geography hazards

8/12/2019 geography Hazards

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Earthquakes

Most earthquakes occur along zones where the Earth's crust isundergoing deformation. Deformation results from plate tectonic forcesand gravitational forces. The type of deformation that takes place duringan earthquake generally occurs along zones where rocks fracture toproduce faults. Before we can understand earthquakes, we first mustexplore deformation of rocks and faulting.

Within the Earth rocks are constantly subjected to forces that tend tobend, twist, or fracture them. When rocks bend, twist or fracture they aresaid to deform or strain (change shape or size). The forces that causedeformation are referred to as stresses. To understand rock deformationwe must first explore stress and strain.

Stress and Strain

Stress is a force applied over an area. One type of stress that we are allused to is a uniform stress, called pressure. A uniform stress is where theforces act equally from all directions. In the Earth the pressure due to the

weight of overlying rocks is a uniform stress and is referred to asconfining stress. If stress is not equal from all directions then the stressis a differential stress. Three kinds of differential stress occur.

1.2.Tensional stress (or extensional stress), which stretches rock;

3.Compressional stress, which squeezes rock; and

Shear stress, which result in slippage and translation.


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Fracture-- irreversible strain wherein the material breaks.

We can divide materials into two classes that depend on their relative

behavior under stress. Brittle materialshave a small to large region of elastic behavior, but

only a small region of ductile behavior before they fracture. Ductile materialshave a small region of elastic behavior and a large

region of ductile behavior before they fracture.

How a material behaves will depend on several factors. Among themare:


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Temperature - At high temperature molecules and their bonds canstretch and move, thus materials will behave in more ductilemanner. At low Temperature, materials are brittle.

Confining Pressure - At high confining pressure materials are lesslikely to fracture because the pressure of the surroundings tends tohinder the formation of fractures. At low confining stress, materialwill be brittle and tend to fracture sooner.

Strain rate -- Strain rate refers to the rate at which the deformationoccurs (strain divided by time). At high strain rates material tendsto fracture. At low strain rates more time is available for individual

atoms to move and therefore ductile behavior is favored.

Composition -- Some minerals, like quartz, olivine, and feldspars arevery brittle. Others, like clay minerals, micas, and calcite are moreductile This is due to the chemical bond types that hold themtogether. Thus, the mineralogical composition of the rock will be afactor in determining the deformational behavior of the rock.Another aspect is presence or absence of water. Water appears to

weaken the chemical bonds and forms films around mineral grainsalong which slippage can take place. Thus wet rock tends tobehave in ductile manner, while dry rocks tend to behave in brittlemanner.


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Evidence of Former Deformation

Evidence of deformation that has occurred in the past is very evident in

crustal rocks. For example, sedimentary layers and lava flows generallyare deposited on a surface parallel to the Earth's surface (nearlyhorizontal). Thus, when we see such layers inclined instead ofhorizontal, evidence of an episode of deformation is present.In order to uniquely define the orientation of a planar feature we firstneed to define two terms - strike and dip. For an inclined plane the

strikeis the compass direction of any horizontal line on the plane. Thedipis the angle between a horizontal plane and the inclined plane,

measured perpendicular to the direction of strike.

In recording strike and dip measurements on a geologic map, a symbol is

used that has a long line oriented parallel to the compass direction of thestrike. A short tick mark is placed in the center of the line on the side towhich the inclined plane dips, and the angle of dip is recorded next tothe strike and dip symbol. For beds with a 90odip (vertical) the shortline crosses the strike line, and for beds with no dip (horizontal) a circlewith a cross inside is used.


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Fracture of Brittle RocksJoints

Joints are fractures in rock that show no slippage or offset along the

fracture. Joints are usually planar features, so their orientation can

be described as a strike and dip. They form from as a result of

extensional stress acting on brittle rock. Such stresses can be

induced by cooling of rock (volume decreases as temperature

decreases) or by relief of pressure as rock is eroded above thus

removing weight.

Joints are zones of weakness,so their presence is critical when

building anything from dams to highways. For dams, the water

could leak out through the joints leading to dam failure. For

highways the joints may separate and cause rock falls and

landslides.

Faults- Faults occur when brittle rocks fracture and there is an

offset along the fracture. When the offset is small, the displacementcan be easily measured, but sometimes the displacement is so large

that it is difficult to measure.


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Types of Faults

Faults can be divided into several different types depending on the

direction of relative displacement. Since faults are planar features,

the concept of strike and dip also applies, and thus the strike anddip of a fault plane can be measured. One division of faults is

between dip-slip faults, where the displacement is measured along

the dip direction of the fault, and strike-slip faults where the

displacement is horizontal, parallel to the strike of the fault.

Dip Slip Faults - Dip slip faults are faults that have an inclined fault

plane and along which the relative displacement or offset has

occurred along the dip direction. Note that in looking at the

displacement on any fault we don't know which side actually movedor if both sides moved, all we can determine is the relative sense of

motion.

For any inclined fault plane we define the block above the fault as

thehanging wall blockand the block below the fault as thefootwall

block.

Normal Faults -

are faults that result from horizontal tensional stressesin brittle rocks and where the hanging-wall block has moved downrelative to the footwall block.

Horsts & Grabens -Due to the tensional stress responsible for normalfaults, they often occur in a series, with adjacent faults dipping inopposite directions. In such a case the down-dropped blocks form


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grabensand the uplifted blocks formhorsts. In areas where tensionalstress has recently affected the crust, the grabens may formrift valleysand the uplifted horst blocks may form linear mountain ranges. The EastAfrican Rift Valley is an example of an area where continental extension

has created such a rift. The basin and range province of the western U.S.(Nevada, Utah, and Idaho) is also an area that has recently undergonecrustal extension. In the basin and range, the basins are elongatedgrabens that now form valleys, and the ranges are uplifted horst blocks.

Reverse Faults - are faults that result from horizontal compressionalstresses in brittle rocks, where the hanging-wall block has moved uprelative the footwall block.


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A Thrust

Fault isa specialcase of areversefaultwherethe dip ofthe fault

is lessthan 45o.Thrustfaults canhaveconsiderabledisplacement,measuringhundredsofkilometers, andcan resultin older

strataoverlyingyoungerstrata.


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Strike Slip Faults- are faults where the relative motion on

the fault has taken place along a horizontal direction. Suchfaults result from shear stresses acting in the crust. Strike slip

faults can be of two varieties, depending on the sense ofdisplacement. To an observer standing on one side of thefault and looking across the fault, if the block on the otherside has moved to the left, we say that the fault is a left-lateral strike-slip fault. If the block on the other side hasmoved to the right, we say that the fault is aright-lateral

strike-slip fault. The famous San Andreas Fault in Californiais an example of a right-lateral strike-slip fault.

Displacements on the San Andreas fault are estimated at over600 km.


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Transform-Faultsare a special class of strike-slip faults. These areplate boundaries along which two plates slide past one another in ahorizontal manner. The most common type of transform faults occurwhere oceanic ridges are offset. Note that the transform fault only

occurs between the two segments of the ridge. Outside of this area thereis no relative movement because blocks are moving in the samedirection. These areas are called fracture zones. The San Andreas faultin California is also a transform fault.


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Blind Faults - are faults that does not break the surface - rocks abovethe fault have behaved in ductile fashion and folded over the tip of thefault.

Active Faults- An active fault is one that has shown recent displacementand likely has the potential to produce earthquakes. Since faulting is partof the deformation process, ancient faults can be found anywhere thatdeformation has taken place in the past. Thus, not every fault one sees isnecessarily an active fault.

Surface Expression of Faults- Where faults have broken the surfacethey are shown on maps as fault lines or fault zones. Recent ruptures ofdip slip faults at the surface show a cliff that is called a fault scarp.

Strike slip faults result in features like linear valleys, offset surfacefeatures (roads, stream channels, fences, etc.) or elongated ridges

How Faults Develop- When tectonic forces generate stress, rocks startto deform elastically. Eventually small cracks to form along the faultzone. When rupture occurs, the stored elastic energy is released asseismic waves.


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Earthquakes

Earthquakes occur when energy stored in elastically strained rocks issuddenly released. This release of energy causes intense ground shakingin the area near the source of the earthquake and sends waves of elasticenergy, called seismic waves, throughout the Earth. Earthquakes can begenerated by bomb blasts, volcanic eruptions, and sudden slippagealong faults. Earthquakes are definitely a geologic hazard for thoseliving in earthquake prone areas, but the seismic waves generated byearthquakes are invaluable for studying the interior of the Earth.

Origin of Earthquakes

Most natural earthquakes are caused by sudden slippage along a

fault zone. The elastic rebound theorysuggests that if slippage along

a fault is hindered such that elastic strain energy builds up in the

deforming rocks on either side of the fault, when the slippage doesoccur, the energy released causes an earthquake.

This theory was discovered by making measurements at a number ofpoints across a fault. Prior to an earthquake it was noted that the rocksadjacent to the fault were bending. These bends disappeared after anearthquake suggesting that the energy stored in bending the rocks was


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suddenly released during the earthquake.

Friction between the blocks then keeps the fault from moving again untilenough strain has accumulated to overcome the friction and generateanother earthquake. Once a fault forms, it becomes a zone of weakness -so long as the tectonic stresses continue to be present more earthquakesare likely to occur on the fault. Thus faults move in spurts and thisbehavior is referred to as Stick Slip.

If there is large displacement during an earthquake, a large earthquakewill be generated. Smaller displacements generate smallerearthquakes. Note that even for small displacements of only a millimeterper year, after 1 million years, the fault will accumulate 1 km ofdisplacement.

Fault Creep- Some faults or parts of faults move continuously withoutgenerating earthquakes. This could occur if there is little friction on thefault & tectonic stresses are large enough to move the blocks in oppositedirections. This is called fault creep. If creep is occurring on one part ofa fault, it is likely causing strain to build on other parts of the fault.


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Seismology, The Study of Earthquakes

When an earthquake occurs, the elastic energy is released sending outvibrations that travel throughout the Earth. These vibrations are calledseismic waves. The study of how seismic waves behave in the Earth is

calledseismology.

The source of anearthquake iscalled the

focus, which isan exact

location withinthe Earth wereseismic wavesare generatedby suddenrelease ofstored elasticenergy. The

epicenteristhe point onthe surface ofthe Earthdirectly abovethe focus.Sometimes themedia getthese twotermsconfused.

Seismic waves emanating from the focus can travel in several ways,

and thus there are several different kinds of seismic waves.


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Body Waves - emanate from the focus and travel in all directionsthrough the body of the Earth. There are two types of body waves: P-waves and S-waves:

P - wavesare Primary waves. They travel with avelocity that depends on the elastic properties of therock through which they travel.

Where, Vpis the velocity of the P-wave, K is the incompressibilityof the material, is the rigidity of the material, and is the density of the material.

P-waves are the same thing as sound waves. They move through thematerial by compressing it, but after it has been compressed itexpands, so that the wave moves by compressing and expandingthe material as it travels. Thus the velocity of the P-wave dependson how easily the material can be compressed (theincompressibility), how rigid the material is (the rigidity), and the


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density of the material. P-waves have the highest velocity of allseismic waves and thus will reach all seismographs first.

S-Waves- Secondary waves, also called shear waves. They travel with a

velocity that depends only on the rigidity and density of the materialthrough which they travel:

S-waves travel through material by shearing it or changing its shape inthe direction perpendicular to the direction of travel. The resistance toshearing of a material is the property called the rigidity. It is notable thatliquids have no rigidity, so that the velocity of an S-wave is zero in aliquid. (This point will become important later). Note that S-wavestravel slower than P-waves, so they will reach a seismograph after the P-wave.

Surface Waves- Surface waves differ from body waves inthat they do not travel through the Earth, but instead travelalong paths nearly parallel to the surface of the Earth.

Surface waves behave like S-waves in that they cause up anddown and side to side movement as they pass, but they travelslower than S-waves and do not travel through the body ofthe Earth. Surface waves are often the cause of the mostintense ground motion during an earthquake.


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Seismometer

s - Seismicwavestravel

through theEarth asvibrations.Aseismomete

ris aninstrumentused to

record thesevibrations,and theresultinggraph thatshows thevibrations iscalled a

seismogram. Theseismometer must beable tomove withthevibrations,yet part of it

must remainnearlystationary.


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This is accomplished by isolating the recording device (like a pen)from the rest of the Earth using the principal of inertia. Forexample, if the pen is attached to a large mass suspended by awire, the large mass moves less than the paper which is attached to

the Earth, and on which the record of the vibrations is made.Modern instruments are digital and dont require the paper.

The record of an earthquake, a seismogram, as recorded by a

seismometer, will be a plot of vibrations versus time. On theseismograph, time is marked at regular intervals, so that we can

determine the time of arrival of the first P-wave and the time ofarrival of the first S-wave. (Note again, that because P-waves havea higher velocity than S-waves, the P-waves arrive at theseismographic station before the S-waves)


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Locating theEpicenters ofEarthquakes -To determine

the location ofan earthquakeepicenter, weneed to haverecorded aseismographof theearthquake

from at leastthreeseismographicstations atdifferentdistances fromthe epicenter.In addition, we

need onefurther pieceof information- that is thetime it takesfor P-wavesand S-waves totravel throughthe Earth and

arrive at aseismographicstation. Suchinformationhas beencollected overthe last 80 or


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Thus the S-P interval tells us the distance to the epicenter from theseismographic station where the earthquake was recorded. Thus at eachstation we can draw a circle on a map that has a radius equal to thedistance from the epicenter. Three such circles will intersect in a point

that locates the epicenter of the earthquake.


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Magnitude of Earthquakes - Whenever a large destructiveearthquake occurs in the world the press immediately wants toknow where the earthquake occurred and how big the earthquakewas (in California the question is usually - Was this the Big One?).

The size of an earthquake is usually given in terms of a scalecalled the Richter Magnitude. Richter Magnitude is a scale ofearthquake size developed by a seismologist named CharlesRichter. The Richter Magnitude involves measuring the amplitude(height) of the largest recorded wave at a specific distance fromthe earthquake. While it is correct to say that for each increase in 1in the Richter Magnitude, there is a tenfold increase in amplitudeof the wave, it is incorrectto say that each increase of 1 in Richter

Magnitude represents a tenfold increase in the size of theEarthquake (as is commonly incorrectly stated by the press).

A better measure of the size of an earthquake is the amountof energy released by the earthquake. While this is muchmore difficult to determine, Richter gave a means by whichthe amount of energy released can be estimated:

Log E = 11.8 + 1.5 MWhere Log refers to the logarithm to the base 10, E is the energy

released in ergs, and M is the Richter Magnitude.Anyone with a hand calculator can solve this equation byplugging in various values of M (magnitude) and solving forE, the energy released. I've done the calculation for you inthe following table:

Magnitude Energy (ergs) Factor

1 2.0 x 1013 31 x

2 6.3 x 1014


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3 2.0 x 1016 31 x

4 6.3 x 1017

5 2.0 x 1019 31 x

6 6.3 x 1020

7 2.0 x 1022 31 x

8 6.3 x 1023

From these calculations you can see that each increase in 1 inMagnitude represents a 31 fold increase in the amount of energyreleased. Thus, a magnitude 7 earthquake releases 31 times more

energy than a magnitude 6 earthquake. A magnitude 8 earthquakereleases 31 x 31 or 961 times as much energy as a magnitude 6earthquake.


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Although the Richter Magnitude is the scale most commonlyreported when referring to the size of an earthquake, it has beenfound that for larger earthquakes a more accurate measurement ofsize is themoment magnitude, Mw. The moment magnitude is a

measure of the amount of strain energy released by the earthquakeas determined by measurements of the shear strength of the rockand the area of the rupture surface that slipped during theearthquake.

Note that it usually takes more than one seismographic station tocalculate the magnitude of an earthquake. Thus you will hearinitial estimates of earthquake magnitude immediately after an

earthquake and a final assigned magnitude for the same earthquakethat may differ from initial estimates, but is assigned afterseismologists have had time to evaluate the data from numerousseismographic stations.

The moment magnitude for large earthquakes is usually greater thanthe Richter magnitude for the same earthquake. For example theRichter magnitude for the 1964 Alaska earthquake is usuallyreported as 8.6, whereas the moment magnitude for thisearthquake is calculated at 9.2.

The largest earthquake ever recorded was in Chile in 1960 with amoment magnitude of 9.5, The Summatra earthquake of 2004 hada moment magnitude of 9.0. Sometimes a magnitude is reportedfor an earthquake and no specification is given as to whichmagnitude (Richter or moment) is reported. This obviously cancause confusion. But, within the last few years, the tendency hasbeen to report the moment magnitude rather than the Richtermagnitude.

The Hiroshima atomic bomb released an amount of energyequivalent to a moment magnitude 6 earthquake.

Note that magnitude scales are open ended with no maximum orminimum. The largest earthquakes are probably limited by rockstrength. Meteorite impacts could cause larger earthquakes thanhave ever been observed.


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Frequency of Earthquakes of Different Magnitude

Worldwide

Magnitude Number ofEarthquakes per

Year

Description

> 8.5 0.3 Great

8.0 - 8.4 1

7.5 - 7.9 3 Major

7.0 - 7.4 15

6.6 - 6.9 56

6.0 - 6.5 210 Destructive

5.0 - 5.9 800 Damaging

4.0 - 4.9 6,200 Minor

3.0 - 3.9 49,000

2.0 - 2.9 300,000

0 - 1.9 700,000


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II A few people noticemovement if at restand/or on upper floorsof tall buildings

III People indoors feelmovement. Hangingobjects swing back andforth. People outdoorsmight not realize thatan earthquake isoccurring

4.2

IV People indoors feel

movement. Hangingobjects swing. Dishes,windows, and doorsrattle. Feels like aheavy truck hittingwalls. Some peopleoutdoors may feelmovement. Parked carsrock.

4.3 - 4.8

V Almost everyone feelsmovement. Sleepingpeople are awakened.Doors swingopen/close. Dishesbreak. Small objectsmove or are turnedover. Trees shake.Liquids spill from opencontainers

4.9-5.4


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VI Everyone feelsmovement. Peoplehave trouble walking.Objects fall from

shelves. Pictures falloff walls. Furnituremoves. Plaster in wallsmay crack. Trees andbushes shake. Damageslight in poorly builtbuildings.

5.5 - 6.1

VII People have difficultystanding. Drivers feelcars shaking. Furniturebreaks. Loose bricksfall from buildings.Damage slight tomoderate in well-builtbuildings; considerablein poorly builtbuildings.

5.5 - 6.1

VIII Drivers have troublesteering. Houses not

bolted down shift onfoundations. Towers &chimneys twist and fall.Well-built buildingssuffer slight damage.Poorly built structuresseverely damaged. Treebranches break.Hillsides crack ifground is wet. Water

levels in wells change.

6.2 - 6.9


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IX Well-built buildingssuffer considerabledamage. Houses notbolted down move off

foundations. Someunderground pipesbroken. Ground cracks.

Serious damage toReservoirs.

6.2 - 6.9

X Most buildings & theirfoundations destroyed.Some bridgesdestroyed. Damsdamaged. Largelandslides occur. Waterthrown on the banks ofcanals, rivers, lakes.Ground cracks in largeareas. Railroad tracksbent slightly.

7.0 - 7.3

XI Most buildingscollapse. Some bridges

destroyed. Large cracksappear in the ground.Underground pipelinesdestroyed. Railroadtracks badly bent.

7.4 - 7.9

XII Almost everything isdestroyed. Objectsthrown into the air.Ground moves inwaves or ripples. Largeamounts of rock maymove.

>8.0


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The Mercalli Scale is very useful in examining the effects of anearthquake over a large area, because it will is responsive not onlyto the size of the earthquake as measured by the Richter scale forareas near the epicenter, but will also show the effects of the

efficiency that seismic waves are transmitted through differenttypes of material near the Earth's surface.

The Mercalli Scale is also useful for determining the size ofearthquakes that occurred before the modern seismographicnetwork was available (before there were seismographic stations, itwas not possible to assign a Richter Magnitude).

Earthquake Risk

Many seismologists have said that "earthquakes don't kill people,buildings do". This is because most deaths from earthquakes are causedby buildings or other human construction falling down during anearthquake.Earthquakes located in isolated areas far from human population rarely

cause any deaths.Thus, earthquake hazard risk depends on

Population densityConstruction standards (building codes)Emergency preparednessExamples:Worst earthquake in recorded history occurred in 1556 in Shaaxi,China. Killed 830,000 people, most living in caves excavated in poorlyconsolidated loess (wind deposited silt and clay).Worst earthquake in the 20th century also occurred in China (T'ang ShanProvince), killed 240,000 in 1976. Occurred at 3:42 AM, Magnitude 7.8Earthquake and magnitude 7.1 aftershock. Deaths were due to collapseof masonry (brick) buildings.


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Worst earthquake so far in the 21st Century was a magnitude 7.0earthquake that occurred in Haiti on January 12, 2010 with an estimateddeath toll of 230,000! (The death toll in this earthquake is still

debated.The Hatian government claims 316, 000 deaths, while U.S.estimates suggest something between 46,000 and 86,000).Contrast - In earthquake prone areas like California, in order to reduceearthquake risk, there are strict building codes requiring the design andconstruction of buildings and other structures that will withstand a largeearthquake. While this program is not always completely successful, onefact stands out to prove its effectiveness. In 1989 an earthquake nearSan Francisco, California (The Loma Prieta, or World Series

Earthquake) with a Moment Magnitude of 6.9 killed about 62 people.Most were killed when a double decked freeway in Oakland collapsed.About 10 months earlier, an earthquake with moment magnitude 6.8occurred in Armenia, where no earthquake-proof building codes existed.The death toll in the latter earthquake was about 25,000!

Similarly the Moment Magnitude 7.0 2010 earthquake in Haiti had ahuge death toll mainly because of the lack of earthquake-resistent

structures. Most buildings were made of poorly reinforced concrete.

Computer simulations for large cities, like San Francisco or LosAngeles, California, indicate that a magnitude >8.0 earthquake wouldcause between 3,000 and 13,000 deaths.

3,000 if at night, when populace is asleep in wood frame houses

13,000 if during day when populace is in masonry buildings and on

freeways.

Architecture and Building CodesWhile architecture and building codes can reduce risk, it should be notedthat not all kinds of behavior can be predicted.


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Although codes are refined each year, not all possible effects can beanticipated. For example different earthquakes show differentfrequencies of ground shaking, different durations of ground shaking,and different vertical and horizontal ground accelerations.

Old buildings cannot cost-effectively be brought up to code, especiallywith yearly refinements to code.Even with construction to earthquake code, buildings fail for otherreasons, like poor quality materials, poor workmanship, etc. that are notdiscovered until after an earthquake.Hazards Associated with Earthquakes

Possible hazards from earthquakes can be classified as follows:

Ground Motion- Shaking of the ground caused by the passage ofseismic waves, especially surface waves, near the epicenter of theearthquake are responsible for the most damage during an earthquakeand is thus a primary effect of an earthquake. The intensity of groundshaking depends on:Local geologic conditions in the area. In general, loose unconsolidatedsediment is subject to more intense shaking than solid bedrock.Size of the Earthquake. In general, the larger the earthquake, the more

intense is the shaking and the duration of the shaking.Distance from the Epicenter. Shaking is most severe near the epicenterand drops off away from the epicenter. The distance factor depends onthe type of material underlying the area. There are, however, strangeexceptions. For example, the 1985 Mexico City Earthquake (magnitude8.1) had an epicenter on the coast of Mexico, more than 350 km to thesouth, yet damage in Mexico City was substantial because Mexico Cityis built on soft unconsolidated sediments that fill a former lake (seeLiquefaction, below).

Damage to structures from shaking depends on the type of construction.Concrete and masonry structures are brittle and thus more susceptible todamagewood and steel structures are more flexible and thus less susceptible todamage.


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Faulting and Ground Rupture- Ground rupture generally occurs onlyalong the fault zone that moves during the earthquake, and are thus aprimary effect. Thus structures that are built across fault zones maycollapse, whereas structures built adjacent to, but not crossing the fault

may survive.Aftershocks- These are smaller earthquakes that occur after a mainearthquake, and in most cases there are many of these (1260 weremeasured after the 1964 Alaskan Earthquake). Aftershocks occurbecause the main earthquake changes the stress pattern in areas aroundthe epicenter, and the crust must adjust to these changes. Aftershocksare very dangerous because they cause further collapse of structuresdamaged by the main shock. Aftershocks are a secondary effect of

earthquakes

Fire- Fire is a secondary effect of earthquakes. Because power linesmay be knocked down and because natural gas lines may rupture due toan earthquake, fires are often started closely following an earthquake.The problem is compounded if water lines are also broken during theearthquake since there will not be a supply of water to extinguish thefires once they have started. In the 1906 earthquake in San Francisco

more than 90% of the damage to buildings was caused by fire.Landslides- In mountainous regions subjected to earthquakes groundshaking may trigger landslides, rock and debris falls, rock and debrisslides, slumps, and debris avalanches. These are secondary effects.

Liquefaction- Liquefaction is a processes that occurs in water-saturatedunconsolidated sediment due to shaking. In areas underlain by such

material, the ground shaking causes the grains to lose grain to graincontact, and thus the material tends to flow.

Liquefaction, because it is a direct result of ground shaking, is a primaryeffect.


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You can demonstrate this process to yourself next time your go thebeach. Stand on the sand just after an incoming wave has passed. Thesand will easily support your weight and you will not sink very deeplyinto the sand if you stand still. But, if you start to shake your body while

standing on this wet sand, you will notice that the sand begins to flow asa result of liquefaction, and your feet will sink deeper into the sand.

Changes in Ground Level- A secondary or tertiary effect that is causedby faulting. Earthquakes may cause both uplift and subsidence of theland surface. During the 1964 Alaskan Earthquake, some areas were

uplifted up to 11.5 meters, while other areas subsided up to 2.3 meters.Tsunami - Tsunami a secondary effect that are giant ocean waves thatcan rapidly travel across oceans, as will be discussed in more detail later.Earthquakes that occur beneath sea level and along coastal areas cangenerate tsunami, which can cause damage thousands of kilometersaway on the other side of the ocean.

Flooding- Flooding is a secondary effect that may occur due to ruptureof human made dams and levees, due to tsunami, and as a result ofground subsidence after an earthquake.

World Distribution of EarthquakesThe distribution of earthquakes is called seismicity. Seismicity ishighest along relatively narrow belts that coincide with plate boundaries.This makes sense, since plate boundaries are zones along which

lithospheric plates move relative to one another.

Earthquakes along these zones can be divided into shallow focusearthquakes that have focal depths less than about 100 km and deepfocus earthquakes that have focal depths between 100 and 700 km.


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Earthquakes at Diverging Plate Boundaries. Diverging plate boundariesare zones where two plates move away from each other, such as atoceanic ridges. In such areas the lithosphere is in a state of tensionalstress and thus normal faults and rift valleys occur. Earthquakes that

occur along such boundaries show normal fault motion, have lowRichter magnitudes, and tend to be shallow focus earthquakes with focaldepths less than about 20 km. Such shallow focal depths indicate that thebrittle lithosphere must be relatively thin along these diverging plateboundaries.

Examples - all oceanic ridges, Mid-Atlantic Ridge, East Pacific rise, andcontinental rift valleys such as the basin and range province of the

western U.S. & the East African Rift Valley.Earthquakes at Transform Fault Boundaries. Transform fault boundariesare plate boundaries where lithospheric plates slide past one another in ahorizontal fashion. The San Andreas Fault of California is one of thelonger transform fault boundaries known. Earthquakes along theseboundaries show strike-slip motion on the faults and tend to be shallowfocus earthquakes with depths usually less than about 100 km. Richtermagnitudes can be large.

Examples - San Andreas Fault, California, South Island of New Zealand,through Port Au Prince, Haiti.Earthquakes at Convergent Plate Boundaries. Convergent plateboundaries are boundaries where two plates run into each other. Thus,they tend to be zones where compressional stresses are active and thusreverse faults or thrust faults are common. There are two types ofconverging plate boundaries. (1) subduction boundaries, where oceaniclithosphere is pushed beneath either oceanic or continental lithosphere;and (2) collision boundaries where two plates with continental

lithosphere collide.

Subduction boundaries -At subduction boundaries cold oceaniclithosphere is pushed back down into the mantle where two platesconverge at an oceanic trench. Because the subducted lithosphere is cold


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it remains brittle as it descends and thus can fracture under thecompressional stress.

When it fractures, it generates earthquakes that define a zone ofearthquakes with increasing focal depths beneath the overriding plate.This zone of earthquakes is called the Benioff Zone. Focal depths ofearthquakes in the Benioff Zone can reach down to 700 km.Examples - Along coasts of South American, Central America, Mexico,Northwestern U.S., Alaska, Japan, Philippines, Caribbean Islands.

Collision boundaries - At collisional boundaries two plates ofcontinental lithosphere collide resulting in fold-thrust mountain belts.Earthquakes occur due to the thrust faulting and range in depth fromshallow to about 200 km.Examples - Along the Himalayan Belt into China, along the Northernedge of the Mediterranean Sea through Black Sea and Caspian Sea intoIraq and Iran.Intraplate Earthquakes - These are earthquakes that occur in the stable

portions of continents that are not near plate boundaries. Many of themoccur as a result of re-activation of ancient faults, although the causes ofsome intraplate earthquakes are not well understood.Examples - New Madrid Region, Central U.S., Charleston SouthCarolina, Along St. Lawrence River - U.S. - Canada Border.Seismic Hazard and Risk MappingThe risk that an earthquake will occur close to where you live dependson whether or not tectonic activity that causes deformation is occurringwithin the crust of that area.

Primary Effects of VolcanismLava Flows

Lava flows are common in Hawaiian and Strombolian type of eruptions,the least explosive.Although lava flows have been known to travel as fast as 64 km/hr, mostare slower and give people time to move out of the way.


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Thus, in general, lava flows are most damaging to property, as they candestroy anything in their path.Control of lava flows has been attempted with limited success bybombing flow fronts to attempt to divert the flow, and by spraying with

water to cool the flow. The latter is credited with saving the fishingharbor during a 1973 eruption of Heimaey in Iceland.

Violent Eruptions and Pyroclastic Activity

Pyroclastic activity is one of the most dangerous aspects of volcanism.Hot pyroclastic flows cause death by suffocation and burning. They can

travel so rapidly that few humans can escape.Lateral blasts knock down anything in their path, can drive flying debristhrough trees.Ash falls can cause the collapse of roofs and can affect areas far fromthe eruption. Althoughash falls blanket an area like snow, they are farmore destructive because tephra deposits have a density more than twicethat of snow and tephra deposits do not melt like snow.Ash falls destroy vegetation, including crops, and can kill livestock that

eat the ash covered vegetation.Ash falls can cause loss of agricultural activity for years after aneruption, a secondary or tertiary effect.

Poisonous Gas Emissions

Volcanoes emit gases that are often poisonous to living organisms.

Among these poisonous gases are: Hydrogen Chloride (HCl), HydrogenSulfide (H2S), Hydrogen Fluoride (HF), and Carbon Dioxide (CO2).

The Chlorine, Sulfur. and Fluorine gases can kill organisms by directingestion, or by absorption onto plants followed by ingestion byorganisms.


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In 1984, CO2 gas escaping from the bottom of Lake Monoun, a craterlake in the African country of Cameroon, killed 37 people.

In 1986 an even larger CO2 gas emission from Lake Nyos in Cameroonkilled more than 1700 people and 3000 cattle.

Secondary and Tertiary Effects of Volcanism

Mudflows (Lahars)Volcanoes can emit voluminous quantities of loose, unconsolidatedtephra which become deposited on the landscape. Such loose depositsare subject to rapid removal if they are exposed to a source of water.

The source of water can be derived by melting of snow or ice during the

eruption, emptying of crater lakes during an eruption, or rainfall thattakes place any time with no eruption.

Thus, mudflows can both accompany an eruption and occur many yearsafter an eruption.

Mudflows are a mixture of water and sediment, they move rapidly downslope along existing stream valleys, although they may easily top banksand flood out into surrounding areas.

They have properties that vary between thick water and wet concrete,and can remove anything in their paths like bridges, highways, houses,etc.


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During the Mt. St. Helens eruption of May 18, 1980, mudflows weregenerated as a result of snow melt on the volcano itself, and depositionof tephra in streams surrounding the mountain.

On November 13, 1985 a mudflow generated by a small eruption onNevado del Ruiz volcano in Columbia flowed down slope anddevastated the town of Armero, 50 km east of the volcano and built onprior mudflow deposits. The town had several hours of warning fromvillages higher up slope, but these warnings were ignored, and 23,000people died in the mudflow that engulfed the town.

Debris Avalanches and Debris FlowsVolcanic mountains tend to become oversteepened as a result of theaddition of new material over time as well due to inflation of themountain as magma intrudes.

Oversteepened slopes may become gravitationally unstable, leading to a

sudden slope failure that results in landslides, debris slides or debrisavalanches. We will cover these types of hazards in more detail later inthe course and in the next lecture.During the May 18, 1980 eruption of Mt. St. Helens, Washington, adebris avalanche was triggered by a magnitude 5.0 earthquake. Theavalanche removed the upper 500 m of the mountain, and flowed intothe Spirit Lake, raising its level about 40 m. It then moved to the westfilling the upper reaches of the North Fork of the Toutle River valley

(see map above).

Debris avalanches, landslides, and debris flowsdo not necessarily occuraccompanied by a volcanic eruption. There are documented cases ofsuch occurrences where no new magma has been erupted.


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Flooding

Drainage systems can become blocked by deposition of pyroclasticflows and lava flows. Such blockage may create a temporary dam that

could eventually fill with water and fail resulting in floods downstreamfrom the natural dam.

Volcanoes in cold climates can melt snow and glacial ice, rapidlyreleasing water into the drainage system and possibly causing floods.Jokaulhlaups occur when heating of a glacier results in rapid outburst ofwater from the melting glacier.

TsunamiDebris avalanche events, landslides, caldera collapse events, andpyroclastic flows entering a body of water may generate tsunami.

During the 1883 eruption of Krakatau volcano, in the straits of Sundabetween Java and Sumatra, several tsunami were generated bypyroclastic flows entering the sea and by collapse accompanying calderaformation. The tsunami killed about 36,400 people, some as far away

from the volcano as 200 km.Volcanic Earthquakes and TremorsEarthquakes usually precede and accompany volcanic eruptions, asmagma intrudes and moves within the volcano.

Although most volcanic earthquakes are small, some are large enough tocause damage in the area immediately surrounding the volcano, andsome are large enough to trigger landslides and debris avalanches, suchas in the case of Mount St. Helens.

Volcanic Tremor(also called harmonic tremor) is a type of continuousrhythmic shaking of the ground that is generated by magma movingunderground.Atmospheric Effects


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Since large quantities of ashand volcanic gases can be injected into theatmosphere, volcanism can have a short-term effect on climate.

Volcanic ash can cause reflection of solar radiation, and thus can cause

the temperatures to be cooler for several years after a large eruption.

The 1815 eruption of Tambora volcano in Indonesia, was the largest inrecorded history. The year following the Tambora eruption (1816) wascalled the "year without summer". Snow fell in New England in July.

Volcanic gases like SO2 also reflect solar radiation. Eruptions in 1981at El Chichn Volcano, Mexico, and 1991 at Pinatubo, Philippines,

ejected large quantities of SO2 into the atmosphere. The effects of theEl Chichn eruption were masked by a strong El Nio in the yearfollowing the eruption, but Pinatubo caused a lowering of averagetemperature by about 1oC for two years following the eruption.

Volcanic gases like CO2 are greenhouse gases which help keep heat inthe atmosphere. During the mid-Cretaceous (about 90 to 120 millionyears ago) the CO2 content of the atmosphere was about 15 times

higher than present. This is thought to have been caused by voluminouseruptions of basaltic magma on the sea floor. Average temperatureswere likewise about 10 to 12oC warmer than present.Famine and DiseaseAs noted above, tephra falls can cause extensive crop damage and killlivestock. This can lead to famine.

Displacement of human populations, breakdown of sewerage and watersystems, cut off of other normal services can lead to disease for years

after an eruption, especially if the infrastructure is not in place to providefor rapid relief and recovery.

Volcanic Fatalaties


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Over the last 500 years, volcanoes have directly or indirectly beenresponsible for over 275,000 deaths. The greatest killers have beenpyroclastic flows, tsunami, lahars, and famine.Beneficial Aspects of Volcanism

Since this course concentrates on the damaging effects of volcanism, wewon't spend too much time on the topic of the beneficial aspects ofvolcanism. We note here, that volcanism throughout Earth history isresponsible for outgasing of the Earth to help produce both theatmosphere and hydrosphere. Volcanism helps renew the soil, and soilsaround active volcanoes are some the richest on Earth. Hydrothermalprocesses associated with volcanism produce rich ore deposits, and theheat rising around magma bodies can sometimes be tapped to produce

geothermal energy.

Mitgation of Volcanic Disasters

The best mitigation against casualties from volcanic eruptions is toprovide warning based on eruption forecasts and knowledge of the pastbehavior of the volcano, and call for evacuations. Little can be done toprotect property as the energy involved in volcanic eruptions is too greatand few structures will survive if subjected to volcanic processes.

As volcanic ash in the atmosphere has been known to cause problemswith airplanes, a system currently exists to keep aircraft out of ashclouds.This can have severe economic consequences as evidenced by the nearshutdown of European airports during the 2010 eruption of a volcano inIceland.Because evacuation plans rely on knowledge of when the volcano mighterupt and how it will behave when it does erupt, we will next discussionpredicting volcanic eruptions and volcanic behavior.

Predicting Volcanic Eruptions and Volcanic BehaviorBefore discussing how we can predict volcanic eruptions, its importantto get some terminology straight by defining some commonly usedterms.


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Active Volcano- An active volcano to volcanologists is a volcano thathas shown eruptive activity within recorded history. Thus an activevolcano need not be in eruption to be considered active.Currently there are about 600 volcanoes on Earth considered to be active

volcanoes.Each year 50 to 60 of volcanoes actually erupt.

Extinct Volcano- An extinct volcano is a volcano that has not shownany historic activity, is usually deeply eroded, and shows no signs ofrecent activity. How old must a volcano be to be considered extinctdepends to a large degree on past activity.For example, Yellowstone Caldera is about 600,000 years old and isdeeply eroded. But fumorolic activity, hot springs, and geysers all point

to the fact that magma still exists beneath the surface. Thus,Yellowstone Caldera is not considered extinct.Other volcanoes that are deeply eroded, smaller, and much younger thanYellowstone, that show no hydrothermal activity may be consideredextinct.

Dormant Volcano- A dormant volcano (sleeping volcano) is somewherebetween active and extinct. A dormant volcano is one that has notshown eruptive activity within recorded history, but shows geologic

evidence of activity within the geologic recent past.Because the lifetime of a volcano may be on the order of a million years,dormant volcanoes can become active volcanoes all of sudden. Theseare perhaps the most dangerous volcanoes because people living in thevicinity of a dormant volcano may not understand the concept ofgeologic time, and there is no written record of activity. These peopleare sometimes difficult to convince when a dormant volcano showssigns of renewed activity.Yellowstone Caldera would be considered a dormant volcano.

Mount St. Helens was a considered a dormant volcano, having noterupted for 123 years, before its reawakening in 1980.Mount Pinatubo in the Philippines had been dormant for over 400 yearsbefore its eruption in 1991.Mount Vesuvius, near Naples, Italy was considered an extinct volcanoprior to its devastating eruption of 79 A.D.


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Long - Term Forecasting and Volcanic Hazards StudiesStudies of the geologic history of a volcano are generally necessary tomake an assessment of the types of hazards posed by the volcano and thefrequency at which these types of hazards have occurred in the past.

The best way to determine the future behavior of a volcano is bystudying its past behavior as revealed in the deposits produced byancient eruptions. Because volcanoes have such long lifetimes relativeto human recorded history, geologic studies are absolutely essential.Once this information is available, geologists can then make forecastsconcerning what areas surrounding a volcano would be subject to thevarious kinds of activity should they occur in a future eruption, and alsomake forecasts about the long - term likelihood or probability of a

volcanic eruption in the area.During such studies, geologists examine sequences of layered depositsand lava flows. Armed with knowledge about the characteristics ofdeposits left by various types of eruptions, the past behavior of a volcanocan be determined. Bore holes often provide data (such as youdiscovered in your homework exercise)

Using radiometric age dating of the deposits the past frequency of events

can be determined.This information is then combined with knowledge about the presentsurface aspects of the volcano to make volcanic hazards mapswhich canaid other scientists, public officials, and the public at large to plan forevacuations, rescue and recovery in the event that short-term predictionsuggests another eruption.Such hazards maps delineate zones of danger expected from the hazardsdiscussed above: lava flows, pyroclastic flows, tephra falls, mudflows,floods, etc.

Short - Term Prediction based on Volcanic MonitoringShort - term prediction of volcanic eruptions involves monitoring thevolcano to determine when magma is approaching the surface andmonitoring for precursor events that often signal a forthcoming eruption.


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Seismic Exploration and Monitoring - Since seismic waves aregenerated by both earthquakes and explosions, and since S-waves cannotpass through liquids, arrays of seismographs can be placed around avolcano and small explosions can be set off to generate seismic waves.

If a magma body exists beneath the volcano, then there will be zonewere no S-waves arrive (an S-wave shadow zone) that can be detected.Monitoring the movement of the S-wave shadow zone can delineate theposition and movement of the magma body.

As noted above, as magma moves and deforms rocks it may beresponsible for the generation of earthquakes. Thus, there is usually an

increase in seismic activity prior to a volcanic eruption. Focal depths ofthese precursor earthquakes may change with time, and if so, themovement of magma can sometimes be tracked. In addition, volcanictremor, as noted above, can also be indication that magma is movingbelow the surface.

Changes in Magnetic Field- Rocks contain minerals such as magnetitethat are magnetic. Such magnetic minerals generate a magnetic field.

However, above a temperature called the Curie Temperature, thesemagnetic minerals show no magnetism. Thus, if a magma body enters avolcano, the body itself will show no magnetism, and if it heats thesurrounding rocks to temperatures greater than the Curie Temperature(about 500oC for magnetite) the magnetic field over the volcano will bereduced. Thus, by measuring changes in the magnetic field, themovement of magma can sometimes be tracked.Changes in Electrical Resistivity- Rocks have resistance to the flow ofelectrical current which is highly dependent on temperature and water

content. As magma moves into a volcano this electrical resistivity willdecrease. Making measurements of the electrical resistivity by placingelectrodes into the ground, may allow tracking of the movement ofmagma.Ground Deformation- As magma moves into a volcano, the structuremay inflate. This will cause deformation of the ground which can be


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monitored. Instruments like tilt meters measure changes in the angle ofthe Earth's surface which are measured in microradians 0.00018o.Other instruments track changes in distance between several points onthe ground to monitor deformation.

Changes in Groundwater System- As magma enters a volcano it maycause changes in the groundwater system, causing the water table to riseor fall and causing the temperature of the water to increase. Bymonitoring the depth to the water table in wells and the temperature ofwell water, spring water, or fumaroles, changes can be detected that maysignify a change in the behavior of the volcanic system.Changes in Heat Flow - Heat is everywhere flowing out of the surface ofthe Earth. As magma approaches the surface or as the temperature of

groundwater increases, the amount of surface heat flow will increase.Although these changes may be small they be measured using infraredremote sensing.Changes in Gas Compositions- The composition of gases emitted fromvolcanic vents and fumaroles often changes just prior to an eruption. Ingeneral, increases in the proportions of hydrogen chloride (HCl) andsulfur dioxide (SO2) are seen to increase relative to the proportion ofwater vapor.

In general, no single event can be used to predict a volcanic eruption,and thus many events are usually monitored so that taken in total, aneruption can often be predicted. Still, each volcano behaves somewhatdifferently, and until patterns are recognized for an individual volcano,predictions vary in their reliability. Furthermore, sometimes a volcanocan erupt with no precursor events at all.

After the catastrophic eruption of Mount St. Helens on May 18, 1980, avolcanic dome began to grow in the crater. Growth of this dome

occurred sporadically, and sometimes small eruptions occurred from thedome. After several years of dedicated monitoring, scientists are nowable to predict with increasing accuracy eruptions from this dome. Anexample is shown in the graphs to the right. In the weeks prior to aneruption on March 19, 1982, the amount of seismic energy released


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increased, the amount dome expansion increased, tilt increased, and SO2emissions increased prior to the event.

Beginning on March 12, a prediction was made that an eruption wouldbe likely within the next 10 days. On March 15, the prediction wasnarrowed to likely within 4 days, and on March 18 scientists predictedthat an eruption would occur within the next two days. On March 19 theeruption did occur.Note that eruption predictions such as in this example are only possibleif constant monitoring of a volcano takes place. Monitoring is anexpensive endeavor, and not all active or potentially active volcanoes are

monitored. Still, if people living around volcanoes are aware of some ofthe precursor phenomena that occur, they may be able to communicatetheir findings of anomalous events to scientists who can beginmonitoring on a regular basis and help prevent a pending disaster.Education and communication is essential in reducing risk from volcanichazards!

Tsunami

Up until December of 2004, the phenomena of tsunami was not on theminds of most of the world's population. That changed on the morningof December 24, 2004 when an earthquake of moment magnitude 9.1occurred along the oceanic trench off the coast of Sumatra in Indonesia.This large earthquake resulted in vertical displacement of the sea floorand generated a tsunami that eventually killed about 230,000 people andaffected the lives of several million people. Although people living on

the coastline near the epicenter of the earthquake had little time orwarning of the approaching tsunami, those living farther away along thecoasts of Thailand, Sri Lanka, India, and East Africa had plenty of timeto move higher ground to escape. But, there was no tsunami warningsystem in place in the Indian Ocean, and although other tsunami warningcenters attempted to provide a warning, there was no effective


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communication system in place. Unfortunately, it has taken a disaster ofgreat magnitude to point out the failings of the world's scientificcommunity and to educate almost every person on the planet abouttsunami.

Even with heightened world awareness of tsunami, disasters still occur.On September 29, 2009, earthquakes in the Samoa region of thesouthwest Pacific Ocean killed nearly 200 people, and as a result of theChilean earthquake of February, 2010, at least 50 casualties resultedfrom a tsunami triggered by a moment magnitude 8.8 earthquake.On March 11, 2011 a Moment Magnitude 9.0 earthquake struck off thenorthern Coast of Japan. The Earthquake generated a tsunami that roseup to 135 feet above sea level and killed over 20,000 people. Because of

Japans familiarity with earthquakes and enforcement of earthquakeresistant building codes, there was only minor destruction from theearthquake itself. But, despite that fact that a tsunami warning systemwas in place, the earthquake was so close to the coast, that little timewas available for people to react.Besides that high death toll, the tsunami caused one of the worst nucleardisasters in history. The Fukushima nuclear power plant, located on thecoast was hit by a 49 ft. tsunami wave that overtopped the tsunami

protection walls that were only 19 feet high, and flooded the backupgenerators for the plant that were somehow placed on the first floor in aknown tsunami zone!hat is a TsunamiA tsunami is a very long-wavelength wave of water that is generated bysudden displacement of the seafloor or disruption of any body ofstanding water. Tsunami are sometimes called "seismic sea waves",although they can be generated by mechanisms other than earthquakes.Tsunami have also been called "tidal waves", but this term should not be

used because they are not in any way related to the tides of the Earth.Because tsunami occur suddenly, often without warning, they areextremely dangerous to coastal communities.

Physical Characteristics of Tsunami


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All types of waves, including tsunami, have a wavelength, a waveheight, an amplitude, a frequency or period, and a velocity.Wavelength is defined as the distance between two identical points on awave (i.e. between wave crests or wave troughs). Normal ocean waves

have wavelengths of about 100 meters. Tsunami have much longerwavelengths, usually measured in kilometers and up to 500 kilometers.

Wave height refers to the distance between the trough of the wave andthe crest or peak of the wave.Wave amplitude- refers to the height of the wave above the still water

line, usually this is equal to 1/2 the wave height. Tsunami can havevariable wave height and amplitude that depends on water depth as weshall see in a momentWave frequency or period- is the amount of time it takes for one fullwavelength to pass a stationary point.Wave velocity is the speed of the wave. Velocities of normal oceanwaves are about 90 km/hr while tsunami have velocities up to 950 km/hr(about as fast as jet airplanes), and thus move much more rapidly across

ocean basins. The velocity of any wave is equal to the wavelengthdivided by the wave period.V = /P

Tsunami are characterized as shallow-water waves. These are differentfrom the waves most of us have observed on a the beach, which arecaused by the wind blowing across the ocean's surface. Wind-generatedwaves usually have period (time between two successive waves) of fiveto twenty seconds and a wavelength of 100 to 200 meters. A tsunami can

have a period in the range of ten minutes to two hours and wavelengthsgreater than 500 km. A wave is characterized as a shallow-water wavewhen the ratio of the water depth and wavelength is very small. Thevelocity of a shallow-water wave is also equal to the square root of theproduct of the acceleration of gravity, g, (10m/sec2) and the depth ofthe water, d.


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The rate at which a wave loses its energy is inversely related to itswavelength. Since a tsunami has a very large wavelength, it will loselittle energy as it propagates. Thus, in very deep water, a tsunami will

travel at high speeds with little loss of energy. For example, when theocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thuscan travel across the Pacific Ocean in less than one day.As a tsunami leaves the deep water of the open sea and arrives at theshallow waters near the coast, it undergoes a transformation. Since thevelocity of the tsunami is also related to the water depth, as the depth ofthe water decreases, the velocity of the tsunami decreases. The change oftotal energy of the tsunami, however, remains constant.

Furthermore, the period of the wave remains the same, and thus morewater is forced between the wave crests causing the height of the waveto increase. Because of this "shoaling" effect, a tsunami that wasimperceptible in deep water may grow to have wave heights of severalmeters or more.If the trough of the tsunami wave reaches the coast first, this causes aphenomenon called drawdown, where it appears that sea level has

dropped considerably. Drawdown is followed immediately by the crestof the wave which can catch people observing the drawdown off guard.When the crest of the wave hits, sea level rises (called run-up). Run-upis usually expressed in meters above normal high tide. Run-ups from thesame tsunami can be variable because of the influence of the shapes ofcoastlines. One coastal area may see no damaging wave activity whilein another area destructive waves can be large and violent. The floodingof an area can extend inland by 300 m or more, covering large areas ofland with water and debris. Flooding tsunami waves tend to carry loose

objects and people out to sea when they retreat. Tsunami may reach amaximum vertical height onshore above sea level, called a run-upheight, of 30 meters. A notable exception is the landslide generatedtsunami in Lituya Bay, Alaska in 1958 which produced a 60 meter highwave.


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Because the wavelengths and velocities of tsunami are so large, theperiod of such waves is also large, and larger than normal ocean waves.Thus it may take several hours for successive crests to reach the shore.(For a tsunami with a wavelength of 200 km traveling at 750 km/hr, the

wave period is about 16 minutes). Thus people are not safe after thepassage of the first large wave, but must wait several hours for all wavesto pass. The first wave may not be the largest in the series of waves. Forexample, in several different recent tsunami the first, third, and fifthwaves were the largest.

How Tsunami are Generated

There is an average of two destructive tsunami per year in the Pacific

basin. Pacific wide tsunami are a rare phenomenon, occurring every 10 -12 years on the average. Most of these tsunami are generated byearthquakes that cause displacement of the seafloor, but, as we shall see,tsunami can be generated by volcanic eruptions, landslides, underwaterexplosions, and meteorite impacts.EarthquakesEarthquakes cause tsunami by causing a disturbance of the seafloor.Thus, earthquakes that occur along coastlines or anywhere beneath the

oceans can generate tsunami. The size of the tsunami is usually relatedto the size of the earthquake, with larger tsunami generated by largerearthquakes. But the sense of displacement is also important. Tsunamiare generally only formed when an earthquake causes verticaldisplacement of the seafloor. The 1906 earthquake near San FranciscoCalifornia had a Richter Magnitude of about 7.1, yet no tsunami wasgenerated because the motion on the fault was strike-slip motion with novertical displacement. Thus, tsunami only occur if the fault generatingthe earthquake has normal or reverse displacement. Because of this,

most tsunami are generated by earthquakes that occur along thesubduction boundaries of plates, along the oceanic trenches. Since thePacific Ocean is surrounded by plate boundaries of this type, tsunami arefrequently generated by earthquakes around the margins of the PacificOcean.


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Examples of Tsunami generated by Earthquakes

Although the December 2004 Indian Ocean tsunami is by far the bestwell known and most deadly (and will be featured in a video in class),we here discuss other disastrous tsunami generated by earthquakes.

April 1, 1946 - A magnitude 7.3 earthquake occurred near UnimakIsland in the Aleutian Islands west of Alaska, near the Alaska Trench.Sediment accumulating in the trench slumped into the trench andgenerated a tsunami. A lighthouse at Scotch Gap built of steelreinforced concrete was located on shore at an elevation of 14 m abovemean low water. The first wave of the tsunami hit the Scotch Gap area20 minutes after the earthquake, had a run-up 30 m and completelydestroyed the lighthouse. 4.5 hours later the same tsunami reached the

Hawaiian Islands after traveling at an average speed of 659 km/hr. As itapproached the city of Hilo on the Big Island, it slowed to about 47km/hr (note that even the fastest human cannot run faster than about 35km/hr) and had a run-up of 18 m above normal high tide. It killed 159people (90 in Hilo) and caused $25 million in property damage.May 22, 1960 - A moment magnitude9.5 earthquake occurred along thesubduction zone off South America. Because the population of Chile isfamiliar with earthquakes and potential tsunami, most people along the

coast moved to higher ground. 15 minutes after the earthquake, atsunami with a run-up of 4.5 m hit the coast. The first wave thenretreated, dragging broken houses and boats back into the ocean. Manypeople saw this smooth retreat of the sea as a sign they could ride theirboats out to sea and recover some of the property swept away by the firstwave. But, about 1 hour later, the second wave traveling at a velocity of166 km/hr crashed in with a run-up of 8 m. This wave crushed boatsalong the coast and destroyed coastal buildings. This was followed by athird wave traveling at only 83 km/hr that crashed in later with a run-up

of 11 m, destroying all that was left of coastal villages. The resultingcausalities listed 909 dead with 834 missing. In Hawaii, a tsunamiwarning system was in place and the tsunami was expected to arrive at9:57 AM. It hit at 9:58 AM and 61 people died, mostly sightseers thatwanted to watch the wave roll in at close range (obviously they were too


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A large Plinian eruption column blasted pumice and ash up to 40 kminto the atmosphere. This Plinian eruption column likely collapsedseveral times to produce pyroclastic flows, any of which could havegenerated a tsunami.

A loud explosive blast was heard as far away as Australia. This blastwas likely caused by a phreatic explosion that occurred as a result ofseawater coming in contact with the magma. The explosion could havegenerated at least one of the tsunami.At some point during the eruption a caldera formed by collapse of thevolcanic island. Areas that were once more than 300 m above sea levelwere found 300 m below sea level after the eruption. The suddencollapse of the volcano to form this caldera could have caused one or

more tsunami.Earthquakes were felt throughout the eruption. Any one of thesesubmarine earthquakes could have caused a tsunami.One of the tsunami had a run-up of about 40 m above normal sea level.A large block of coral weighing about 600 tons was ripped off theseafloor and deposited 100 m inland. One ship was carried 2.5 kminland and was left 24 meters above sea level, with all of its crew sweptinto the ocean.

Landslides

Landslides moving into oceans, bays, or lakes can also generate tsunami.Most such landslides are generated by earthquakes or volcaniceruptions. As previously mentioned, a large landslide or debrisavalanche fell into Lituya Bay, Alaska in 1958 causing a wave with arun-up of about 60 m as measured by a zone completely stripped ofvegetation.Underwater ExplosionsNuclear testing by the United States in the Marshall Islands in the 1940s

and 1950s generated tsunami.Meteorite ImpactsWhile no historic examples of meteorite impacts are known to haveproduced a tsunami, the apparent impact of a meteorite at the end of theCretaceous Period, about 65 million years ago near the tip of what is


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now the Yucatan Peninsula of Mexico, produced tsunami that leftdeposits all along the Gulf coast of Mexico and the United States.Mitigation of Risks and HazardsThe main damage from tsunami comes from the destructive nature of the

waves themselves. Secondary effects include the debris acting asprojectiles which then run into other objects, erosion that can underminethe foundations of structures built along coastlines, and fires that resultfrom disruption of gas and electrical lines. Tertiary effects include lossof crops and water and electrical systems which can lead to famine anddisease.Within the last century, up until the December 2004 tsunami, there were94 destructive tsunami which resulted in 51,000 deaths. Despite the fact

that tsunami warning systems have been in place in the Pacific Oceanbasin since 1950, deaths still result from tsunami, especially when thesource of the earthquake is so close to a coast that there is little time fora warning, or when people do not heed the warning or followinstructions associated with the warning. These factors point out theinadequacy of the world in not having a tsunami warning system inplace in the Indian Ocean, where in one event, the death toll fromtsunami was increased by a factor of 5 over all previous events.

Prediction and Early WarningFor areas located at great distances from earthquakes that couldpotentially generate a tsunami there is usually plenty of time forwarnings to be sent and coastal areas evacuated, even though tsunamitravel at high velocities across the oceans. Hawaii is good example ofan area located far from most of the sources of tsunami, where earlywarning is possible and has saved lives. For earthquakes occurringanywhere on the subduction margins of the Pacific Ocean there is aminimum of 4 hours of warning before a tsunami would strike any of the

Hawaiian Islands.The National Oceanic and Atmospheric Administration (NOAA) has setup a Pacific warning system for areas in the Pacific Ocean, called thePacific Tsunami Warning Center. It consists of an international networkof seismographic stations, and tidal stations around the Pacific basin thatcan all send information via satellite to the Center located in Hawaii.


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When an earthquake occurs somewhere in the region, the Centerimmediately begins to analyze the data looking for signs that theearthquake could have generated a tsunami. The tidal stations are alsomonitored, and if a tsunami is detected, a warning is sent out to all areas

on the Pacific coast. It takes at least 1 hour to assimilate all of theinformation and issue a warning. Thus for an average velocity of atsunami of 750 km/hr, the regional system can provide a warningsufficient for adequate evacuation of coastal areas within 750 km of theearthquake.In order to be able to issue warnings about tsunami generated within 100to 750 km of an earthquake, several regional warning centers have beenset up in areas prone to tsunami generating earthquakes. These include

centers in Japan, Kamchatka, Alaska, Hawaii, French Polynesia, andChile.

These systems have been very successful at saving lives. For example,before the Japanese warning system was established, 14 tsunami killedover 6000 people in Japan. Since the establishment of the warning

system, up until March 2011, 20 tsunami have killed 215 people inJapan.Like all warning systems, the effectiveness of tsunami early warningdepends strongly on local authority's ability to determine that their is adanger, their ability to disseminate the information to those potentiallyaffected, and on the education of the public to heed the warnings andremove themselves from the area.Tsunami Safety Rules

A strong earthquake felt in a low-lying coastal area is a natural warningof possible, immediate danger. Keep calm and quickly move to higherground away from the coast.All large earthquakes do not cause tsunami, but many do. If the quake islocated near or directly under the ocean, the probability of a tsunami


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increases. When you hear that an earthquake has occurred in the oceanor coastline regions, prepare for a tsunami emergency.Tsunami can occur at any time, day or night. They can travel up riversand streams that lead to the ocean.

A tsunami is not a single wave, but a series of waves. Stay out of dangeruntil an "ALL CLEAR" is issued by a competent authority.Approaching tsunami are sometimes heralded by noticeable rise or fallof coastal waters. This is nature's tsunami warning and should beheeded.A small tsunami at one beach can be a giant a few miles away. Do notlet modest size of one make you lose respect for all.Sooner or later, tsunami visit every coastline in the Pacific. All tsunami -

like hurricanes - are potentially dangerous even though they may notdamage every coastline they strike.Never go down to the beach to watch for a tsunami! WHEN YOU CANSEE THE WAVE YOU ARE TOO CLOSE TO ESCAPE. Tsunami canmove faster than a person can run!During a tsunami emergency, your local emergency management office,police, fire and other emergency organizations will try to save your life.Give them your fullest cooperation.

Homes and other buildings located in low lying coastal areas are notsafe. Do NOT stay in such buildings if there is a tsunami warning.The upper floors of high, multi-story, reinforced concrete hotels canprovide refuge if there is no time to quickly move inland or to higherground.If you are on a boat or ship and there is time, move your vessel to deeperwater (at least 100 fathoms). If it is the case that there is concurrentsevere weather, it may safer to leave the boat at the pier and physicallymove to higher ground.

Damaging wave activity and unpredictable currents can affect harborconditions for a period of time after the tsunami's initial impact. Be sureconditions are safe before you return your boat or ship to the harbor.Stay tuned to your local radio, marine radio, NOAA Weather Radio, ortelevision stations during a tsunami emergency - bulletins issued through


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your local emergency management office and National Weather Serviceoffices can save your life.


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Flood Stage

The termstagerefers to the height of a river (or any other body ofwater) above a locally defined elevation. This locally defined

elevation is a reference level, often referred to as datum. Forexample, for the lower part of the Mississippi River, referencelevel or datum, is sea level (0 feet). Currently the MississippiRiver is at a stage of about 3 feet, that is 3 feet above sea level.Other river systems have a reference level that is not sea level.Most rivers in the United States have gaging stations wheremeasurements are continually made of the river's stage anddischarge. These are plotted on a graph called a hydrograph,

which shows the stage or discharge of the river, as measured at thegaging station, versus time. When the discharge of a river increases, the channel may become

completely full. Any discharge above this level will result in theriver overflowing its banks and causing a flood. The stage atwhich the river will overflow its banks is calledbankfull stageor

flood stage. For example, the flood stage of the Mississippi Riverat New Orleans is 17 feet. Discharge that produces a stage over 17feet will result in the water nearing the top of the levee withpotential flooding of the city of New Orleans (the top of the leveeis actually at 25 feet above sea level). (Note that for theMississippi River and other large rivers in Louisiana, the currentstage and flood stage are published on a daily basis in the weathersection of the Times-Picayune newspaper).

Discharge is not linearly related to stage because discharge dependson both the depth and width of the stream channel, or moreprecisely, on the cross-sectional shape of the channel. Stage refers

only to the height of the water above some reference level. Forexample, the graph below is a hydrograph of the Mississippi Riverat St. Louis, Missouri during the time period of the 1993 flood.Discharge is plotted on the Y-axis, and dates are plotted on the x-axis. Note that stages corresponding to various discharges areshown on the left-hand y-axis, and that the spacing between equalunits of stage are not equal along the y-axis.


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Note that for the1993MississippiRiver Flood,

the riverreached floodstage of 30feet abovedatum onabout June 26and peaked (orcrested) at just

under 50 feetabove datumon August 1.The suddendrops seen indischargearound July 15and July 20

correspondedto breaks inthe leveesystemupstream fromSt. Louis thatcaused waterto flow ontothe floodplain

upstream, thusreducing boththe stage anddischargemeasured atSt. Louis.


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Factors that Affect Flooding

As discussed previously the main factors that cause flooding are heavyrainfall, sudden or heavy snow melt, and dam failure. Now that we

understand something about levees and floodplains, we can add to thislist the possibility of levee failure. All of these factors can suddenlyincrease discharge of water into streams, within streams, and out ofstreams. Furthermore, as we have just seen, when the discharge causesthe river to rise above flood stage water runs onto the floodplain. Herewe discuss the main cause of flooding, that is heavy rainfall over a shortperiod of time.

When rain falls on the surface of the Earth, some of the water isevaporated and returns to the atmosphere, some of it infiltrates the soiland moves downward into the groundwater system, and some isintercepted by depressions and vegetation. What remains on the surfaceof the Earth and eventually flows into streams is calledrunoff. Ingeneral, then:

Runoff = Precipitation - Infiltration - Interception - Evaporation

Evaporation tends to be the least of these quantities, particularly overshort periods of time, and thus precipitation, infiltration, andinterception are the most important variables that determine runoff andeventual discharge into streams.

Rainfall Distribution

If rainfall is heavier than normal in a particular area and infiltration,

interception, and evaporation are low then runoff can be high and thelikelihood of flooding will increase. Heavy rainfall can be depictedon maps that show curves of equal rainfall. Such curves are calledisohyets, and the resulting maps are called isohyetal maps.


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Lag Time - Thetime differencebetween whenheavy

precipitationoccurs andwhen peakdischargeoccurs in thestreamsdraining anarea is called

lag time.

Lag time dependson such factorsas the amountof time overwhich the rainfalls and the

amount ofinfiltration andinterceptionthat takesplace along thepath to astream.


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If the amount of rain is high over a short time period, lag

time is short. If the amount of rain is high over a longer time period, lag

time is longer. Lack of infiltration and interception reduce lag time.

Upstream flooding and flash floodsIn areas where large amounts of rain fall over a short period of timewithin a small area, streams in the local area may flood, with little or noeffect on areas downstream. Such floods are referred to as upstream

floods. In such floods, water rises quickly and flows away quickly afterthe storm has passed. Lag times are measured in days.

Flash floodsoccur when the rate of infiltration is low and heavy rainsoccur over a short period of time. They are upstream floods with verylittle lag time (lag times may be only a few hours). Because they comewith little warning, flash floods are the most dangerous to human lives.

Downstream flooding If large amounts of rain fall over an extended period of time over a

large region,downstream floods(also calledregional floods) mayoccur. Lag times are usually longer as tributary streamscontinually increase the discharge into larger streams. Such floodsextend over long periods of time and affect the larger streams aswell as tributary streams. The 1993 flood on the upper MississippiRiver is considered a downstream flood. Water levels rise slowlyand dissipate slowly (in the case of the 1993 flood, the increase in

discharge to the peak occurred over several weeks after severalweeks of intense rainfall, and it took several months for riverstages to return to normal levels).


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Infiltration

Infiltration is controlled by how readily the water can seep into the soil,

be absorbed by the soil, and work its way down to the water table.Several factors determine the rate of infiltration:

Extent of water saturation of the soil If the soil is already saturatedwith water and the water table has risen as a result of rainfall priorto a heavy storm, then little further water can infiltrate the soil, andthe rate of infiltration will be highly decreased.


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Vegetation coverVegetation can aid infiltration by slowing the flow of water over thesurface and providing passageways along root systems for water to enterthe soil. In desert regions or areas that have recently been deforested

either by fires or humans, infiltration will be reduced, thus increasingthe rate of runoff and decreasing the lag time.

Soil types (dependent on climate)Different soil types have different capacities to absorb moisture. Soiltype is to a large extent dependent on climate. For example a type ofsoil that forms in dry, desert-like environments has a thin layer of poorlydeveloped soil overlying a crust of caliche. Caliche is calciumcarbonate that has precipitated out of water infiltrating though the thin

soil. The caliche zone acts as an impermeable layer though which watercan only penetrate with difficulty. Such soils in deserts, combined withthe lack of vegetation make flash flooding in desert areas morecommon.

Frozen ground If the ground is frozen little water can penetrate. Thus rainfall after

a period of cold temperatures may not be able to infiltrate throughthe frozen ground.

Human constructionHumans tend to pave the Earth with such things as parking lots,highways, sidewalks, and plazas that prevent infiltration of water intothe soil. Furthermore they tend to channel the water into storm sewersystems and concrete lined drainages, all of which increase runoff anddecrease infiltration.Interception

Interception involves anything that traps rainwater and prevents it fromcontributing to rnnoff. This includes water that is stored on leaves and

braches of trees until it evaporates and water that is gets stored in pondsor lakes. Thus, removal of vegetation decreases interception and resultsin more runoff. Increasing vegetation or construction of retention ponds,increases interception and results in less runoff.


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Levee Failures

Natural levees are constructed as a result of flooding, as we saw in thediscussion last lecture. But, natural levees tend to be relatively low and

do not offer much protection from large discharge because they caneasily be overtopped. Human made levees, such as we see on theMississippi River along much of its length, are much higher and areconstructed to prevent flooding from high discharges on the River.Most levees are constructed of piles of dirt (rock and soil) with a

concrete cover on the river side of the levee. Such levees often give afalse sense of security for those living on the floodplain the levee wasbuilt to protect, because failure of such levees can lead to flooding,

either because discharge can become great enough to overtop the leveesor the levees can become weakened and fail. Levees can fail for threemain reasons.

2.Overtopping of levees

If high discharge in the river leads to a river stage that is higher thanany point on a levee, the water will overtop the levee and start toflow onto the floodplain. Because the initial gradient from theriver to flood plain is relatively high, the velocity of the stream asit overtops the levee will be high. High velocities can result inhigh rates of erosion, and thus the levee that is initially overtoppedwill soon become eroded and a channel through the levee will soonbe created.

3.Undercutting and slumping of levee

Higher discharge in the river will lead to higher velocities with thestream trying to increase its width and depth. Higher velocities canlead to higher rates of erosion along the inner parts of levees and


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thus lead to undercutting and slumping of the levee into the river.Heavy rainfall or seepage into the levee from the river can increasefluid pressure in the levee and lead to slumping on the outer partsof the levee. If the slumps grow to the top of the levee, large

sections of the levee may slump onto the floodplain and lower theelevation of the top of the levee, allowing it to be more easilyovertopped.

4.Seepage and Piping

Increasing levels of water in the river will cause the water table in thelevee to rise. This will also increase fluid pressure and may result

in seepage (water being pushed through the levee to rise as springson the surrounding flood plains). If a high rate of flow isdeveloped due to the increased fluid pressures, then a high velocitypathway to the flood plain may develop piping may occur. Pipingwill erode the material under the levee, undermining it and causingits collapse and failure.

Dam Failures

Failure of natural dams or human made dams results in floodingdownstream from the dam. Natural dams result from natural events thatblock streams, such as landslides, lava flows, or pyroclastic flows intostreams. Humans build dams for flood control, water storage, and the forthe generation of electricity.

Hazards Associated with FloodingHazards associated with flooding c

geography hazards

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