure 6.10b). Because liquids can-not spring back when subjected to this type of mo-tion, called sideways shear, S waves cannot movethrough liquids.13

When seismic waves reach the surface, complexsurface waves (R waves) are produced. Surface waves,which move along Earth’s surface, travel more slowlythan either P or S waves and cause much of the

Earthquake Shaking 185

earthquake damage to buildings and other structures.Damage occurs because surface waves have a com-plex horizontal and vertical ground movement orrolling motion that may crack walls and foundationsof buildings, bridges, and roads. An important surface

wave is the R wave, shown in Figure 6.10c. Therolling motion moves in the direction opposite tothat of the wave propagation (direction wave is mov-ing) and the vertical motion, or amplitude, is at rightangles to the direction of propagation.

(a)

(b)

FIGURE 6.9 Tectonic creep (a) This

concrete culvert at the Almaden Vine-

yards in Hollister, California, is being split

by creep on the San Andreas fault. (James

A. Sugar/NGS Image Collection) (b) Creep

along the Hayward fault (a branch of the

San Andreas fault) is slowly deforming

the football stadium at the University of

California at Berkeley from goalpost to

goalpost. (Courtesy of Richard Allen)


SeismographA seismogram is a written or digital record of anearthquake. In written form, it is a continuous linethat shows vertical or horizontal Earth motions re-ceived at a seismic recording station and recorded by

a seismograph. The components of a simple seismo-graph are shown in Figure 6.11a, and a photographof an older seismograph is shown in Figure 6.11b.An idealized written record, or seismogram, isshown in Figure 6.11c. Notice that, for Figure 6.11c,

Surface of Earth

Rolling Motion

A

A

A B C

C

Time 1

Time 2

Time 3

S wave: Direction of propagation

(b)

Time 1

P wave: Direction of propagation

R wave: Direction of propagation of surface wave

(a)

(c)

Time 2

Time 3

A

A

A

Push Pull

Push PullPull

PushPull Pull

FIGURE 6.10 Seismic waves Idealized diagram showing differences between P waves and S waves. (a) Visualize the P wave as di-

lation (pulling apart) and contraction (pushing together) of the rings of a Slinky plastic spring. If you extend it about 3 m (about 10 ft) hor-

izontally on a flat surface and then push from the left end, the zone of compression (i.e., contraction) will propagate to the right, as

shown. The rate of propagation of P waves through rocks such as granite is about 6 km/sec (3.7 mi/sec). Through liquids, P waves move

much slower, about 1.5 km/sec (0.9 mi/sec) through water. (b) To visualize S waves, stretch a rope about 7 m (about 23 ft) between two

chairs, or use a clothesline if you have one. Pull up the left end and then release it; the wave will propagate to the right, in this case, up

and down. Displacement for S waves is at right angles to the direction of wave propagation, just the opposite of P wave displacements

that push and pull in the same direction as the wave propagates. Points A, B, and C are the positions of a specific part of the wave at dif-

ferent times. S waves travel through rocks, such as granite, approximately 3 km/sec (1.9 mi/sec). They cannot travel through liquids.

(c) Surface, or R, waves. Notice that the vertical motion is at right angles to the direction of wave propagation, and that the elliptical

motion is opposite to the direction of propagation. Surface waves are often the most damaging of the seismic waves.

Pasadena Needles

Goldstone

C A L I F O R N I A

0

100 Kilometers500

50 100 Miles

Pasadena 38 km (24 mi)from epicenter

Needles 356 km (222 mi)from epicenter

Goldstone 195 km (122 mi)from epicenter

7 sec

35 sec

65 sec

0 1 2 3

PS R

Time S – P =50 seconds

Time (minutes)

BackgroundAmplitude (0)

Seismic stations

Epicenter 1994Northridge earthquakeApproximate lengthof time between occurenceof earthquake and arrival ofP waves

(d)

(b)(a)

(c)

No. of sec.

Lightsource

Light beam direct

Pendulum

Mirror

Tie wire

Supportingcolumn

Boom Pivot

Bedrock

ReflectedRecording drumand seismograph

Concrete base

FIGURE 6.11 Seismograph (a) Simple seismograph, showing how it works. (b) Modern seismograph at the Pacific Geoscience Centre

in Canada. The earthquake is the February 28, 2001, Seattle earthquake (M 6.8). (Ian McKain/AP Photo) (c) Idealized seismogram for an

earthquake. The amplitude of the R (surface) waves is greater than that of P and the S waves. The S – P time of 50 seconds tells us that the

earthquake epicenter was about 420 km (261 mi) from the seismograph. (d) Differences in arrival time and amount of shaking at three seis-

mic stations located from 38 to 356 km (24 to 222 mi) from the 1994 Northridge, California, earthquake. Notice that, with distance, the time

of arrival of shaking increases, and the amplitude of shaking decreases. (Modified from Southern California Earthquake Center)

187


the P waves arrive 50 seconds before the S wavesand about 1 minute 40 seconds before the R waves.R waves have the largest amplitude and often causethe most damage to buildings.

The effect of distance on the seismogram isshown in Figure 6.11d for the 1994 M 6.7 North-ridge, California, earthquake. Three seismographs,from close to the epicenter at Pasadena (38 km[24 mi]) to far away at Needles (356 km [221 mi]),are shown. The shaking starts with the arrival of theP wave. Notice that the shaking arrives sooner andis more intense at Pasadena than at Needles.

The difference in the arrival times shown byseismograms (S – P) can be used to locate the epi-center of an earthquake. Locating an epicenterrequires at least three seismograms from three lo-cations. For example, the S – P value of 50 secondson Figure 6.11c suggests that the distance betweenthe epicenter and the seismograph was about420 km (261 mi). This is calculated algebraically,using the S – P value and the velocity for P andS waves mentioned earlier; however, this calcula-tion is beyond the scope of our discussion. Al-though we now know that the epicenter was about420 km (261 mi) from the seismograph, this dis-tance could be in any direction. When the recordsfrom seismographs from numerous seismic sta-tions are analyzed using computer models, the epi-center for an earthquake can be located. The modelis used to calculate the travel times of seismicwaves to each seismograph from various depths offocus (Figure 6.12). Distances and depths to thefocus are adjusted to provide the best fit of thedata from all the seismographs. By a process of

approximation, the location of the epicenter anddepth to the focus are determined.

Frequency of Seismic WavesAn important characteristic of earthquakes andtheir seismic waves is their frequency, measured incycles per second, or hertz (Hz). To understandwave frequency, visualize water waves travelingacross the surface of the ocean, analogous to seismicwaves. If you are at the end of a pier and record thetime when each wave peak moves by you towardshore, the average time period between wave peaksis the wave period and is usually measured in sec-onds. Consider the passing of each wave by you to beone cycle. If one wave passes the end of the pier onits way toward the shore every 10 seconds, the waveperiod is 10 seconds, and there are 6 waves perminute. The frequency of the waves in cycles persecond is 1 cycle divided by 10 seconds, or 0.1 Hz.Returning to earthquake waves, most P and S waveshave frequencies of 0.5 to 20 Hz, or 0.5 to 20 cyclesper second. Surface waves have lower frequenciesthan P and S waves, often less than 1 Hz.

Why is the wave frequency important? During anearthquake, seismic waves with a wide range of fre-quencies are produced, and intense ground motionnear the epicenter of a large earthquake is observed.High-frequency shaking causes low buildings tovibrate, and low-frequency shaking causes tall build-ings to vibrate. It is the vibrating or shaking thatdamages buildings. Two points are important inunderstanding the shaking hazard: (1) Near theepicenter, both shorter buildings and taller buildings

2 4 6 8 10

PACIFICOCEAN

San Francisco

Seismic recordingstation—seismograph

Epicenter

Earthquake focus

Time of expandingwavefront in seconds2

FIGURE 6.12 Locating an earthquake Idealized

diagram of San Francisco, California, region, with expanding

wavefront of seismic waves from the focus of an earthquake.

The arrival of waves at seismic recording stations may be used

to mathematically calculate the location of the epicenter and its

depth of focus. (U.S. Geological Survey)


may be damaged by high- and low-frequency seismicwaves. (2) With increasing distance from the epicen-ter, the high-frequency waves are weakened or re-moved by a process called attenuation. Rapid shak-ing dies off quickly with distance. As a result, nearbyearthquakes are often described as “jolting” and far-away earthquakes as “rolling.” Low-frequency seismicwaves of about 0.5 to 1.0 Hz can travel long dis-tances without much attenuation. Therefore, theycan damage tall buildings far from the epicenter.This fact has importance in planning to reduceearthquake damages and loss of life. Tall buildingsneed to be designed to withstand seismic shaking,even if they are located hundreds of kilometers fromlarge faults that are capable of producing strong togreat earthquakes.

Material AmplificationDifferent Earth materials, such as bedrock, alluvium(sand and gravel), and silt and mud, respond differ-ently to seismic shaking. For example, the intensityof shaking or strong ground motion of unconsoli-dated sediments may be much more severe thanthat of bedrock. Figure 6.13 shows how the amplitudeof shaking, or the vertical movement, is greatlyincreased in unconsolidated sediments, such assilt and clay deposits. This effect is called materialamplification.

The Mexico City earthquake (M 8.1) demon-strated that buildings constructed on materialslikely to accentuate and increase seismic shakingare extremely vulnerable to earthquakes, even if theevent is centered several hundred kilometers away.

Although seismic waves originating offshore ini-tially contained many frequencies, those arriving atthe city were low-frequency waves of about 0.5 to1.0 Hz. It is speculated that, when seismic wavesstruck the lake beds beneath Mexico City, the am-plitude of shaking may have increased at the sur-face by a factor of 4 or 5. Figure 6.14 shows thegeology of the city and the location of the worstdamage. The intense regular shaking caused build-ings to sway back and forth, and, eventually, manyof them pancaked as upper stories collapsed ontolower ones.14

The potential for amplification of surface waves tocause damage was again demonstrated with tragicresults during the 1989 Loma Prieta earthquake (M 7.1), which originated south of San Francisco.Figure 6.15 (page 191) shows the epicenter and theareas that greatly magnified shaking. The collapse ofa tiered freeway, which killed 41 people, occurred ona section of roadway constructed on bay fill and mud(Figure 6.16, page 192). Less shaking occurredwhere the freeway was constructed on older,stronger alluvium; in these areas, the structure sur-vived. Extensive damage was also recorded in theMarina district of San Francisco (Figure 6.17, page192). This area was primarily constructed on bay filland mud, as well as debris dumped into the bay dur-ing the cleanup following the 1906 earthquake.15

DirectivityRupture of rocks on a fault plane starts at a pointand radiates, or propagates, from that point. Thelarger the area of rupture, the larger the earthquake.

Low HighAmplification of shaking (surface waves)

Hard igneousrock

Sedimentaryrock

Alluvium Silt,mud

FIGURE 6.13 Amplification of shakingGeneralized relationship between near-surface

Earth material and amplification of shaking

during a seismic event.


Directivity, the intensity of seismic shaking in-creases in the direction of the fault rupture. Forexample, fault rupture of the 1994 Northridgeearthquake was upwards along the fault (north) andto the west, resulting in stronger ground motions

from seismic shaking to the northwest (Figure 6.18,page 193). The stronger ground motion to thenorthwest is believed to have resulted from themovement of a block of Earth, the hanging-wallblock, upward and to the northwest.11

Ancient Lake Texcoco

Ancient LakeTexcoco deposits

Volcanic deposits

Alluvial sediments

Volcanic rocks

Sedimentary rocks

Mexico CityUrban Area

Mexico City

100,000 years

2 million years

70 million years

Qua

tern

ary

Terti

ary

Cre

tace

ous

Rio FrioSierra

3000 m2000 m1000 m

0 m

Mexico CityLas CrucesSierra

Extreme damage

Approximate location of cross section belowSevere damage

(a)

(b)

0

5 10 Kilometers0

5 10 Miles

FIGURE 6.14 Earthquakedamage to Mexico City(a) Generalized geologic map of

Mexico City, showing the ancient

lake deposits where the greatest

damage occurred. The zone of

extreme damage is represented

by the solid red area, and the

severe damage zone is outlined

in red. (b) One of many buildings

that collapsed during the 1985

(M 8.1) earthquake. (Both courtesy

of T. C. Hanks and Darrell

Herd/USGS)


Ground Acceleration DuringEarthquakesStrong ground motion from earthquakes may bedescribed in terms of the speed or velocity at whichprimary, secondary, and surface waves travelthrough rocks or across the surface of Earth. Ingeneral, these waves move a few kilometers per sec-

ond. This velocity is analogous to the units used todescribe the speed at which you drive a car, for ex-ample, 100 km (62 mi) per hour. Earthquake waves,however, travel much faster than cars, at a velocityof about 20,000 km (12,428 mi) per hour, similar tothat of the U.S. Space Shuttle when it orbits Earth.Damage to structures from strong ground motion isrelated to both the amplitude of seismic surfacewaves and the rate of velocity change of the seismicwaves with time. The rate of change of velocity withtime is referred to as acceleration. You may havelearned in physical science that the acceleration dueto gravity on Earth is 9.8 meters per second per sec-ond (written as 9.8 m/sec2, or 32 ft/sec2). This ac-celeration is known as 1 g. If you parachute from anairplane and free fall for several seconds, you willexperience the acceleration of gravity (1 g), and, bythe end of the first second of free fall, your velocitywill be 9.8 m/sec. At the end of 2 seconds of free fall,your velocity will be about 20 m/sec, and it will beabout 30 m/sec at the end of 3 seconds. This increase,or rate of change, in velocity with time is the accelera-tion. Earthquake waves cause the ground to acceler-ate both vertically and horizontally, just as your caraccelerates horizontally when you step on the gaspedal. Specially designed instruments, called ac-celerometers, measure and record the acceleration ofthe ground during earthquakes. The units of acceler-ation in earthquake studies are in terms of the accel-eration of gravity. If the ground accelerates at 1 g, the value is 9.8 m/sec2. If the acceleration of the ground is 0.5 g, the value is roughly 5 m/sec2.One reason we are interested in acceleration of theground from earthquakes is that, when engineersdesign buildings to withstand seismic shaking, thebuilding design criteria are often expressed interms of a maximum acceleration of the ground,since it is the horizontal acceleration of the groundthat causes most damage to buildings. For example,earthquakes with M 6.0 to 6.9, typical magnitudesfor strong southern California earthquakes, will pro-duce horizontal accelerations of about 0.3 to 0.7 g,with localized values near the epicenter exceeding1.0 g. Horizontal accelerations in excess of about0.3 g cause damage to some buildings, and, at 0.7 g,damage is widespread, unless buildings are designedand constructed to withstand strong groundmotion. Therefore, if we wish to design buildings towithstand M 6 to 6.9 earthquakes, we need toconservatively design them to withstand ground

1

85 17

84

92

24

80

80

580

280

280

880

880

680

580101

101

PACIFIC

OCEANSan Francisco Bay

San Andreas fault

SanFrancisco

Collapsedfreeway

Oakland

Marinadistrict

1989 Epicenter

Bay fill and mud——greatly magnifies shaking—liquefaction may occur—structures built on these materials may suffer significant damage during an earthquake

Older alluvium——moderate shaking is likely—well-built structures generally survive an earthquake

Bedrock——relatively less shaking and damage from earthquake

0

4 8 Kilometers0

4 8 Miles

FIGURE 6.15 Loma Prieta earthquake San Francisco

Bay region, showing the San Andreas fault and the epicenter of

the 1989 earthquake, which had a magnitude of 7.1. The most

severe shaking was on bay fill and mud, where a freeway col-

lapsed and the Marina district was damaged. (Modified after T.

Hall. Data from U.S. Geological Survey)

192

FIGURE 6.16 Collapse of afreeway (a) Generalized

geologic map of part of the San

Francisco Bay, showing bay fill

and mud and older alluvium.

(Modified after Hough, S. E., et al.

1990. Nature 344(6269):853–855.

Copyright © Macmillan Maga-

zines Ltd., 1990. Used by permis-

sion of the author) (b) Collapsed

freeway, as a result of the 1989

earthquake. (Courtesy of Dennis

Laduzinski)

FIGURE 6.17 Earthquake damageDamage to buildings in the Marina district

of San Francisco, resulting from the 1989

M 7.1 earthquake. (John K. Nakata/USGS)

CHAPTER 6 Earthquakes

24

13

80

80

580880

RedwoodRegional

Park

Nimitz Fwy

Warren Fwy

Bay

Bridge M

acArthur Fwy

San Leandro

Alameda

Oakland

San Francisco Bay

1 2 Kilometers0

OaklandSan Francisco

PACIFICOCEAN

Collapse of two-tiersection of Nimitz Freeway

Bay fill and mudGreatly magnifies shaking—liquefaction may occur. Structures built on thesematerials may suffer significantdamage during an earthquake.

Older alluvium Moderate shaking is likely. Well-built structures generallysurvive an earthquake.

(a)

(b)


accelerations of about 0.6 to 0.7 g. To put this inperspective, consider that homes constructed ofadobe, which are common in rural Mexico, SouthAmerica, and the Middle East, can collapse undera horizontal acceleration of 0.1 g.13 Unreinforced,prefabricated concrete buildings are also very

vulnerable, at 0.2 to 0.3 g, as was tragically illus-trated in the 1995 Sakhalin, Russia, earthquake(M 7.5). Two thousand of the 3,000 people in thetown of Neftegorsk were killed when 17 prefabri-cated apartment buildings collapsed into rubble(Figure 6.19).

Intensityof shaking

High

Low

Epicenter

Los Angeles N

Slip on faultsurface (m)

3.2

1.6

0Focus

Fault plane

5 km

19 km

40°

10 kilometers

FIGURE 6.18 Epicenter and rupture path Aerial view of the Los Angeles region from the south, showing the epicenter of the

M 6.7 1994 Northridge earthquake and showing peak ground motion in centimeters per second and the fault plane in its subsurface

position. The path of the rupture is shown, along with the amount of slip in meters along the fault plane. The area that ruptured along the

fault plane is approximately 430 km2 (166 mi2), and the fault plane is dipping at approximately 40 degrees to the south–southwest.

Notice that the maximum slip and intensity of shaking both occur to the northwest of the epicenter. The fault rupture apparently began

at the focus in the southeastern part of the fault plane and proceeded upward and to the northwest, as shown by the arrow.

(U.S. Geological Survey. 1996. USGS Response to an Urban Earthquake, Northridge ‘94. U.S. Geological Survey Open File Report 96–263)


FIGURE 6.19 Buildingscollapse in Russia Rows

of prefabricated, unreinforced,

five-story apartment buildings

destroyed by the 1995 earth-

quake that struck the island of

Sakhalin in Russia (M 7.5).

(Tanya Makeyeva/AP Photo)

SupershearSupershear occurs when the propagation of rupture isfaster than the velocity of shear waves or surfacewaves produced by the rupture. The effect is analo-gous to supersonic aircraft breaking the sound bar-rier, producing a sonic boom. Supershear can produceshock waves that produce strong ground motionalong the fault. The increased shear and shock maysignificantly increase the damage from a large earth-quake. Supershear has been observed or suspectedduring several past earthquakes, including the 1906San Francisco earthquake and the 2002 Denaliearthquake in Alaska. Apparently, supershear is mostlikely to occur with strike-slip earthquakes that rup-ture a long straight fault segment of several tens to ahundred or more kilometers in length.

Depth of FocusRecall that the point or area within Earth where anearthquake rupture starts is called the focus (seeFigure 6.1). The depth of focus of an earthquakevaries from just a few kilometers deep to almost700 km (435 mi) below the surface. The deepest

earthquakes occur along subduction zones, whereslabs of oceanic lithosphere sink to great depths.The 2001 Seattle, Washington, earthquake (M 6.8)is an example of a relatively deep earthquake. Al-though the Seattle event was a large quake, aboutthe same size as the 1994 Northridge earthquake,the focus was deeper, at about 52 km (32 mi)beneath Earth’s surface. The focus was within thesubducting Juan de Fuca plate (see Figures 2.6 and2.10). Seismic waves had to move up 52 km (32 mi)before reaching the surface, and they lost much oftheir energy in the journey. As a result, the earthquakecaused relatively little damage, considering its mag-nitude. In contrast, most earthquakes in southernCalifornia have focal depths of about 10 to 15 km(6 to 9 mi), although deeper earthquakes have oc-curred. These relatively shallow earthquakes aremore destructive than deeper earthquakes of compa-rable magnitude because they are deep enough togenerate strong seismic shaking, yet sufficientlyclose to the surface to cause strong surface shaking.A M 7.5 earthquake on strike-slip faults in the Mo-jave Desert near Landers, California, in 1992 had afocal depth of less than 10 km (6 mi). This event

Earthquake Cycle 195

caused extensive ground rupture for about 85 km(53 mi).16 Local vertical displacement exceeded 2 m(6.5 ft), and extensive lateral displacements of about5 m (16 ft) were measured (Figure 6.20). If the Lan-ders event had occurred in the Los Angeles Basin, ex-tensive damage and loss of life would have occurred.

6.8 Earthquake CycleObservations of the 1906 San Francisco earthquake(M 7.7) led to a hypothesis known as the earthquakecycle. The earthquake cycle hypothesis proposesthat there is a drop in elastic strain after an earth-quake and there is a reaccumulation of strain beforethe next event.

Strain is deformation resulting from stress.Elastic strain may be thought of as deformation thatis not permanent, provided that the stress is re-leased. When the stress is released, the deformedmaterial returns to its original shape. If the stresscontinues to increase, the deformed material even-tually ruptures, making the deformation perma-nent. For example, consider a stretched rubberband or a bent archery bow; continued stress willsnap the rubber band or break the bow. When astretched rubber band or bow breaks, it experiencesa rebound, in which the broken ends snap back,

releasing their pent-up energy. A similar effect, re-ferred to as elastic rebound, occurs after an earth-quake (Figure 6.21). At time 1, rocks on either sideof a fault segment have no strain built up and showno deformation. At time 2, elastic strain begins tobuild, caused by the tectonic forces that pull therocks in opposite directions, referred to as shearstress. The rocks begin to bend. At time 3, elasticstrain is accumulating, and the rocks have bent;however, they are still held together by friction.When the deformed rocks finally rupture, at time 4,the stress is released, and the elastic strain sud-denly decreases as the sides of the fault “snap” intotheir new, or permanently deformed, position.After the earthquake, it takes time for sufficientelastic strain to accumulate again in order to pro-duce another rupture.17

Stages of the Earthquake CycleIt is speculated that a typical earthquake cycle hasthree or four stages. The first is a long period ofseismic inactivity following a major earthquake andassociated aftershocks, earthquakes that occur any-where from a few minutes to a year or so after themain event. This is followed by a second stage, char-acterized by increased seismicity, as accumulatedelastic strain approaches and locally exceeds the

(a)

(b)

FIGURE 6.20 Ground rupture from faulting (a) Cracking

of the ground and damage to a building caused by the M 7.5 Landers

earthquake, California. (b) Fence and dirt road offset to the right.

(Photos by Edward A. Keller)


strength of the rock. The accumulated elastic straininitiates faulting that produces small earthquakes.The third stage of the cycle, which may occur onlyhours or days before the next large earthquake, con-sists of foreshocks. Foreshocks are small to moderateearthquakes that occur before the main event. Forexample, a M 6 earthquake may have foreshocks of

about M 4. In some cases, this third stage may notoccur. After the major earthquake, considered to bethe fourth stage, the cycle starts over again.17 Al-though the cycle is hypothetical and periods be-tween major earthquakes are variable, the stageshave been identified in the occurrence and reoccur-rence of large earthquakes.

Rock units thatcross the fault

Fault showingdirection of displacement,in this case, right-lateral strike-slip

Fault

Time 1

Time 2

Time 3

Time 4

No strain andno displacement

Elastic strainbegins (rocksbegin to bend)

Elastic strainaccumulates(rocks bend)

Rupture (earthquake)occurs and rocksrebound. Elastic strain(bending) is replaced byhorizontal displacementknown as fault slip.

FIGURE 6.21 Earthquake cycleIdealized (map view) diagram illustrating the

earthquake cycle. Moving from time 1 to

time 4 may take from hundreds to thou-

sands of years.

Earthquakes Caused by Human Activity 197

The Dilatancy-Diffusion ModelAlthough we have many empirical observationsconcerning physical changes in Earth materialsbefore, during, and after earthquakes, no generalagreement exists in terms of a physical model toexplain the observations. One model, known as thedilatancy-diffusion model,13,18 assumes that thefirst stage in earthquake development is an in-crease of elastic strain in rocks that causes them todilate (undergo an inelastic increase in volume)after the stress on the rock reaches one-half therock’s breaking strength. During dilation, openfractures develop in the rocks, and it is at this stagethat the first physical changes that might indicatea future earthquake take place.

Briefly, the model assumes that the dilatancy andfracturing of the rocks are first associated with a rela-tively low water pressure in the dilated rocks, whichhelps to produce lower seismic velocity (velocity ofseismic waves through the rocks near a fault), moreEarth movement, and fewer minor seismic events. Aninflux of water then enters the open fractures, caus-ing the pore pressure to increase, which increases theseismic velocity and weakens the rocks. Radon gasdissolved in the water may also increase. The weaken-ing facilitates movement along the fractures, which isrecorded as an earthquake. After the movement andrelease of stress, the rocks resume many of their orig-inal characteristics.18

Considerable controversy surrounds the validityof the dilatancy-diffusion model. One aspect of themodel gaining considerable favor is the role of fluidpressure (force per unit area exerted by a fluid) inearthquakes. As we learn more about rocks at seis-mogenic (earthquake generating) depths, it is appar-ent that much water is present. Deformation of therocks and a variety of other processes are thought toincrease the fluid pressure at depth, and this resultsin a lowering of the strength of the rocks. If the fluidpressure becomes sufficiently high, this can facilitatethe occurrence of earthquakes. Empirical data fromseveral environments, including subduction zonesand active fold belts, suggest that high fluid pressuresare present in many areas where earthquakes occur.Thus, there is increasing interest in the role of fluidflow in fault displacement and the earthquake cycle.The fault-valve mechanism19 hypothesizes thatfluid (usually water) pressure rises until failure oc-curs, thus triggering an earthquake, along with up-

ward fluid discharge. This may explain why, follow-ing some earthquakes, springs discharge more waterand dry streams may start flowing (i.e., groundwaterseeps into channels). Increased flow (i.e., discharge)of groundwater may occur for months following alarge earthquake.

6.9 Earthquakes Caused by Human Activity

Several human activities are known to increase orcause earthquake activity. Damage from human-caused earthquakes is regrettable, but the lessonslearned may help to control or stop large cata-strophic earthquakes in the future. Three ways thatthe actions of people have caused earthquakes are:

• Loading the Earth’s crust, as in building a damand reservoir

• Disposing of waste deep into the groundthrough disposal wells

• Setting off underground nuclear explosions

Reservoir-Induced SeismicityDuring the 10 years following the completion ofHoover Dam on the Colorado River in Arizona andNevada, several hundred local tremors occurred.Most of them were very small, but one was M 5 andtwo were about M 4.17 An earthquake in India, ap-proximately M 6, killed about 200 people after theconstruction and filling of a reservoir. Evidently,fracture zones may be activated both by the increasedload of water on the land and by the increased waterpressure in the rocks below the reservoir, resultingin faulting.

Deep Waste DisposalFrom April 1962 to November 1965, several hun-dred earthquakes occurred in the Denver, Colorado,area. The largest earthquake was M 4.3 and causedsufficient shaking to knock bottles off shelves instores. The source of the earthquakes was eventuallytraced to the Rocky Mountain Arsenal, which wasmanufacturing materials for chemical warfare. Liq-uid waste from the manufacturing process wasbeing pumped down a deep disposal well to a depthof about 3,600 m (11,800 ft). The rock receiving the


waste was highly fractured metamorphic rock, andinjection of the new liquid facilitated slippage alongfractures. Study of the earthquake activity revealeda high correlation between the rate of waste injec-tion and the occurrence of earthquakes. When theinjection of waste stopped, so did the earthquakes(Figure 6.22).20 Fluid injection of waste as anearthquake trigger was an important occurrencebecause it directed attention to the fact that earth-quakes and fluid pressure are related.

Nuclear ExplosionsNumerous earthquakes with magnitudes as largeas 5.0 to 6.3 have been triggered by underground

nuclear explosions at the Nevada Test Site. Analy-sis of the aftershocks suggests that the explosionscaused some release of natural tectonic strain. Thisled to discussions by scientists about whether nu-clear explosions might be used to prevent largeearthquakes by releasing strain before it reaches acritical point.

6.10 Effects of EarthquakesShaking is not the only cause of death and damage inearthquakes: Catastrophic earthquakes have a widevariety of destructive effects. Primary effects—those caused directly by fault movement—include

1962 1963 1964

No wasteinjected

1965

(b)

(a)

0

20

40

60

80

Earth

quak

es/m

onth

05

10152025303540

Inje

ctio

n ra

te10

00s

m3 /

mon

th

3600 m

Sedimentary rock

Fractured metamorphic rock

ColoradoSt. Capitol

Fault

Focus of earthquakes

Frequency of Earthquakes

Waste Injection

Arsenal disposalWell

North Denver

0 5 km

FIGURE 6.22 Human-causedearthquakes (a) Generalized block

diagram showing the Rocky Mountain

Arsenal well. The red line encompasses an

area where most of the earthquakes oc-

curred and a natural fault is present about

10 km southwest of the disposal well. (b)

Graph showing the relationship between

earthquake frequency and the rate of in-

jection of liquid waste. Injection of waste

stoppen in 1966 and the well was perma-

nently sealed in 1986. Earthquake activity

has settled back to the natural frequency.

An earthquake magnitude of 4.1 occurred

in 1982, suggesting the faulting remains

active (Data from www.geosurvey.state.

co.us. Rocky Mountain Arsenal--The most

famous example of induced earthquakes.

Accessed 12/5/10)

www.geosurvey.state.co.us

www.geosurvey.state.co.us

Effects of Earthquakes 199

ground shaking and its effects on people and struc-tures and surface rupture. Secondary effects inducedby the faulting and shaking include liquefaction ofthe ground, landslides, fires, disease, tsunami, andregional changes in ground and surface water, as wellas land elevation.

Shaking and Ground RuptureThe immediate effects of a catastrophic earthquakecan include violent ground shaking, accompanied bywidespread surface rupture and displacements. Sur-face rupture with a vertical component produces afault scarp. A fault scarp is a linear, steep slope thatlooks something like a curb on a street. Fault scarpsare often approximately 1 m (3.3 ft) or more highand extend for a variable distance, from a few tensof meters to a few kilometers along a fault (Figure6.23). The 1906 San Francisco earthquake (M 7.7)produced 6.5 m (21.3 ft) of horizontal displacement,or fault-slip, along the San Andreas fault north ofSan Francisco and reached a maximum ModifiedMercalli Intensity of XI.13 At this intensity, surfaceaccelerations can snap and uproot large trees andknock people to the ground. The shaking maydamage or collapse large buildings, bridges, dams,tunnels, pipelines, and other rigid structures.The great 1964 Alaskan earthquake (M 8.3) causedextensive damage to transportation systems, rail-roads, airports, and buildings. The 1989 Loma Pri-eta earthquake (M 7.1) was much smaller than theAlaska event, yet it caused about $5 billion of dam-

age. The 1994 Northridge earthquake (M 6.7)caused 57 deaths and inflicted about $40 billion indamage, making it one of the most expensive haz-ardous events ever in the United States. The North-ridge earthquake caused so much damage because therewas so much there to be damaged. The Los Angelesregion is highly urbanized, with a high populationdensity, and the seismic shaking was intense.

LiquefactionLiquefaction is the transformation of water-saturatedgranular material, or sediments, from a solid to a liq-uid state. During earthquakes, liquefaction may resultfrom compaction of sediments during intense shak-ing. Liquefaction of near-surface water-saturated siltsand sand causes the materials to lose their strengthand to flow. As a result, buildings may tilt or sink intothe liquefied sediments, while tanks or pipelinesburied in the ground may rise buoyantly.21

LandslidesEarthquake shaking often triggers landslides, a com-prehensive term for several types of hill slope failure,in hilly and mountainous areas. Landslides can beextremely destructive and can cause great loss of life,as demonstrated by the 1970 Peru earthquake. Inthat event, more than 70,000 people died, and, ofthose, 20,000 were killed by a giant landslide thatburied the cities of Yungan and Ranrahirca. Both the1964 Alaskan earthquake and the 1989 Loma Prieta

FIGURE 6.23 Fault scarpFault scarp produced by the 1992

M 7.5 Landers earthquake in Cali-

fornia. (Edward A. Keller)


earthquake caused extensive landslide damage tobuildings, roads, and other structures. The 1994Northridge earthquake and aftershocks triggeredthousands of landslides. A giant landslide from the

side of a mountain was triggered by the 2002 Alaskaearthquake (Figure 6.24). Thousands of other land-slides were also triggered by the earthquake, most ofwhich were on the steep slopes of the Alaska Range.

A large landslide associated with the January 13,2001, El Salvador earthquake (M 7.6) buried thecommunity of Las Colinas, killing hundreds of peo-ple (Figure 6.25). Tragically, the landslide couldprobably have been avoided if the slope that failedhad not previously been cleared of vegetation forthe construction of luxury homes.

The 2008 M 7.9 earthquake in China caused manylandslides. Very large landslides (Figure 6.26) buriedvillages and blocked rivers, forming “earthquakelakes” that produced a serious flood hazard, shouldthe landslide dam fail by overtopping or erosion.Channels were excavated to slowly drain lakes andreduce the flood hazard.22

FiresFire is a major hazard associated with earthquakes.Shaking of the ground and surface displacements canbreak electrical power and gas lines, thus startingfires. In individual homes and other buildings, appli-ances such as gas heaters may be knocked over, caus-ing gas leaks that could be ignited. The threat fromfire is intensified because firefighting equipment may

FIGURE 6.24 Earthquake-triggered landslides This

giant landslide was one of thou-

sands triggered by the 2002 (M 7.9)

Alaskan earthquake. Landslide de-

posits cover part of the Black Rapids

Glacier. (USGS)

FIGURE 6.25 Landslide in El Salvador The brown area in

the center of this image is the landslide that buried more than

500 homes in the Las Colinas neighborhood of Santa Tecla, a

western suburb of San Salvador, the capital of El Salvador. The

landslide, which killed more than 500 people, was triggered by a

January 2001 earthquake. (USGS)

Effects of Earthquakes 201

as the “San Francisco Fire,” and, in fact, 80 percent ofthe damage from that event was caused by firestormsthat ravaged the city for several days. The 1989 LomaPrieta earthquake also caused large fires in the Ma-rina district of San Francisco.

FIGURE 6.26 M 7.9 earthquake in China A 2008 earthquake produced a

huge landslide that blocked a river, producing an “earthquake lake.”Channels in the

landslide dam were excavated to lower the lake level and reduce a potential flood

hazard by overtopping or erosion. (AP Photo)

become damaged, and essential water mains may bebroken during an earthquake. Earthquakes in bothJapan and the United States have been accompaniedby devastating fires (Figure 6.27). The San Franciscoearthquake of 1906 has been repeatedly referred to

FIGURE 6.27 Earthquake andfire Fires associated with the 1995

Kobe, Japan, earthquake caused

extensive damage to the city. (Corbis)


DiseaseLandslides from the 1994 Northridge earthquakeraised large volumes of dust, some of which con-tained fungi spores that cause valley fever. Windscarried the dust and spores to urban areas, includingSimi Valley, where an outbreak of valley fever oc-curred during an 8-week period after the earth-quake. Two hundred cases were diagnosed, which is16 times the normal infection rate; of these, 50 peo-ple were hospitalized and 3 died.11 Earthquakes canrupture sewer and water lines, causing water to be-come polluted by disease-causing organisms. Thedeath of animals and people buried in earthquakedebris also produces potential sanitation problemsthat may result in an outbreak of disease.

Regional Changes in Groundwaterand Land ElevationEarthquakes can alter groundwater levels andstream flow. Earth materials, such as fine sediment(i.e., sand, silt) when shaken can consolidate, forc-ing water out (i.e., initiating spring or stream flow).Earthquakes can also fracture rock, increasing porespaces or cleaning out fractures to increase ground-water flow. Changes in water level and stream flowtend to increase with earthquake magnitude.Groundwater change from a large earthquake canoccur hundreds to thousands of kilometers from theepicenter, while changes in stream flow (usually in-creases) can occur tens to hundreds of kilometersfrom the epicenter.23,24

Vertical deformation, including both uplift andsubsidence, is another effect of some large earth-quakes. Such deformation can cause regional changesin groundwater levels. The great 1964 Alaskanearthquake (M 8.3) caused regional uplift and subsi-dence.25 The uplift, which was as much as 10 m(33 ft), and the subsidence, which was as much as2.4 m (7.8 ft), caused effects ranging from severelydisturbing or killing coastal marine life to changes ingroundwater levels. In areas of uplift, canneries andfishermen’s homes were displaced above the high-tide line, rendering docks and other facilities inop-erable. The subsidence resulted in flooding of somecommunities. In 1992, a major earthquake (M 7.1)near Cape Mendocino in northwestern Californiaproduced approximately 1 m (3.3 ft) of uplift at theshoreline, resulting in the death of marine organ-isms exposed by the uplift.26

TsunamiTsunami (from the Japanese word for “large harborwaves”) are produced by the sudden vertical dis-placement of ocean water. Tsunami (the subject ofChapter 7) are a serious natural hazard that cancause catastrophes thousands of kilometers fromwhere they originate.

6.11 Earthquake Risk andEarthquake Prediction

The great damage and loss of life associated withearthquakes are due, in part, to the fact that theyoften strike without warning. A great deal of re-search is being devoted to anticipating earthquakes.The best we can do at present is to use probabilisticmethods to determine the risk associated with aparticular area or with a particular segment of afault. Such determinations of risk are a form oflong-term prediction: We can say that an earth-quake of a given magnitude or intensity has a highprobability of occurring in a given area or fault seg-ment within a specified number of years. These pre-dictions assist planners who are considering seismicsafety measures or people who are deciding whereto live. However, long-term prediction does nothelp residents of a seismically active area to antici-pate and prepare for a specific earthquake over aperiod of days or weeks before an event. Short-termprediction, specifying the time and place of theearthquake, would be much more useful, but theability to make such predictions has eluded us. Pre-dicting imminent earthquakes depends to a largeextent on observation of precursory phenomena, orchanges preceding the event.

Estimation of Seismic RiskThe earthquake risk associated with a particular areais shown on seismic hazard maps, which are preparedby scientists. Some of these maps show relative haz-ard—that is, where earthquakes of a specified magni-tude have occurred. However, a preferable method ofassessing seismic risk is to calculate the probability ofeither a particular event or the amount of shakinglikely to occur. Figure 6.28a shows an earthquakehazard map for the United States. It was prepared onthe basis of the probability of horizontal groundmotion. The darkest areas on the map represent theregions of greatest seismic hazard because those areas

Earthquake Risk and Earthquake Prediction 203

have the highest probability of experiencing thegreatest seismic shaking. Regional earthquake hazardmaps are valuable; however, considerably more dataare necessary to evaluate hazardous areas in order toassist in the development of building codes and thedetermination of insurance rates. Figure 6.28bshows that the probability of one or more M 6.7 orlarger earthquakes occurring over a 30 year period inCalifornia is a near certainty. The probability for a M7.5 is also high; for M 8, the probability is about 1 in25.27,28 Four sources of data were used to estimatethe probability of earthquakes:27

• Geology: the mapped location of faults withdocumented offsets

• Paleoseismology: the study of past earthquakehistory from trenches excavated across faultsto determine dates and amount of displace-ment from past events

• Geodesy: the determination, most often fromglobal positioning systems (GPSs), of how fastEarth’s tectonic plates are moving

• Seismology: the occurrence of past earthquakesdetected by seismographs located at many lo-cations that monitor when earthquakes occurand how strong they are

Short-Term PredictionThe short-term prediction, or forecast, of earthquakesis an active area of research. Similarly to a weather

forecast, an earthquake forecast specifies a relativelyshort time period in which the event is likely to occurand assigns it a probability of occurring. The basicprocedure for predicting earthquakes was oncethought to be as easy as “one-two-three.”29 First, de-ploy instruments to detect potential precursors of afuture earthquake; second, detect and recognize theprecursors in terms of when an earthquake will occurand how big it will be; and, third, after review of yourdata, publicly predict the earthquake. Unfortunately,earthquake prediction is much more complex thanfirst thought.29

The Japanese made the first attempts at earthquakeprediction with some success, based on the frequencyof microearthquakes (M less than 2), repetitive sur-veys of land levels, and a change in the local magneticfield of Earth. They found that earthquakes in theareas they studied were nearly always accompanied byswarms of microearthquakes that occurred severalmonths before the major shocks. Furthermore, groundtilt was correlated strongly with earthquake activity.

Chinese scientists made the first successful pre-diction of a major earthquake in 1975. The February4, 1975, Haicheng earthquake (M 7.3) destroyed ordamaged about 90 percent of buildings in the city,which had a population of 9,000. The short-termprediction was based primarily on a series of fore-shocks that began 4 days before the main event.On February 1 and 2, several shocks with a magni-tude of less than 1 occurred. On February 3, less

ATLANTICOCEAN

PACIFICOCEAN

Lowest hazardHighest hazard

FIGURE 6.28a Earthquake hazardmap A probabilistic approach to the

seismic hazard in the United States. Colors

indicate level of hazard. (From U.S. Geological

Survey Fact Sheet 2008–3027)


than 24 hours before the main shock, a foreshock ofM 2.4 occurred, and, in the next 17 hours, eightshocks with a magnitude greater than 3 occurred.Then, as suddenly as it began, the foreshock activitybecame relatively quiet for 6 hours, until the mainearthquake occurred.30 Haicheng’s population of9,000 was saved by a massive evacuation from po-tentially unsafe housing just before the earthquake.

Unfortunately, foreshocks do not always precedelarge earthquakes. In 1976, one of the deadliest earth-quakes in recorded history struck near the mining townof Tanshan, China, killing several hundred thousand

people. There were no foreshocks. Earthquake predic-tion is still a complex problem, and it will probably bemany years before dependable short-range prediction ispossible. Such predictions most likely will be basedupon precursory phenomena such as:

• Patterns and frequency of earthquakes, such asthe foreshocks used in the Haicheng prediction

• Preseismic deformation of the ground surface• Emission of radon gas• Seismic gaps along faults• Anomalous animal behavior (?)

San Francisco Bay

Pacific Ocean

MontereyBay

360

880

280

101

101

1

17

580

SanJose

SanMateo

PaloAlto

Gilroy

Salinas

Monterey

Santa CruzWatsonville

Bay

Pacifica

San Francisco

San Rafael

Novato

Petaloma1

101

SantaRosa

NapaSanoma

AntiochStockton

Sacramento

Tracy

LivermorePleasanton

Hayward

Oakland Dauville

WalnutCreek

Vallejo

N

27%

3%

11%

GREENVILLE

FAULT

CALAVERAS

MT. DIABLO

THRUST FAULT

CON

CORD

GREEN

VALLEY FAULT

FAULT

EXTENT OF RUPTURE

IN LOMA PRIETA QUAKE

SAN

GREG

ORIO

FAULT

SAN ANDREAS FAULT

HAYWARD

FAULT

RODG

ERS CREEK FAULT

80 Probability for one or moremagnitude 6.7 or greaterearthquakes from 2003 to 2032.This result incorporates 14% oddsof quakes not on shown faults.

62%

SAN FRANCISCO BAY REGIONEARTHQUAKE PROBABILITY

Probability of magnitude6.7 or greater quakesbefore 2032 on theindicated fault

Increasing probabilityalong fault segments

Expanding urban areas

%

10%

Half Moon

21%

4%

3%

20 Miles0

0 20 Kilometers

FIGURE 6.28b Probabilities ofearthquakes in California Probability of at least one M 6.7 or

greater earthquake occurring over the

next 30 year period (2003–2032) on a

particular fault. (Field, E. H., Milner, K. R.,

and the 2007 Working Group on Califor-

nia Earthquake Probabilities. 2008. Fore-

casting California’s Earthquakes—

What Can We Expect in the Next 30

Years? U.S. Geological Survey Fact Sheet

2008–3027)

Earthquake Risk and Earthquake Prediction 205

Preseismic Deformation of the Ground SurfaceRates of uplift and subsidence, especially when theyare rapid or anomalous, may be significant in pre-dicting earthquakes. For more than 10 years beforethe 1964 earthquake near Niigata, Japan (M 7.5),there was a broad uplift of Earth’s crust of severalcentimeters near the Sea of Japan coast. Similarly,broad slow uplift of several centimeters occurredover a 5 year period before the 1983 Sea of Japanearthquake (M 7.7).18

Preinstrument uplifts of 1 to 2 m (3.3 to 6.6 ft)preceded large Japanese earthquakes in 1793,1802, 1872, and 1927. Although these uplifts arenot well understood, they could have been indica-tors of the impending earthquakes. These upliftswere recognized by sudden withdrawals of the seafrom the land, often as much as several hundredmeters in harbors. For example, on the morning ofthe 1802 earthquake, the sea suddenly withdrewabout 300 m (984 ft) from a harbor in response toa preseismic uplift of about 1 m (3.3 ft). Fourhours later, the earthquake struck, destroyingmany houses and uplifting the land anothermeter, causing the sea to withdraw an even greaterdistance.31

Emission of Radon GasThe levels of radon, a radioactive gas, have been ob-served to increase significantly before some earth-quakes. It is believed that, before an earthquake,rocks expand, fracture, and experience an influx ofwater. Radon gas, which is naturally present inrocks, moves with the water. There was a significantincrease in radon gas measured in water wells amonth or so before the 1995 Kobe, Japan, earth-quake (M 7.2).32

Seismic GapsSeismic gaps are defined as areas along active faultzones or within regions that are likely to producelarge earthquakes but have not caused one recently.The lack of earthquakes is interpreted to be tempo-rary; furthermore, these areas are thought to storetectonic strain and are, thus, candidates for futurelarge earthquakes.33 Seismic gaps have been usefulin medium-range earthquake prediction. At least10 large plate boundary earthquakes have been

successfully forecast from seismic gaps since 1965,including one in Alaska, three in Mexico, one inSouth America, three in Japan, and one in Indonesia.In the United States, seismic gaps along the San An-dreas fault include one near Fort Tejon, California,that last ruptured in 1857 and one along theCoachella Valley, a segment that has not produced agreat earthquake for several hundred years. Bothgaps are likely candidates to produce a great earth-quake in the next few decades.17,33

As Earth scientists examine patterns of seismicity,two ideas are emerging. First, there are sometimes re-ductions in small or moderate earthquakes prior to alarger event. For example, prior to the 1978 (M 7.8)Oaxaca, Mexico, earthquake, there was a 10-year pe-riod (1963 to 1973) of relatively high seismicity ofearthquakes (mostly M 3 to 6.5), followed by a quies-cence period of 5 years. Renewed activity beginning10 months before the M 7.8 event was the basis of asuccessful prediction.31 Second, small earthquakesmay tend to ring an area where a larger event mighteventually occur. Such a ring (or donut) was noticedin the 16 months prior to the 1983 M 6.2 event inCoalinga, California. Unfortunately, hindsight isclearer than foresight, and this ring was not noticedor identified until after the event.

Anomalous Animal Behavior (?)Anomalous animal behavior has often been re-ported before large earthquakes. Reports have in-cluded dogs barking unusually, chickens refusing tolay eggs, horses or cattle running in circles, ratsperching on power lines, and snakes crawling out inthe winter and freezing. Anomalous behavior of an-imals was evidently common before the Haichengearthquake.30 Years ago, it was suggested that,sometime in the future, we might have animals,such as ground squirrels or snakes in cages, alongfaults, and that somehow they would tell us whenan earthquake is likely to occur in the near future.Undoubtedly, some animals are much more sensi-tive to Earth movements and possible changes inEarth before an earthquake or landslide than arepeople. However, the significance and reliability ofanimal behavior are very difficult to evaluate. Thereis little research going on to further explore anom-alous animal behavior in the United States, but itremains one of the interesting mysteries surround-ing earthquakes.

6.12 Toward EarthquakePrediction

We are still a long way from a working, practicalmethodology with which to reliably predict earth-quakes. However, a good deal of information is cur-rently being gathered concerning possible precursorphenomena associated with earthquakes. To date,the most useful precursor phenomena have beenpatterns of earthquakes, particularly foreshocks,and seismic gaps. Optimistic scientists around theworld today believe that we will eventually be able tomake consistent long-range forecasts (tens to a fewthousand years), medium-range predictions (a fewyears to a few months), and short-range predictions(a few days or hours) for the general locations andmagnitudes of large, damaging earthquakes.

Although progress on short-range (days tomonths) earthquake prediction has not matched ex-pectations, medium- to long-range forecast (years todecades), based on the probability of an earthquakeoccurring on a particular fault, has progressed fasterthan expected. The October 28, 1983, Borah Peakearthquake in central Idaho (M 7.3) has been laudedas a success story for medium-range earthquakehazard evaluation. Previous evaluation of the LostRiver fault suggested that the fault was active.34 Theearthquake killed two people and caused approxi-mately $15 million in damages. Fault scarps up toseveral meters high and numerous ground fracturesalong the 36 km (22 mi) rupture zone of the faultwere produced as a result of the earthquake. The im-portant fact was that the scarp and faults producedduring the earthquake were superimposed on previ-ously existing fault scarps, validating the usefulnessof careful mapping of scarps produced from prehis-toric earthquakes. Remember: Where the groundhas broken before, it is likely to break again!

6.13 Sequence of Earthquakes in Turkey: Can OneEarthquake Set up Another?

There is considerable controversy regarding patternsof repetitive earthquakes. Do earthquakes on a givenfault have return periods that are relatively constantwith relatively constant-magnitude earthquakes, ordo they tend to occur in clusters over a period of


several hundred to several thousand years? Un-doubtedly, one of the most remarkable sites in theworld for paleoseismic studies is Pallett Creek, lo-cated approximately 55 km northeast of Los Angeleson the Mojave segment of the San Andreas fault. Thesite contains evidence of 10 large earthquakes, 2 ofwhich occurred in historical time (1812 and 1857),and prehistoric events extending back to approxi-mately A.D. 671.35 High-precision radiocarbon datingmethods provide accurate dating of most of the 8prehistoric events. When the dates and their accom-panying error bars are plotted, it is apparent that theearthquakes are not evenly distributed through timebut tend to cluster. Most of the earthquakes withinclusters are separated by a few decades, but the timebetween clusters varies from approximately 160 to360 years. The earthquakes were identified throughcareful evaluation of natural exposures and faulttrenches at Pallett Creek.35 Figure 6.29 shows partof the trench wall and the offset of organic-rich sedi-mentary layers that correspond to an earthquakethat occurred in approximately A.D. 1100. Naturally,the question we wish to ask is, when will the next bigearthquake occur on the Mojave segment of the SanAndreas fault? Studies using conditional probabilitysuggest that probability is approximately 60 percentfor the next 30-year period (see Figure 6.28b). As-suming that the two historic events are a cluster and,if clusters can be separated by approximately 160 to360 years, then the next large earthquake might beexpected between the years 2017 and 2207. On theother hand, the data set at Pallett Creek, althoughthe most complete record for any fault in the world,is still a study of small numbers.

A study of the San Andreas fault, about 200 km(125 mi) northwest of Pallett Creek, discovered evi-dence for six earthquakes since A.D. 1360, with aver-age return period of about a century (88�41 years).The size distribution of these events is not well con-strained. Some could be strong earthquakes (M 6.5to 7), while others could be major earthquakes (M7.5 to 7.9).36 The more we learn about the San An-dreas fault, the more variable past earthquakesseem to be in terms of both time and magnitude.However, major events appear to occur about every150 years on average, which is about the time sincethe last big one in 1857.

Some faults, evidently, do produce earthquakes ofsimilar magnitude over relatively constant return pe-riods. However, as we learn more about individual

206

faults, we learn that there is a great deal of variabilityconcerning magnitudes of earthquakes and their re-turn periods for a given fault system. For example, inthe twentieth century, a remarkable series of earth-quakes with magnitude greater than 6.7 generallyoccurred from east to west on the North Anatolian

Sequence of Earthquakes in Turkey: Can One Earthquake Set up Another?

fault in Turkey, resulting in surface ruptures along a1,000 km (621 mi) section of the fault (Figure 6.30).Two events in 1999—the Izmit (M 7.4) and theDuzce (M 7.1) earthquakes—were particularly severe,causing billions of dollars in property damage andthousands of deaths. The sequence of earthquakes

207

1299

53 6467 57 44

43 42 3949

66

Izmit

ANATOLIAN PLATE

ARABIAN PLATE

AFRICAN PLATE

EURASIAN PLATE

North Anatolian Fault

Med i t e r r a nean Sea

MarmaraSea

Aegean Sea

B l a c k S ea

0

100 200 Kilometers0

100 200 Miles

Reverse fault, teeth onupthrown block

Strike-slip fault

Year of rupture (1900's),length indicated by arrows

12

FIGURE 6.30 Sequence ofearthquakes Earthquakes

(M greater than 6.7) on the

North Anatolian fault in the

twentieth century. Events of

1992 and 1951 are not shown.

Year 1999 shows the rupture

length of two events com-

bined (Izmit, August; and

Duzce, November). (Modified

after Reilinger, R., Toksot, N.,

McClusky, S., and Barka, A. 2000.

1999 Izmit, Turkey earthquake

was no surprise. GSA Today

10(1):1–5)

Not faulted

Faulted

22 cm scale

Fault

FIGURE 6.29 Pallett Creek Part

of the trench wall crossing the San

Andreas fault at Pallett Creek, showing

a 30 cm offset. Notice that the strata

above the 22 cm scale (central part of

photograph) are not offset. The offset

sedimentary layers (white bed) and

the dark organic layers were pro-

duced by an earthquake that oc-

curred about 800 years ago. (Courtesy

of Kerry Sieh)

has been described as a “falling-domino scenario,” inwhich one earthquake sets up the next, eventuallyrupturing nearly the entire length of the fault, in acluster of events.37,38 Clusters of earthquakes thatform a progressive sequence of events in a relativelyshort period of time are apparently separated by aperiod of no earthquakes for several hundred years.A similar sequence or clustering may be occurringalong the fault bordering the Sumatra Plate bound-ary in Indonesia. Three earlier large earthquakesfrom 1600 to 1833, discovered from the geologicrecord, preceded the 2004 event, and a M 8.7 eventoccurred in 2005. That earthquake did not produce atsunami, but it collapsed buildings and took about1,000 lives.7 Understanding processes related to clus-tering of earthquakes along a particular fault is veryimportant if we are to plan for future seismic events ina given region. The fact that there may not have beena large damaging earthquake for several hundredyears may not be as reassuring as we once thought.

6.14 The Response toEarthquake Hazards

Responses to seismic hazards in earthquake-proneareas include development of hazard-reduction pro-grams, careful siting of critical facilities, engineeringand land-use adjustments to earthquake activity,and development of a warning system. The extentto which these responses occur depends, in part, onpeople’s perception of the hazard.

Earthquake Hazard-ReductionProgramsIn the United States, the U.S. Geological Survey(USGS), as well as university and other scientists,are developing the National Earthquake Hazard Re-duction Program. The major goals of the programare to:

• Develop an understanding of the earthquakesource: This requires an understanding of thephysical properties and mechanical behavior offaults, as well as development of quantitativemodels of the physics of the earthquake process.

• Determine earthquake potential: This deter-mination involves characterizing seismically ac-tive regions, including determining the rates ofcrustal deformation; identifying active faults;


determining characteristics of paleoseismicity;estimating strong ground motion, slip rate, andearthquake recurrence interval; calculatinglong-term probabilistic forecasts; and develop-ing methods of intermediate- and short-termprediction of earthquakes. (See A Closer Look:Earthquake Hazard Evaluation: Ground Mo-tion and Slip Rate.)

• Predict effects of earthquakes: Predicting ef-fects includes gathering the data necessary forpredicting ground rupture and shaking and forpredicting the response of structures that webuild in earthquake-prone areas and evaluat-ing the losses associated with the earthquakehazard. (See A Closer Look: The Alaska Earth-quake of 2002 and the Value of Estimating Po-tential Ground Rupture.)

• Apply research results: The program is inter-ested in transferring knowledge about earth-quake hazards to people, communities, states,and the nation. This knowledge concerns whatcan be done to better plan for earthquakes andto reduce potential losses of life and property.

Adjustments to Earthquake ActivityThe mechanism of earthquakes is still poorly under-stood; therefore, such adjustments as warning sys-tems and earthquake prevention are not yet reliablealternatives. There are, however, reliable protectivemeasures we can take:

• Structural protection, including the constructionof large buildings and other structures, such asdams, power plants, and pipelines able to ac-commodate moderate shaking or surface rup-ture. This measure has been relativelysuccessful in the United States. (See A CloserLook: The Alaska Earthquake of 2002 and theValue of Estimating Potential Ground Rupture.)The 1988 Armenia earthquake (M 6.8) wassomewhat larger than the 1994 Northridgeevent (M 6.7), but the loss of life and destruc-tion in Armenia were staggering. At least45,000 people were killed, compared with 57 inCalifornia, and near-total destruction occurredin some towns near the epicenter. Most build-ings in Armenia were constructed of unrein-forced concrete and instantly crumbled intorubble, crushing or trapping their occupants.The 2005 Pakistan M 7.6 earthquake killedmore than 80,000 people. Many of the deathsoccurred as apartment buildings with little orno steel reinforcement collapsed to resemble a

208

The Response to Earthquake Hazards 209

The primary purpose of earthquakehazard assessment is to provide asafeguard against loss of life andmajor structural failures rather than tolimit damage or to maintain func-tional aspects of society, includingroads and utilities.39 Thebasic philosophy behindthe evaluation of seismichazard is to have what isknown as a design basis

ground motion, defined asthe ground motion that hasa 10 percent chance ofbeing exceeded in a 50-yearperiod (equivalent to 1chance in 475 of beingexceeded each year). This isdetermined in California bya series of maps that showstrong-motion peak groundacceleration (PGA) forearthquake waves of a 0.3-second period for one-and two-story buildingsand a 1.0-second period forbuildings taller than twostories.40 The U.S. GeologicalSurvey has programs topredict peak accelerationand velocity of shakingthrough generation of anearthquake-planning sce-nario for a particular fault.For example, Figure 6.C

shows a scenario for a M 7.2 event on the San Andreas fault at San Francisco, California.41

Assessment ofearthquake hazard at a par-ticular site includes identifi-cation of the tectonicframework (i.e., geometryand spatial pattern of faultsor seismic sources), in

order to predict earthquake slip rateand ground motion (Figure 6.D). Theprocess of predicting slip rate andground motion from a given earth-quake may be illustrated by consider-

ing a hypothetical example. Figure

6.Ea shows an example of a dam andreservoir site. The objective is to pre-dict strong ground motion at thedam from several seismic sources

ACloserLook

Earthquake Hazard Evaluation: Ground Motion andSlip Rate

Cloverdales

Bodega Bay

San Francisco

Sacramento

Placerville

StocktonVallejoAntioch

Walnut CreekOakland Tracy

Livermote Modesto

Merced

Santa Rosa

Sonoma NapaPetaluma

NovatoSan Rafael

Half Moon Bay

Santa CruzWatsonville San Juan Bautista

GilroyMorgan Hill

San JosePalo Alto

San Mateo Hayward

SalinasMonterey

38.5�

38�

37.5�

37�

36�

36.5�

–124� –123� –122�

20 40 60 Kilometers0

I II–III IV V VI VII VIII IX X+

< .17 .17–1.4 1.4–3.9 3.9–9.2 9.2–18 18–34 34–65 65–124 > 124

< 0.1 0.1–1.1 1.1–3.4 3.4–8.1 8.1–16 16–31 31–66 66–116 > 116

none none none Very light Light Moderate Moderate/Heavy

Heavy VeryHeavy

Not felt


Peak VEL(cm/s)

Peak ACC(%g)

Potentialdamage

Perceivedshaking

Weak Light Moderate Strong VeryStrong

Severe Violent Extreme

FIGURE 6.C Earthquake planning scenario This scenario is for a M 7.2 event on the San

Andreas fault in San Francisco, California. Instrumental intensity refers to the Mercalli Scale

(see Table 6.4). (www.earthquake.usgs.gov)

www.earthquake.usgs.gov


TC

Dam

River

A'

0 10 km

UD

Mw = 6.5 Consistent with rupture length of 30 km and slip of 1 m

Mw = 7 Consistent with rupture length of 50 km and slip of 2 m

A

A

TC

Dam

Reverse faultNorth side up (U) displacement,fault dip 30° North

Strike-slip fault, right-lateraldisplacement, fault dip vertical

Anticline

Syncline

Alluvium (gravel and sand)

Sedimentary rocks

Igneous rocks (granite)

Trench site

Focus of earthquake energy at 10 km below surface

A'

42 km32 km

Reservoir

(a)

(b)

FIGURE 6.E Tectonicframework for ahypothetical dam site(a) Geologic map. (b) Cross

section A-A’, showing seis-

mic sources and distances

of possible ruptures to the

dam.

Tectonicframework

(geometry and spatial variability

of faultspresent)

Focal mechanisms

(type of faulting;strike-slip,normal, or reverse slip)

Energy release

(earthquakeoccurs)

Seismic waveradiation

and propagation

Influence of local soil conditions

(may increaseamplitude of

seismic waves)

Earthquake hazard

assessment by predictive

modeling of earthquake

groundmotion

Urban area

Mountain range and fault

Energy release

Local conditions influence ground motions

Propagation of seismic waves

FIGURE 6.D Assessmentof earthquake hazardOne way to assess earthquake

hazard is to model ground

motion. (After Vogel, 1988. In

A. Vogel and K. Brandes (eds.),

Earthquake Prognostics.

Brannschweig/Weisbaden:

Friedr. Vieweg & Sohn, 1–13.)


Earth materials, folds, and faults.Assuming that earthquakes wouldoccur at depths of approximately10 km, the distances from the dam tothe two seismic sources (the reversefault and the strike-slip fault) are42 km and 32 km, respectively. Thus,for this area, two focal mechanismsare possible: reverse faulting andstrike-slip faulting.

Another step in the process is toestimate the largest earthquakeslikely to occur on these faults. Assumethat fieldwork in the area revealedground rupture and other evidenceof faulting in the past, suggestingthat, on the strike-slip fault, approxi-mately 50 km of fault length mightrupture in a single event, with right-lateral strike-slip motion of 2 m. The

(i.e., faults) in the area. The tectonicframework shown consists of anorth-dipping reverse fault and anassociated fold (an anticline) locatedto the north of the dam, as well as aright-lateral strike-slip fault locatedto the south of the dam. Figure 6.Ebshows a cross section through thedam, illustrating the geologic envi-ronment, including several different

Total vertical component of slip in 4,000 yrs about 4.0 m in 4,000 yrs or 1 mm/yr (1 m/1000 years)Most recent event about 1.5 m in 2,000 yrs or 0.8 mm/yrLast two events about 2.8 m in 3,000 yrs or 0.9 mm/yr

S

Average return period 3 events in 4,000 yrs, about 1,300 year return period

Surface fault scarp

NBuried faultscarp Sand & clay

2,000 years old

Gravel & sand 3,000 years old

Sand & clay 4,000 years old

2m

2m

0

FIGURE 6.G Trench log The trench log

(stratigraphy) for the trench site on Figure 6.Ea.

++

+

++

++

++

+

**

*

9

8

7

6

5

41 10 100 103 1 10 100 103

Mom

ent m

agni

tude

(m

)

Surface rupture length (km) Surface rupture length (km)

Strike-slipReverseNormal

a b

Strike-slipReverseNormal

77 EQs

FIGURE 6.F Relationship between moment magnitude of an earthquake andsurface rupture length Data for 77 events are shown in (a). The solid line is the “best-fit” or regres-

sion line, and dashed lines are error bars at significant difference between the lines. The regression lines

for the three types of faults are nearly the same (b). (After Wells and Coppersmith, 1994. Bulletin of the Seis-

mological Society of America 84:974-1002 )


reverse fault with surface rupturelength of 30 km, the magnitude of apossible earthquake is estimated tobe Mw 6.5. Notice that, in Figure 6.F,the regression line that predicts themoment magnitude is for strike-slip,normal, and reverse faults. Statisticalanalyses have suggested that the re-lation between moment magnitudeand length of surface rupture is notsensitive to the style of faulting.42

Slip rate (the vertical component)and average return period may beestimated from geologic data col-lected in a trench excavated across afault. Figure 6.G shows the stratigra-phy (trench log) for the site shownin Figure 6.Ea. The slip rate is about1 mm/yr (1 m/1,000 years), and the average return period is about1,300 years.

fieldwork also revealed that thelargest rupture likely on the reversefault would be 30 km of fault length,with vertical displacement of about1.5 m. Given this information, themagnitudes of possible earthquakeevents can be estimated from graphssuch as those shown in Figure 6.F.42

Fifty kilometers of surface rupture areassociated with an earthquake ofapproximately Mw 7. Similarly, for the

stack of pancakes7 (also see the case historyopening this chapter). This is not to say that theNorthridge earthquake was not a catastrophe. Itcertainly was; the Northridge earthquake left25,000 people homeless, caused the collapse ofseveral freeway overpasses, injured approxi-mately 8,000 people, and inflicted many billionsof dollars in damages to structures and build-ings (Figure 6.A). However, since most buildingsin the Los Angeles Basin are constructed withwood frames or reinforced concrete, thousandsof deaths were avoided in Northridge.

• Land use planning, including the siting of im-portant structures such as schools, hospitals,and police stations in areas away from activefaults or sensitive Earth materials that arelikely to increase seismic shaking. This plan-ning involves zoning the ground’s response toseismic shaking on a block-by-block basis. Zon-ing for earthquakes in land-use planning isnecessary because ground conditions canchange quickly in response to shaking. Inurban areas, where property values may be ashigh as millions to billions of dollars per block,we need to produce detailed maps of groundresponse to accomplish zonation. These mapswill assist engineers when designing buildingsand other structures that can better withstandseismic shaking. Clearly, zonation requires asignificant investment of time and money;however, the first step is to develop methodsthat adequately predict the ground motionfrom an earthquake at a specific site.

• Increased insurance and relief measures to helpadjustments after earthquakes. After the 1994Northridge earthquake, total insurance claims

were very large, and some insurance compa-nies terminated earthquake insurance.

We hope to be able to predict earthquakes eventu-ally. The federal plan for issuing prediction and warn-ing is shown in Figure 6.31. Notice how it is relatedto the general flow path for issuance of a disaster pre-diction shown in Figure 5.14. The general flow of in-formation moves from scientists to a predictioncouncil for verification. Once verified, a predictionthat a damaging earthquake of a specific magnitude

DATA

Scientists

USGSEarthquakePredictionCouncil

Stateearthquakepredictionreview group

USGSheadquarters

Statedisasterresponsegroup

Localofficials

HeadquartersFederal offices

Regionaloffices

GovernorOfficials

THE PUBLIC

PREDICTI

ON

WARNING

Earthquake prediction and warningproposed information flow

FIGURE 6.31 Issuing a prediction or warning A federal

plan for issuance of earthquake predictions and warning: the flow

of information. (From McKelvey, V. E. 1976. Earthquake Prediction—

Opportunity to Avert Disaster. U.S. Geological Survey Circular 729)


early 1970s, as part of an evaluationof the Trans-Alaskan pipeline thattoday supplies approximately 17 per-cent of the domestic oil supply forthe United States. Where the pipelinecrosses the Denali fault, geologistsdetermined that the fault zone wasseveral hundred meters wide andmight experience a 6 m horizontaldisplacement from a magnitude 8earthquake. These estimates wereused in the design of the pipeline,which included long, horizontal steelbeams with Teflon shoes that allowedthe pipeline to slide horizontally ap-proximately 6 m. This was accompa-nied by the utilization of a zigzagpattern of the pipeline that allowed,

along with the steel beams, the hori-zontal movement (Figure 6.H). The2002 earthquake occurred within themapped zone of the fault and sus-tained about 4.3 m (14 ft) of right-lat-eral strike-slip. As a result, thepipeline suffered little damage, andthere was no oil spill. The cost in 1970of the engineering and constructionwas about $3 million. Today, thatlooks very cost-effective, consideringthat approximately $25 million worthof oil per day is transported via thepipeline. If the pipeline had beenruptured, the cost of repair andcleanup of the environment mighthave been several hundred milliondollars.44

ACloserLook

On November 3, 2002, a magnitude7.9 earthquake occurred in earlyafternoon on the Denali fault insouth-central Alaska. That event pro-duced approximately 340 km ofsurface rupture, with maximum right-slip displacement of just over 8 m(Figure 6.H). The earthquake struck aremote part of Alaska where fewpeople live, and, although it causedthousands of landslides and numerousexamples of liquefaction and intenseshaking, little structural damage andno deaths were recorded.43

The Denali fault earthquakedemonstrated the value of seismichazard and earthquake hazard evalu-ation. The fault was studied in the

The Alaska Earthquake of 2002 and the Value ofEstimating Potential Ground Rupture

Denali fault rupture

Oil pipeline

Bering Sea Gulf ofAlaska

ARCTIC OCEAN

PACIFIC OCEAN

RUSSIA

CANADA

Alaska

160°W 150°W 140°W

60°N

70°N

Arctic Circle

Fairbanks

Valdez

Prudhoe Bay

Anchorage

FIGURE 6.H Trans-Alaska oil pipeline survives (M 7.9) earthquake The Alaskan pipeline was designed to withstand

several meters of horizontal displacement on the Denali fault. The 2002 earthquake caused a rupture of 4.3 m (14 ft) beneath the

pipeline. The built-in bends on slider beams with Teflon shoes accommodated the rupture, as designed. This was strong confirmation of

the value of estimating potential ground rupture and taking action to remove the potential threat and damage. (Alyeska Pipeline Service

Company [ASPC])


would occur at a particular location during a specifiedtime would be issued to state and local officials. Theseofficials are responsible for issuing a warning to thepublic to take defensive action that, one hopes, hasbeen planned in advance. Potential response to a pre-diction depends upon lead time, but even a few dayswould be sufficient to mobilize emergency service,shut down important machinery, and evacuate par-ticularly hazardous areas.

Earthquake Warning SystemsTechnically, it is feasible to develop an earthquakewarning system that would provide 10 seconds ormore warning to California cities before the arrival ofdamaging earthquake waves from an event severalhundred kilometers away. This type of system is basedon the principle that the warning sent by a radio sig-nal via satellite relay travels much faster than seismicwaves. The Japanese have had a system (with morethan 1,000 seismometers) for nearly 20 years thatprovides earthquake warnings for high-speed trains;train derailment by an earthquake could result in the

loss of hundreds of lives. A proposed system forCalifornia involves a network of seismometers (thereare currently about 300) and transmitters along theSan Andreas and other faults. This system would firstsense motion associated with a large earthquake andthen send a warning to Los Angeles, which would thenrelay the warning to critical facilities, schools, and thegeneral population (Figure 6.32). The warning timewould vary from as little as 10 seconds to as long asabout 1 minute, depending on the location of the epi-center of the earthquake. This could be enough timefor people to shut down machinery and computersand take cover.45,46 Note that this earthquake warn-ing system is not a prediction tool; it only warns thatan earthquake has already occurred.

A potential problem with a warning system is thechance of false alarms. For the Japanese system, thenumber of false alarms is less than 5 percent. How-ever, because the warning time is so short, somepeople have expressed concern about whether muchevasive action could be taken. There is also concernabout liability issues resulting from false alarms,warning system failures, and damage and suffering

Monitoring instruments(seismometer) Data acquisition and

processing (warningissued from here)

User of system

Fault rupture

Satellite transponder

Ground relay station

Signal keyWarning signalDetection signal

FIGURE 6.32 Earthquake warning Idealized diagram showing an earthquake warning sys-

tem. Once an earthquake is detected, a signal is sent ahead of the seismic shaking to warn people

and facilities. The warning time depends on how far away an earthquake occurs. It could be long

enough to shut down critical facilities and for people to take cover. (After Holden, R., Lee, R., and

Reichle, M. 1989. California Division of Mines and Geology Bulletin 101)

resulting from actions taken as the result of falseearly warning. The next step for California is to deter-mine how big the seismic network will need to be, in-stall the necessary seismometers, and upgrade thecommunication system so warnings can be sent outreliably.46

Perception of Earthquake HazardThe fact that terra firma is not so firm in places isdisconcerting to people who have experienced evena moderate earthquake. The large number of people,especially children, who suffered mental distressafter the San Fernando and Northridge earthquakesattests to the emotional and psychological effects ofearthquakes. These events were sufficient to influ-ence the decision of a number of families to moveaway from Los Angeles.

The Japanese were caught off guard by the 1995Kobe earthquake, and the government was criti-cized for not mounting a quick and effective re-sponse. Emergency relief did not arrive until about10 hours after the earthquake! They evidently be-lieved that their buildings and highways were rela-tively safe, compared with those that had failed inNorthridge, California, a year earlier.

The Response to Earthquake Hazards

As mentioned earlier, a remarkable sequence ofearthquakes in Turkey terminated in 1999 with twolarge damaging earthquakes (Figure 6.30). The firstoccurred on August 17 and leveled thousands ofconcrete buildings. As a result of the earthquake,600,000 people were left homeless, and approxi-mately 38,000 people died. Some of the extensivedamage to the town of Golcuk in western Turkey isshown in Figure 6.33. Note that the very oldmosque on the left is still standing, whereas manymodern buildings collapsed, suggesting that earlierconstruction was more resistant to earthquakes. Al-though Turkey has a relatively high standard fornew construction to withstand earthquakes, there isfear that poor construction was a factor in the col-lapse of the newer buildings, due to the intense seis-mic shaking. It has been alleged that some of theTurkish contractors bulldozed the rubble from col-lapsed buildings soon after the earthquake, perhapsin an effort to remove evidence of shoddy construc-tion. If that allegation is true, these contractors alsotied up bulldozers that could have been used to helprescue people trapped in collapsed buildings.

The lessons learned from Northridge, Kobe, Turkey,Pakistan, and Haiti were bitter ones that illuminateour modern society’s vulnerability to catastrophic loss

215

FIGURE 6.33 Collapse of buildings in Turkey Damage to the town of Golcuk in west-

ern Turkey from the M 7.4 earthquake in August 1999. The very old mosque on the left remains

standing, whereas many modern buildings collapsed. (Enric Marti/AP Photo)

from large earthquakes. Older, unreinforced concretebuildings or buildings not designed to withstand strongground motion are most susceptible to damage. InKobe, reinforced concrete buildings constructed post-1990 with improved seismic building codes experi-enced little damage, compared with those constructedin the mid-1960s or earlier. Minimizing the hazardrequires new thinking about the hazard. In addition,microzonation is instrumental in the engineeringresponse to design structures that are less vulnerableto ground motion from earthquakes.

Personal and CommunityAdjustments: Before, During, and After an EarthquakeA group of individuals living in an area define a com-munity, whether it is a village, town, or city. In areaswith an earthquake hazard, it is important for peo-ple to prepare for the hazard in terms of what can bedone before, during, and after an earthquake.

At the community level, one important aspect ofearthquake preparedness is enforcing building codes,such as the Uniform Building Code of California. Theobjective of the earthquake section of the code is toprovide safeguards against loss of life and major struc-tural failures through better design of buildings toallow them to better withstand earthquake shaking.Another important task is the inspection of olderbuildings to determine if a “retrofit” is necessary to in-crease the strength of the building in order to betterwithstand earthquakes. In fact, the state of Californiahas regulated building retrofits. A recent study con-cluded that many of the hospitals in southern Califor-nia are in need of extensive and costly retrofitting.

Education is also an important component ofearthquake preparedness at the community level.Government, state, and local agencies prepare pam-phlets and videos concerning the earthquake hazardand help ensure that this information is distributedto the public. Workshops and training meetings con-cerning the most up-to-date information on how tominimize earthquake hazards are available to pro-fessionals in engineering, geology, and planning. Agreat deal of information at the community level isalso available on the World Wide Web from a varietyof sources, including the U.S. Geological Survey andscientific organizations, such as the Southern Cali-fornia Earthquake Center, as well as other agenciesinterested in earthquake hazard reduction.47


In schools, it is important to practice what issometimes called the “earthquake drill”: Everyonepretends that an earthquake is happening, and stu-dents “duck, cover, and hold.” After about 15 or 20seconds, students emerge from their duck-and-coverlocation, take five slow, deep breaths, practice calm-ing down, and then walk to a designated safe area.

A massive earthquake drill was held on November13, 2008, for the Los Angeles region. Known as theGreat Southern California Shake Out, the drill involvedmillions of people. The scenario (potential) earthquakewas a M 7.8 event on the southern San Andreas faultthat resulted in 2,000 deaths, 50,000 injuries, and$200 billion in losses, with extensive and long-lastingdisruption. The drill helped evaluate emergency prepa-ration for a large earthquake and made people moreaware of what we know is coming. We will know howsuccessful the drill was after the real event occurs.

At the personal level in homes and apartments, itis estimated that billions of dollars of damages fromearthquakes could be avoided if our buildings andcontents were better secured to withstand shakingfrom earthquakes. Before an earthquake occurs,homeowners and apartment dwellers should com-plete a thorough check of the building, includingrooms, foundations, garages, and attics. The follow-ing are some items of a home safety check:47

• Be sure your chimney has been reinforced towithstand shaking from earthquakes. After the1994 Northridge earthquake, one of the mostcommonly observed types of damage was col-lapsed chimneys.

• Be sure your home is securely fastened to thefoundation and that there are panels of ply-wood between the wooden studs in walls. Makesure that large openings, such as garage doorswith a floor above, are adequately braced.

• Within the house, be sure that anything that isheavy enough to hurt you if it falls on you, or isfragile and expensive, is secured. There areways to secure tabletop objects, television sets,tall furniture, and cabinet doors.

• Large windows and sliding doors may be coveredwith strong polyester (Mylar) films to makethem safer and reduce the hazard from brokenglass that is shattered during earthquakes.

• Ensure that your gas water heater is strapped tothe building so that it will not fall over and starta fire. If necessary, change to flexible gas con-nections that can withstand some movement.

216

Probably the most important and rational thingyou can do before an earthquake is to prepare a planof exactly what you will do if a large earthquake oc-curs. This might include the following points:47

• Teach everyone in your family to “duck, cover,and hold.” For each room in your home, identi-fy safe spots, such as under a sturdy desk ortable or near strong interior walls.

• Instruct everyone who might be home how toturn off the gas. However, the gas should beturned off only if a leak is detected through ei-ther hearing or smelling the leaking gas.

• After an earthquake, out-of-area phone callsare often much easier to place and receive thanlocal calls. Therefore, establish an out-of-areaperson who can be contacted.

• Be sure you have necessary supplies, such asfood, water, first-aid kit, flashlights, and somecash to tide you over during the emergencyperiod when it may be difficult to obtain someitems.

• Canvass your neighborhood and identify elder-ly or disabled neighbors who may need yourhelp in the event of an earthquake and help ed-ucate others in the neighborhood about how toprepare their own plans.

During an earthquake, the strong ground motionwill greatly restrict your motion, and, as a result, yourstrategy should be to “duck, cover, and hold.” Givenyour knowledge of earthquakes, you may also try torecognize how strong the earthquake is likely to beand to predict what may happen during the event. Forexample, you know there will be several types of waves,including P, S, and R waves. The P waves will arrivefirst, and you may even hear them coming. However,the S and R waves, which soon follow, have bigger dis-placement and cause most of the damage. The lengthof shaking during an earthquake will vary with themagnitude. For example, during the 1994 Northridgeearthquake, the shaking lasted approximately 15 sec-onds, but the time of shaking in a great earthquakemay be much longer. For example, during the 1906 SanFrancisco earthquake, shaking lasted nearly 2 minutes.In addition to the “duck, cover, and hold” strategy, youneed to remain calm during an earthquake and try toprotect yourself from appliances, books, and other ma-terials possibly sliding or flying across the room. Agood strategy would be to crouch under a desk or table,roll under a bed, or position yourself in a strong doorway.

The Response to Earthquake Hazards

During the earthquake, there may be explosive flashesfrom transformers and power poles, and you must ob-viously avoid downed power lines. At all costs, resistthe natural urge to panic.47

After the shaking stops, take your deep breathsand organize your thinking in accordance with theplan you developed. Check on your family membersand neighbors, and check for gas leaks and fires.Your telephones should be used only for emergencycalls. Examine your chimney, as chimneys are partic-ularly prone to failure from earthquakes. The chim-ney may separate from the roof or the walls alongthe side of the chimney. If you are caught in a the-ater or stadium during a large earthquake, it is im-portant to remain in your seat and protect yourhead and neck with your arms; do not attempt toleave until the shaking has stopped. Remain calmand walk out slowly, keeping a careful eye out forobjects that have fallen or may fall. If you are in ashopping mall, it is important to “duck, cover, andhold” away from glass doors and display shelves ofbooks or other objects that could fall on you. If youare outdoors and an earthquake occurs, it is pru-dent to move to a clear area where you can avoidfalling trees, buildings, power lines, or other haz-ards. If you are in a mountainous area, be aware oflandslide hazards, since an earthquake may gener-ate slides that occur during and for some time afterthe earthquake. If you are in a high-rise building,you need to “duck, cover, and hold,” as in any otherindoor location, avoiding any large windows. It islikely that the shaking may activate fire alarms andwater sprinkler systems. Streets lined with tallbuildings are very dangerous locations during anearthquake; glass from these buildings often shat-ters and falls to the street below, and the razor-sharp shards can cause serious damage and deathto people below.47

Finally, after an earthquake, be prepared for af-tershocks. There is a known relationship betweenthe magnitude of the primary earthquake and thedistribution of aftershocks in hours, days, months,or even years following the earthquake. If the earth-quake has a magnitude of about 7, then severalmagnitude 6 aftershocks can be expected. Manymagnitude 5 and 4 events are likely to occur. In gen-eral, the number and size of aftershocks decreasewith time from the main earthquake event, and themost hazardous period is in the minutes, hours, anddays following the main shock.

217

The good news concerning living in earthquakecountry in the United States is that large earth-quakes are survivable. Our buildings are generallyconstructed to withstand earthquake shaking,and our woodframe houses seldom collapse. It is,


however, important to be well informed and pre-pared for earthquakes. For that reason, it is ex-tremely important to develop and become familiarwith your personal plan of what to do before, dur-ing, and after an earthquake.

218

Making The ConnectionLinking the Opening Case History About Recent Earthquakes in Italy and Haiti to the Fundamental Concepts

Consider and discuss thefollowing questions:

1. What is the main lesson from

the recent earthquakes in Italy

and Haiti?

2. How are science and values

linked to the two earthquakes

discussed?

3. Italy is a wealthy country com-

pared to Haiti. How important is

the wealth of a country to reduc-

ing the earthquake hazard?

SummaryLarge earthquakes rank among na-ture’s most catastrophic and devastat-ing events. Most earthquakes occur intectonically active areas where lithos-pheric plates interact along theirboundaries, but some large damagingintraplate earthquakes also occur.

A fault is a fracture or fracture sys-tem along which rocks have been dis-placed. Strain builds up in the rockson either side of a fault as the sidespull in different directions. When thestress exceeds the strength of therocks, they rupture, giving rise toseismic, or earthquake, waves thatshake the ground.

Strike-slip faults exhibit horizontaldisplacement and are either right orleft lateral. Dip-slip faults exhibit ver-tical displacement and are either nor-mal or reverse. Some faults are buriedand do not rupture the surface, evenwhen their movement causes largeearthquakes. Recently, a new funda-mental Earth process known as slowearthquakes has been discovered.Slow earthquakes may last from days

to months, with large areas of faultrupture but small displacements.

A fault is usually considered activeif it has moved during the past 10,000years and potentially active if it hasmoved during the past 1.65 millionyears. Some faults exhibit tectoniccreep, a slow displacement not accom-panied by felt earthquakes.

The area within Earth where faultrupture begins is called the focus ofthe earthquake and can be from a fewkilometers to almost 700 km (435 mi)deep. The area of the surface directlyabove the focus is called the epicenter.Seismic waves of different kindstravel away from the focus at differ-ent rates; much of the damage fromearthquakes is caused by surfacewaves. The severity of shaking of theground and buildings is affected bythe frequency of the seismic wavesand by the type of Earth material pre-sent. Buildings on unconsolidatedsediments or landfill, which tend toamplify the shaking, are highly sub-ject to earthquake damage.

The magnitude of an earthquake isa measure of the amount of energy re-leased. The measure of the intensityof an earthquake, the Modified Mer-calli Scale, is based on the severity ofshaking as reported by observers andvaries with proximity to the epicenterand local geologic and engineeringfeatures. Following an earthquake,shake maps, based on a dense net-work of seismographs, can quicklyshow areas where potentially damag-ing shaking occurred. This informationis needed quickly to assist emergencyefforts. Ground acceleration duringan earthquake is important informa-tion necessary to design structuresthat can withstand shaking.

The hypothesized earthquake cyclefor large earthquakes has four stages.A period of seismic inactivity, duringwhich elastic strain builds up in therocks along a fault, is followed by aperiod of increased seismicity as thestrain locally exceeds the strength ofthe rocks, initiating local faulting andsmall earthquakes. The third stage,

Revisiting Fundamental Concepts 219

potential aid in earthquake predic-tion. A potential problem of predict-ing earthquakes is that their patternof occurrence is often variable, withclustering or sequencing of eventsbeing separated by longer periods oftime with reduced earthquake activity.

Reduction of earthquake hazardswill be a multifaceted program, in-cluding recognition of active faultsand Earth materials sensitive toshaking and development of im-proved ways to predict, control, warnof and adjust to earthquakes, includ-ing designing structures to betterwithstand shaking. Many communi-ties in the U.S. and other countriesare developing emergency plans torespond to a predicted or unexpectedcatastrophic earthquake. Seismiczoning, including microzonation andother methods of hazard reduction,are active areas of research. At a per-sonal level, there are steps an individ-ual can take before, during, and afteran earthquake to reduce the hazardand ease recovery.

uplift of landmasses, as well as re-gional changes in groundwater levelsand surface water flow. Large togreat submarine earthquakes cangenerate a damaging catastrophictsunami (see Chapter 7).

Prediction of earthquakes is a sub-ject of serious research. To date, long-term and medium-term earthquakeprediction, based on probabilisticanalysis, has been much more suc-cessful than short-term prediction.Long-term prediction provides impor-tant information for land-use plan-ning, developing building codes, andengineering design of critical facili-ties. Some scientists believe that wewill eventually be able to make long-,medium-, and short-range predic-tions, based on previous patterns andfrequency of earthquakes, as well asby monitoring the deformation ofland, the release of radon gas, andexisting seismic gaps. Although notcurrently being pursued, reports sug-gest that anomalous animal behaviorbefore an earthquake may offer

which does not always occur, consistsof foreshocks. The fourth stage is themajor earthquake, which occurs whenthe fault segment ruptures, producingthe elastic rebound that generatesseismic waves.

Human activity has caused increas-ing earthquake activity by loadingEarth’s crust through construction oflarge reservoirs; by disposal of liquidwaste in deep disposal wells, whichraises fluid pressures in rocks and fa-cilitates movement along fractures;and by setting off underground nu-clear explosions. The accidental dam-age caused by the first two activitiesis regrettable, but what we learn fromall the ways we have caused earth-quakes may eventually help us tocontrol or stop large earthquakes.

Effects of earthquakes include vio-lent ground motion, accompanied byfracturing, which may shear or collapselarge buildings, bridges, dams, tunnels,and other rigid structures. Other ef-fects include liquefaction, landslides,fires, and regional subsidence and

Revisiting Fundamental ConceptsHuman Population GrowthHuman population growth, espe-cially in large cities in seismicallyactive regions, is placing more andmore people and property at riskfrom earthquakes.

Sustainability Minimizing thedamages from earthquakes to publicand private property is a componentof sustainable development. The goalis to produce stable communities thatare less likely to experience cata-strophic losses as a result of poorearthquake preparation.

Earth as a System Earthquakesare produced by Earth’s internal tec-tonic systems. Landforms, includingocean basins and mountains, are theproducts of continental movementand resultant earthquakes. Moun-tains cause changes to atmospheric

processes that create deserts and regional patterns of rainfall and,thus, affect vegetation and erosion.This is an example of the principleof environmental unity: Onechange, in this case the develop-ment of mountains, causes a chainof other events.

Hazardous Earth Processes,Risk Assessment, and Percep-tion We cannot control processes thatproduce earthquakes, but how we per-ceive the earthquake hazard greatly in-fluences the actions we take to mini-mize the risk of loss of life. If weperceive earthquakes as a real risk toour lives and those of our family andfriends, we will take the necessary stepsto prepare for future earthquakes.

Scientific Knowledge andValues Scientific knowledge about

earthquakes in terms of how they areproduced, where and why they occur,and how to design buildings to betterwithstand earthquake shaking hasgrown dramatically in recent years.Important lessons were learned fromthe 1999 earthquakes in Turkey thatkilled about 38,000 people. Turkeyhas relatively high building standards,and more buildings should have sur-vived the quakes. Some people believethat improper construction con-tributed significantly to the loss ofbuildings. Contractors may have de-stroyed evidence of inadequate con-struction after the earthquake oc-curred. Community values result inbuilding regulations that help reduceearthquake losses. If buildings areproperly constructed to withstandearthquakes, then loss of life may begreatly reduced in the future.


active fault (p. 181)

design basis ground motion (p. 209)

dilatancy-diffusion model (p. 197)

dilate (p. 197)

directivity (p. 190)

earthquake (p. 179)

earthquake cycle (p. 195)

earthquake segment (p. 181)

epicenter (p. 170)

fault (p. 179)

fault segmentation (p. 181)

fault-valve mechanism (p. 197)

fault zone (p. 181)

fluid pressure (p. 197)

focus (p. 170)

liquefaction (p. 199)

material amplification (p. 189)

Modified Mercalli Scale (p. 173)

moment magnitude (p. 170)

P wave (p. 184)

recurrence interval (p. 182)

shake map (p. 173)

slip rate (p. 182)

slow earthquake (p. 184)

surface wave (p. 184)

S wave (p. 184)

tectonic creep (p. 184)

tectonic framework (p. 209)

Key Terms

Review Questions1. What is the difference between the

focus and the epicenter of anearthquake?

2. How is Richter magnitude determined?

3. What are slow earthquakes?

4. What factors determine theModified Mercalli Scale?

5. What are the main differencesbetween the Richter, momentmagnitude, Modified Mercalli,and instrumental scales?

6. Define fault.

7. What are the major types of faults?

8. What is the difference between ananticline and a syncline?

9. Define active fault.

10. What is tectonic creep?

11. What are the main types of seis-mic wave?

12. Describe the motions of P, S, and Rwaves. How do their physical prop-erties account for their effects?

13. What is a shake map, how is it pro-duced, and why are they important?

14. What is material amplification?

15. Define the earthquake cycle andillustrate it with a simple example.

16. How has human activity causedearthquakes?

17. What are some of the major effects of earthquakes?

18. What is supershear?

19. What are some of the precursoryphenomena likely to assist us inpredicting earthquakes?

20. What are the major goals ofearthquake hazard-reduction programs?

21. What are the main adjustmentspeople make to seismic activityand the occurrence of earthquakes?

1. Assume that you are working forthe Peace Corps and are in a devel-oping country where most of thehomes are built out of unrein-forced blocks or bricks. There hasnot been a large damaging earth-quake in the area for several hun-dred years, but earlier there wereseveral earthquakes that killedthousands of people. How wouldyou present the earthquake hazard

to the people living where you areworking? What steps might betaken to reduce the hazard?

2. You live in an area that has a sig-nificant earthquake hazard. Thereis ongoing debate as to whether anearthquake warning system shouldbe developed. Some people areworried that false alarms will causea lot of problems, and others pointout that the response time may

not be very long. What are yourviews? Do you think it is a respon-sibility of public officials to financean earthquake warning system, as-suming such a system is feasible?What are potential implications ifa warning system is not developed,and a large earthquake results indamage that could have partiallybeen avoided with a warning sys-tem in place?

Critical Thinking Questions