UNDERSTANDING EARTHQUAKE HAZARDS IN THE SAN FRANCISCO BAY REGION

Major Quake Likely to Strike Between 2000 and 2030

U.S. Department of the InteriorU.S. Geological Survey

USGS Fact Sheet-152-99Reprinted 2001

On the basis of researchconducted since the 1989

Loma Prieta earthquake, U.S.Geological Survey (USGS) andother scientists conclude thatthere is a 70% probability of atleast one magnitude 6.7 orgreater quake, capable of caus-ing widespread damage, strikingthe San Francisco Bay regionbefore 2030. Major quakes mayoccur in any part of this rapidlygrowing region. This empha-sizes the urgency for all commu-nities in the Bay region to con-tinue preparing for earthquakes.

Just before dawn, residents of a baysideurban area, thought to be well prepared forearthquakes, were jolted from their beds bya magnitude 6.9 quake. This 1995 temblorkilled more than 6,000 people and caused$100 billion in damage. The quake struckKobe, Japan, but similar losses could haveoccurred in the San Francisco Bay region in1989 had the magnitude 6.9 Loma Prieta earth-quake been centered in an urbanized area.

Damaging earthquakes are inevitable inthe Bay region, but taking actions based onthe odds of future quakes will help savelives and protect property. Following theLoma Prieta quake, the U.S. GeologicalSurvey’s (USGS) Working Group on Cali-fornia Earthquake Probabilities reassessedthe likelihood of large quakes striking theBay region and issued a report in 1990.

Since then, scientists have gained newinsights into Bay region earthquakes, pro-viding a better basis for determiningquake odds. In 1997, the USGS workinggroup, now known as WG99, was ex-panded to include more than 100 scien-tists from Federal and California Stategovernments, consulting firms, industry,and universities.

Earthquake probabilities are based onbalancing the continual motions of the

plates that make up the Earth’s outer shellwith the slip on faults, which occurs prima-rily during earthquakes. To determine Bayregion earthquake probabilities, WG99gathered new data, developed analyticaltools, and debated a wide variety of inter-pretations about how future temblors mayoccur (see inside pages for further details).

WG99 determined that there is a 70%chance (±10%) of at least one magnitude6.7 or greater earthquake striking the SanFrancisco Bay region between 2000 and2030. This result is the most important out-come of WG99’s work, because any majorquake can cause damage throughout the re-gion. This was [continued on back page]

280

580

880

680

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1

1

80

101

101

101

21%

6%

10%

18%

6%

32%

odds ( 10%) for one or moremagnitude 6.7 or greaterearthquakes from 2000 to 2030.This result incorporates 9% oddsof quakes not on shown faults.

SAN FRANCISCO BAY REGIONEARTHQUAKE PROBABILITY

70%

4%

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MT. DIABLO

THRUST FAULT

MT. DIABLO

THRUST FAULT

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SA

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ay

New odds of magnitude 6.7 or greater quakes before 2030 on the indicated fault

Odds for faults that were not previously included in probability studies

Expanding urban areas

Increasing quake odds along fault segments

Individual fault probabilities are uncertain by 5 to 10%

0

0 20 KILOMETERS

20 MILES

NNN

21%

18%

San Francisco

Half MoonBay

Pacifica

Oakland

Sacramento

Stockton

DanvilleDanville

Antioch

PaloAlto

SanMateo

WalnutCreek

WalnutCreek

LivermorePleasanton

Hayward

Tracy

Santa CruzWatsonville

GilroyGilroy

MontereySalinas

SanJoseSanJose

SantaRosa

Petaluma

NovNovato

SanRafael

NapaSonoma

Vallejo

EXTENT OF RUPTURE

IN LOMA PRIETA QUAKE

EXTENT OF RUPTURE

IN LOMA PRIETA QUAKE

The threat of earthquakes extends across the entire San Francisco Bay region, and a major quake is likely be-fore 2030. Knowing this will help people make informed decisions as they continue to prepare for future quakes.

High

LowM7

M7.8

M6.5

EARTHQUAKEODDS

M7 M6.5

Plate motionscontinually load strain

onto faults

Fault slip occursduring earthquakes,

releasing strain

THE ''EARTHQUAKE PROBABILITY BALANCE''

I

Quake probabilities are derived by balancingtwo processes—(1) the continual motions ofthe plates that make up the Earth’s outer shell(represented by pouring sand onto the left tray)and (2) the slip on faults, which occurs prima-rily during earthquakes (equivalent to addingballs to the right tray). The slip on faults overtime must balance the strain built up by theplate motions. The total amount of slip duringan earthquake, shown here by the proportionalvolumes of the spheres, depends on its magni-tude (M). The larger the quake, the more strainreleased.

Balancing plate motions and earthquakesQuake probabilities for the San Francisco

Bay region are derived by balancing two pro-cesses—(1) the motions of the plates that makeup the Earth’s outer shell and (2) the slip onfaults, which occurs primarily during earth-quakes. The continual northwestward motionof the Pacific Plate past the North AmericanPlate loads strain onto the network of activefaults that slice through the region. Earthquakessporadically release and redistribute this strain.WG99 combined geology, physics, and statis-tics to balance these processes and calculatequake odds.

One side of the balance is the rate at whichplate motions load strain onto faults. Develop-ment of the Global Positioning System (GPS)has allowed geophysicists to make accuratemeasurements of how the current plate mo-tions—totaling 1.5 inches per year across theentire region—distributes strain onto individualfaults. Geologic studies also contribute to thisunderstanding by documenting long-term faultmotions, which must match the strain-loadingrate. For example, on the San Gregorio Fault

near Moss Beach, a buried stream channel hasbeen offset about 1,000 feet over the past80,000 years. This indicates that the strain ratethere is about one-sixth of an inch per year, asmall fraction of the total regional plate mo-tions. Offshore studies reveal other activestrands of the San Gregorio Fault that accountfor additional strain.

The other side of the balance is the slip onfaults, which over time must account for thestrain built up by the plate motions. Slip onfaults (movement of one side of a fault relativeto the other) can occur either during earth-quakes or during slow, aseismic (without earth-quakes) creep. For example, creep on the Hay-ward Fault is slowly offsetting street curbs,even in the absence of large quakes. Whereaseismic creep occurs, it affects the balance be-tween plate motions and earthquakes by reliev-ing strain, which can either change the likeli-hood or lower the magnitude of future quakes.Nevertheless, most slip on faults occurs duringearthquakes—the larger the quake, the greaterthe slip.

Earthquake historyKnowledge of past earthquakes indicates

what sizes of quakes to expect in the future.The most accurate locations and magnitudesexist for quakes recorded on seismographs,which came into widespead use about 1900.However, this record is too short to under-stand the pattern of earthquakes over geo-logic time. Historical accounts of damagehelp identify and locate quakes that oc-curred before there were adequate seismo-graphic records (seismograms), but such ac-counts are fragmentary in northernCalifornia before 1850. Scientists reana-lyzed seismograms and historical accountsas part of WG99’s efforts.

To go back even further in time, geolo-gists dig trenches across faults to uncoverearthquake ruptures that once reached thesurface but now are buried. Many newtrenches across Bay region faults expose theancient earthquake history. For example, atrench in El Cerrito revealed evidence offour to seven large quakes on the HaywardFault during the past 2,200 years.

Understanding faultsData from trenches, historical accounts,

and seismograms provide incomplete infor-mation about quakes in the San Francisco

Bay region. Supplementing this with up-to-date knowledge about how faults work al-lows scientists to make better projections ofthe expected sizes of future earthquakes.

Many Earth scientists believe that faultsare composed of segments that may ruptureindividually or in groups of adjacent seg-ments during an earthquake, and WG99used this concept. Larger quakes rupturegreater fault lengths and produce more slip.For example, the magnitude 7.8 San Fran-cisco earthquake of 1906 ruptured 300 milesof the San Andreas Fault and produced asmuch as 30 feet of slip, whereas the magni-tude 6.9 Loma Prieta quake in 1989 rup-tured only 25 miles of fault and producedonly about 6 feet of slip. If scientists can

CALCULATING THE EARTHQUAKE ODDSn 1997, the U.S. Geological Survey’s(USGS) Working Group on California

Earthquake Probabilities was expanded toinclude more than 100 geologists, seis-mologists, geophysicists, and mathemati-cians. This group, known as WG99, calcu-lated new quake odds for the SanFrancisco Bay region based on insightsgained since the 1989 Loma Prieta earth-quake. WG99 concluded that there is a70% probability (±10%) of at least onemagnitude 6.7 or greater quake, capable ofcausing widespread damage, striking theregion before 2030. The process used todetermine these odds is described below.

Knowledge of past earthquakes is essential for estimatingthe odds of future temblors. This knowledge comes from(top to bottom) historical damage accounts, fault rupturesexposed in trenches, and seismographic records.

Damage from1868 HaywardFault earth-quake

Fault rupture from1906 San Franciscoearthquake

Seismogram from 1989 Loma Prieta earthquake

identify the lengths of fault segments thatmay fail together in an earthquake, they canestimate the magnitudes and amounts of slipfor possible future quakes.

Earth scientists identify fault segments bystudying bends, intersections, and gaps infaults, past earthquakes, and major changesin rock types along faults. One example ofhow past earthquakes define segments is the1868 magnitude 6.9 quake on the HaywardFault. This quake ruptured only the southernpart of the fault, defining that part as a seg-ment that can rupture separately. However,WG99 concluded that an even-larger earth-quake might rupture the entire Hayward-Rodgers Creek Fault.

Finding new faultsIn addition to identifying fault segments,

WG99 looked for previously unknownfaults. Most faults in the San Francisco Bayregion have “strike-slip” motion, in whichthe two sides of the fault slip horizontallypast each other. In contrast, ramp-like“thrust” faults have vertical motion and of-ten do not reach the Earth’s surface, makingthem difficult to find. The importance of lo-cating these hidden faults was underscoredby the devastating 1994 Northridge earth-quake in southern California, which oc-curred on a previously unknown thrust fault.

Of the known thrust faults in the Bay re-gion, only the Mount Diablo Thrust Faulthas a high enough slip rate to be included inWG99’s calculations. To account for thesmall percentage of large earthquakes that oc-cur on minor or unknown faults (both strikeslip and thrust), WG99 estimated probabilitiesfor such quakes on the basis of the rate of simi-lar quakes in the historical record.

A new method for determining quake oddsScientists can best determine earthquake

probabilities for a fault once they know when

it last ruptured, the sizes of possible quakes,and the rates of plate motions. For example,if plate motions are loading a fault at 1 inchper year, the fault will accumulate 100inches of strain in 100 years. If each earth-quake on that fault releases 100 inches ofslip and there is no aseismic creep, then onequake can be expected every 100 years. Ifthe occurrence of that earthquake is equallylikely at any time, the odds of it strikingwould be 1% in any given year. However,many earthquake experts believe that once afault slips, plate motions must load strainback onto that fault before the next quakecan occur. If quakes occurred with perfectregularity, then in this example they wouldoccur exactly 100 years apart, and the oddsof a quake would be 0% for the first 99years and 100% for the final year.

Earthquakes, however, are not that pre-dictable. WG99 therefore developed a newset of models that use both physics and sta-tistics. In these models, quakes are causedby a combination of constant plate motionsand a random process that accounts forvariations in earthquake sizes and occur-rence. These models closely mimic the oc-currence of quakes around the world.

Because every earthquake changes strainon nearby faults, another important elementin the WG99 method is the inclusion of in-teractions between faults in the San Fran-cisco Bay region. For example, the rate oflarge quakes in the region was high in thelate 1800’s but abruptly dropped after the1906 San Francisco earthquake. Scientistsbelieve that this rate dropped because theSan Andreas Fault slipped so much oversuch a great length in 1906 that the strainwas reduced on most faults throughout theBay region. Because plate motions are con-tinuous, strain has been slowly building upagain, and strong earthquakes began to oc-cur again in the 1980’s. However, the level

of seismic activity has not yet reached that ofthe late 1800’s (see diagram on back page).

The WG99 method for determiningearthquake probabilities involves makingmany decisions, such as defining fault seg-ments and choosing among alternative sta-tistical models. Every such decision is uncer-tain, but WG99 members assigned weights tothe various choices so that all of them were in-cluded in their overall calculations.

This process helps ensure that the WG99probabilities are reliable estimates of theearthquake threat faced by the San Fran-cisco Bay region between 2000 and 2030.The WG99 method will also allow theUSGS to update these probabilities as newinsights are gained.❖

The San Francisco Bay region lies on the boundaryzone between two of the tectonic plates that makeup the Earth’s outer shell. The relentless motion ofthese plates builds up strain that will eventually bereleased in earthquakes on the region’s many faults.The lengths of fault that slipped in the 1868 Haywardand 1989 Loma Prieta magnitude 6.9 earthquakesare shown in yellow.

Oakland

Sacramento

San Jose

MossBeach

El Cerrito

San Francisco

SantaRosa

SantaCruz

Monterey19891989

18681868

80

101

NORTHAMERICAN

PLATE

PACIFICPLATE

SA

NG

REG

OR

IO

RODGERS

CREEK

SAN AN

DREAS

HAYWARD

MTDIABLOTHRUST

0

0 30 MILES

30 KILOMETERS

NN

FAULTS AND PLATE MOTIONS INTHE SAN FRANCISCO BAY REGION

Major faults

AREAOF MAP

CALIF

Questionsare posed by scientists about how quakes in the Bay Region are caused

New data are collected and panels ofexperts develop probabilities for multiple quake scenarios

Quake scenario probabilities are combined to calculate 30-year odds for the region

70% probability of at least one magnitude6.7 or greater quake before 2030

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70% odds

for at least one magnitude

6.7 or greater quake

2000 to 2030

1850

19001906

1906

1950

1950

2000

2000

The future

6.0 to 6.5

Greater than 6.5

EARTHQUAKE MAGNITUDES

5.5 to 5.9

Increasing quake odds

For more information contact:Earthquake Information Hotline (650) 329-4085

U.S. Geological Survey, Mail Stop 977345 Middlefield Road, Menlo Park, CA 94025

http://quake.usgs.gov/See also Progress Toward a Safer Future Since the 1989 Loma

Prieta Earthquake (USGS Fact Sheet 151-99)

COOPERATING ORGANIZATIONSAssociation of Bay Area Governments

California Division of Mines and GeologyCalifornia Governor’s Office of Emergency Services

Federal Emergency Management AgencyLawrence Livermore National Laboratory

Geomatrix Consultants Inc.Pacific Gas and Electric Company

University of California at BerkeleyWilliam Lettis & Associates

Many other institutions, organizations, and firms

Andrew J. Michael, Stephanie L. Ross,David P. Schwartz, James W. Hendley II, and

Peter H. Stauffer

Graphic design bySusan Mayfield, Sara Boore, and Michael F. Diggles

The rate of large earthquakes in the San Francisco Bay region abruptly dropped after the Great 1906 Earth-quake. The San Andreas Fault slipped so much over such a great length in that quake that the strain was re-duced on most faults throughout the region. Strain has been slowly building up again. However, the level ofseismic activity has not yet reached that of the late 1800’s.

dramatically demonstrated when the 1989Loma Prieta earthquake caused severe dam-age in Oakland and San Francisco, morethan 50 miles from the fault rupture. Al-though earthquakes can inflict damage at aconsiderable distance, shaking will be veryintense near the fault rupture. Therefore,temblors located in urbanized areas of theregion have the potential to cause muchmore damage than the 1989 quake.

In the Bay region’s rapidly growing easternvalleys, four faults slice through Contra Costa,Alameda, Solano, Santa Clara, San Benito, andNapa Counties. WG99 calculated the odds ofmajor quakes on these faults for the first time.They determined that there is a 30% chance ofone or more magnitude 6.7 or greater quakesoccurring somewhere on the Calaveras, Con-cord-Green Valley, Mount Diablo Thrust, andGreenville Faults before 2030.

Residents living near the Pacific coast inburgeoning San Mateo, Santa Cruz, andMonterey Counties are sandwiched betweenthe San Andreas and San Gregorio Faults. Newdata have allowed WG99 to calculate the firstearthquake probabilities for the San GregorioFault and to better estimate probabilities for theSan Andreas Fault. Combined, these two faultshave a 25% chance of producing one or moremagnitude 6.7 or greater quakes in thesecoastal areas before 2030.

When the 1990 USGS probability reportwas released, earthquake odds could be esti-mated only for the San Andreas Fault and theHayward-Rodgers Creek Fault, although thedanger posed by other faults was recognized.WG99 found that, of all the faults in the Bayregion, these two and the Calaveras pose thegreatest threat, because they have high quakeodds and run through the region’s urban core.

There are important differences betweenthe 1990 and WG99 studies. WG99 ana-lyzed five additional faults, which would be

expected to increase the estimated regionalprobability of major quakes. This expectedincrease was largely compensated for, how-ever, by two effects not included in the 1990report: (1) slip on faults in the absence ofearthquakes and (2) the effect of the 1906earthquake in reducing quake activitythroughout the region.

Additionally, the 1990 study consideredonly earthquakes of about magnitude 7 indetermining there was a 67% chance of ma-jor quakes in the Bay region between 1990and 2020. WG99 decided to focus on earth-quakes of magnitude 6.7 and greater in theircalculations, because the 1994 Northridgequake in southern California was only mag-nitude 6.7 yet killed 57 people and causedmore than $20 billion in damage.

Magnitude 6.7 or greater quakes can causedamage throughout the Bay region, but evensmaller quakes could be serious if centered inan urbanized area. WG99 found an 80%chance of one or more magnitude 6 to 6.6

Before the next quake, learn:• What to do during a quake,• What supplies to have on

hand, and• How to make sure your

home, office, and schoolsare safe.

Some good places to get pre-paredness information are thefront of telephone directories,libraries, the Red Cross, the Cali-fornia Governor’s Office of Emer-gency Services (510-286-0873),and http://quake.usgs.gov.

SIMPLE STEPS TO EARTHQUAKE PREPAREDNESSquakes occurring in theBay region before 2030.

WG99’s conclusionsfrom their 2-year effortare to appear in USGSCircular 1189, “Earth-quake Probabilities inthe San Francisco BayRegion: 2000 to 2030.”Their finding that amajor temblor is morelikely than not empha-sizes the ongoing needfor the Bay region toprepare for earth-quakes.

Large earthquakes in the San Francisco Bayregion can produce sudden and tremendousloss of life and property, threatening the socialand economic fabric of this region. Althoughquakes cannot be prevented, the damage theydo can be greatly reduced through prudentplanning and preparedness. Much preparationhas already been done, but because a largequake is likely and could happen at any mo-ment, further preparations should not be de-layed. WG99’s results will help business, gov-ernment, and the public make informeddecisions as they continue their preparations.

The work of USGS and other scientists inevaluating earthquake probabilities for the SanFrancisco Bay region is an ongoing part of theNational Earthquake Hazard ReductionProgram’s efforts. These efforts help to safe-guard lives and property from the earthquakesthat will inevitably strike in northern Californiaand elsewhere in the United States.

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