U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 039–03

2003

Is a Powerful Quake Likely to Strike in the Next 30 Years?

UNDERSTANDING EARTHQUAKE HAZARDS IN THE SAN FRANCISCO BAY REGION

Using newly collected data and 

evolving theories of earthquake 

occurrence, U.S. Geological Survey 

(USGS) and other scientists now conclude that there is a 62% probability of at 

least one magnitude 6.7 or greater quake, 

capable of causing widespread damage, 

striking somewhere in the San Francisco 

Bay region before 2032. A major quake 

can occur in any part of this densely 

populated region. Therefore, there is an 

ongoing need for all communities in the 

Bay region to continue preparing for the 

quakes that will strike in the future.

Since the great earthquake of 1906, 

no major quake has been centered near a 

densely urbanized part of the San Francisco 

Bay region. Although the 1989 magnitude 

6.9 Loma Prieta quake killed more than 40 

people in the region’s urban core, it was 

centered in mountainous country 50 miles 

south of San Francisco. In 1995, when a 

quake of the same magnitude struck Kobe, 

Japan, another bayside urban area thought 

to be well prepared for earthquakes, more 

than 6,000 people died and the damage 

amounted to $100 billion. Had the Loma 

Prieta quake been centered in San Jose, 

Oakland, or San Francisco, similar losses 

could have occurred.

Damaging earthquakes are inevitable in 

the Bay region, but taking actions based on 

the likelihood of future quakes will help save 

lives and protect property. Following the 

Loma Prieta quake, the U.S. Geological Survey’s (USGS) Working Group on California 

Earthquake Probabilities reassessed the likelihood of large quakes striking the Bay region 

and issued a report in 1990.

Since then, scientists have gained new insights into Bay region earthquakes, providing 

a better basis for determining future quake 

probabilities. The USGS working group was 

expanded to include about 100 scientists from 

Federal and State of California agencies, 

consulting firms, industry, and universities. 

In 1999, results from this expanded working 

group were published in USGS Fact Sheet 

152-99 and USGS Open-File Report 99-

517. The efforts of this continuing working 

group, now called WG02, have produced a 

new assessment of Bay region earthquake 

probabilities that is published in this Fact 

Sheet and more fully in a USGS Open-File 

Report. WG02 determined that there is a 

62% chance of at least one magnitude 6.7 or 

greater earthquake striking the San Francisco 

Bay region between 2003 and 2032.

The population of this region is projected 

to exceed 8.2 million people by 2025—an 

increase of more than 1.4 million from the 

2000 census level. Nearly all of this growth 

is expected to occur in the northern and eastern parts of the region. Solano, Napa, and 

Sonoma Counties are anticipated to show 

the highest growth, adding more than 30% 

to their populations. Contra Costa, Alameda, 

and Santa Clara Counties are projected to 

see a 15 to 20% increase in population. 

The threat of earthquakes extends across the entire San Francisco Bay region, and a major quake is likely before 2032. Knowing this will help people make informed decisions as they continue to prepare for future quakes.
This eastward and northward growth of 

the region will occur in areas of significant 

seismic hazard, a fact not fully appreciated 

until recently. The cumulative 30-year probability of an earthquake of magnitude 6.7 or 

greater occurring somewhere within these 

rapidly growing northern and eastern areas 

alone is nearly 50%.

Residents living near the Pacific coast in 

San Mateo, Santa Cruz, and Monterey Counties are sandwiched between the San Andreas 

and San Gregorio Faults. These two faults 

have a combined 34% chance of producing 

one or more magnitude 6.7 or greater quakes 

in these coastal areas before 2032.

When the 1990 USGS probability report 

was released, earthquake likelihood could be 

estimated only for the San Andreas Fault and 

the Hayward-Rodgers Creek Fault, although 

the danger posed by other faults was recognized. WG02 found that, of all the faults in 

the Bay region, these two and the Calaveras 

Fault still pose the greatest threat, because 

they have high quake odds and run through 

the region’s urban core.

Since 1990, new data have allowed five 

additional faults to be included in the probability calculations. Furthermore, there is 

now a focus on events of magnitude 6.7 and 

greater rather than 7 and greater, because 

the 1994 Northridge quake in southern California was only magnitude 6.7 yet killed 57 

people and caused more than $20 billion in 

damage. Including more faults and smaller 

events would be expected to increase the estimated regional probability of major quakes. 

This expected increase was more than compensated for, however, by two effects not included in the 1990 report: (1) gradual slip on 

faults in the absence of earthquakes and (2) 

the effect of the 1906 earthquake in reducing 

quake activity throughout the region.

The rate of large earthquakes in the Bay 

region dropped abruptly after the 1906 earthquake and has only slightly increased during 

the last few decades. How long it will take to 

again reach the level of activity seen in the 

late 1800’s is a question that WG02 carefully 

evaluated. The new average regional quake 

probability of 62% is a result of considering 

several earthquake theories, which produce 

probabilities from 37 to 87%. As earthquake 

science progresses, this and other uncertainties should diminish and the average probability may change.

Any magnitude 6.7 or greater quake can 

cause damage throughout the Bay region, 

but even a smaller quake could have serious effects if centered in an urbanized area. 

WG02 found at least an 80% chance of one 

or more magnitude 6 to 6.6 quakes occurring 

in the Bay region before 2032.

Earthquake probabilities are only one component in the evaluation of earthquake hazard 

and risk. Local soil conditions, bedrock type, 

quality of construction, susceptibility to 

earthquake-induced ground failures or flooding (caused by dam or levee failure) can all 

profoundly affect the possibility of damage 

at a particular site in an earthquake. This was 

dramatically demonstrated by the 1989 Loma 

Prieta earthquake, which devastated vulnerable parts of Oakland and San Francisco more 

than 50 miles from the fault rupture.

WG02 has examined possible earthquake 

scenarios on all of the major Bay region 

faults and assigned probabilities to the occurrence of these events over the next 30 

years. A next step in hazard assessment is 

the estimation of intensity of ground shaking from each such scenario earthquake, 

taking into account the location, length, and 

magnitude of the anticipated rupture and the 

ground conditions (bedrock and soil types) 

at various locations around the Bay region. 

For example, a shock on the southern Hayward Fault, which last ruptured in 1868 in an 

earthquake of magnitude about 6.9, would 

produce damaging shaking over a wide area.

The estimated distribution of shaking intensities in the Bay region from a scenario event, 

combined with the probability of that event 

happening in a given time period, yields an 

estimated frequency of occurrence of that disThe San Francisco Bay region lies 

on the boundary zone between two 

of the tectonic plates that make 

up the Earth’s outer shell. The 

relentless motion of these plates 

builds up strain that will eventually 

be released in earthquakes on the 

region’s many faults. Inset shows 

the northwestward movement of the 

Farallon Islands relative to Columbia 

in eastern California, as documented 

by Global Positioning System (GPS) 

data from those two sites. The 

lengths of fault that slipped in the 

MOTION OF FARALLON ISLANDS IN N36°W DIRECTION RELATIVE

TO COLUMBIA, CALIFORNIA

TIME (YEARS)

Velocity = 40 ± 1.5 mm/year

(1.6 inches/year)

INCHES

MILLIMETERS

5.0

4.0

3.0

2.0

1.0

2004 2003 2002 2001 2000

140

120

100

80

60

40

20

0

1868 Hayward 

and 1989 

Loma Prieta 

magnitude 6.9 

earthquakes are 

shown in yellow.

AREA

OF MAP

CALIF

This “ShakeMap” shows the shaking 

intensities expected for a hypothetical scenario earthquake (magnitude 

6.7) on the southern Hayward Fault, 

which last ruptured in 1868 in an 

event of magnitude about 6.9. Note 

that damaging shaking would occur 

over a wide tract and be especially 

severe around the margins of the 

Bay in areas underlain by bay mud. 

The 1868 shock destroyed a number 

of buildings in San Francisco. The 

photo below shows one wrecked 

building in what is now San 

Francisco’s Financial District, mostly 

built on bay mud and artifi cial fi ll. 

(Photograph courtesy of the Karl V. 

Steinbrugge Collection, Earthquake 

Engineering Research Center, University of California, Berkeley.)
EARTHQUAKE ODDS—A BALANCING ACT

Quake probabilities for the San Francisco 

Bay region are derived by balancing two 

processes—(1) the motions of the plates that 

make up the Earth’s outer shell and (2) the 

slip on faults, which occurs primarily during 

earthquakes. The continual northwestward 

motion of the Pacific Plate past the North 

American Plate loads strain onto the network 

of active faults that slice through the region. 

Earthquakes sporadically release and redistribute this strain.

One side of the balance is the rate at which 

plate motions load strain onto faults. Development of the Global Positioning System (GPS) 

has allowed geophysicists to make accurate 

measurements of how the current plate motions—totaling 1.6 inches per year across the 

entire Bay region—distribute strain onto individual faults. Geologic studies also contribute 

to this understanding by documenting longterm fault slip, which must match the strainloading rate. For example, on the San Gregorio Fault near Moss Beach, a buried stream 

channel has been offset about 1,000 feet over 

the past 80,000 years, giving a local slip rate 

of about one-sixth of an inch per year.

On the other side of the balance is slip on 

faults, primarily during earthquakes. Data 

from seismograms, historical accounts, and 

trenches that reveal buried fault ruptures 

provide information about past quakes in 

the San Francisco Bay region. U.S. Geological Survey and other scientists reanalyzed 

seismograms and historical accounts and 

excavated many new trenches across faults 

as part of recent reassessments of earthquake 

hazards in the region. For example, a trench 

in El Cerrito revealed evidence of four to 

seven large quakes on the Hayward Fault 

during the past 2,200 years. Unfortunately, 

seismogram records only go back to about 

1900, historical accounts in this region are 

fragmentary before 1850, and trenches are 

effective only in some locations.

Supplementing these data with up-to-date 

knowledge about how faults work, scientists 

are able to make better projections of the expected sizes and locations of future quakes. 

If Earth scientists can identify the lengths of 

fault segments that may rupture in an earthquake, they can estimate the magnitudes and 

amounts of slip for possible future quakes. 

1906 that the strain was reduced on most 

faults throughout the region. Because plate 

motions are continuous, strain has been slowly building up again, and strong earthquakes 

began to reoccur in the 1980’s. However, the 

level of seismic activity in the Bay region has 

not yet reached that of the late 1800’s. Some 

earthquake models forecast that this relatively low level of activity should continue for 

some time and that earthquake probabilities 

therefore will be lower than average. Other 

models, in which earthquakes on different 

faults recur on somewhat regular schedules, 

yield higher than average probabilities for the 

next few decades.

At some point, the rate of earthquakes in 

the Bay region must increase so that the balance will once again even out. Earth scientists 

do not agree on how soon this may happen, 

because they do not yet sufficiently understand the process by which strain reductions 

are imposed by large earthquakes and are 

removed by plate motions and the slow deformation of the Earth. We know what is on the 

two sides of the balance, but do not know all 

the details of how the balance really works.

Given the uncertainty in how to model 

future earthquake behavior, determining 

earthquake probabilities involves making many decisions, such as defining fault 

segments and choosing among alternative 

statistical models. Because such decisions 

are uncertain, recent reassessments of Bay 

region earthquake probabilities have assigned weights to the various choices so that 

all were included in the overall calculations. 

These weights led to an average result of 

62%, but this value could be as low as 37% 

or as high as 87%, given the uncertainties in 

these choices. Regardless, the San Francisco 

Bay region has substantial earthquake hazard.

For example, the magnitude 7.8 San Francisco earthquake of 1906 ruptured 300 miles of 

the San Andreas Fault and produced as much 

as 25 feet of slip, whereas the magnitude 6.9 

Loma Prieta quake in 1989 ruptured only 25 

miles of fault and produced only about 6 feet 

of slip. Scientists identify fault segments by 

studying bends, intersections, and gaps in 

faults, past earthquake ruptures, and major 

changes in rock types along faults.

Reading the Balance

Over millennia the balance between plate 

motions and slip on faults must even out, and 

therefore average long-term earthquake probabilities can be reliably calculated. However, 

calculating accurate odds for a short time 

period such as 30 years is more difficult, and 

alternative models lead to different results. 

For example, the rate of large quakes in the 

Bay region was high in the late 1800’s but 

dropped abruptly after the 1906 San Francisco earthquake. Scientists believe that this 

rate dropped because the San Andreas Fault 

slipped so much over such a great length in 

The rate of large earthquakes in the San Francisco Bay region dropped abruptly after the Great 1906 

Earthquake. The San Andreas Fault slipped so much over such a great length in that quake that the strain 

was reduced on most faults throughout the region. Strain has been slowly building up again. However, the 

level of seismic activity has not yet reached that of the late 1800’s.

Quake probabilities are derived by balancing two 

processes—(1) the continual motions of the 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 primarily during earthquakes 

(equivalent to adding balls to the right tray). The slip 

on faults over time must balance the strain built up 

by the plate motions. The total amount of slip during 

an earthquake, shown here by the proportional 

volumes of the spheres, depends on its magnitude 

(M). The larger the quake, the more strain released.

Knowledge of past earthquakes is essential 

for estimating the odds of future temblors. This 

knowledge comes from historical damage accounts, fault ruptures exposed in trenches, and 

seismographic records.
Printed on recycled paper

Andrew J. Michael, Stephanie L. Ross, Robert W. Simpson, 

Mary Lou Zoback, David P. Schwartz, Michael L. Blanpied,

and Working Group 2002

Edited by

Peter H. Stauffer and James W. Hendley II

Graphic design by

Susan Mayfi eld and Sara Boore 

COOPERATING ORGANIZATIONS

Association of Bay Area Governments

California Geological Survey

California Governor’s Offi ce of Emergency Services

Federal Emergency Management Agency

Lawrence Livermore National Laboratory

Geomatrix Consultants Inc.

Pacifi c Gas and Electric Company

University of California at Berkeley

William Lettis & Associates

Many other institutions, organizations, and fi rms

For more information contact:

Earthquake information Hotline (650) 329-4085

U.S. Geological Survey, Mail Stop 977

345 Middlefi eld Road, Menlo Park, CA 94025

http://earthquake.usgs.gov

See also The USGS Earthquake Hazards Program in NEHRP—Investing in a Safer Future (USGS Fact Sheet 017-03), “ShakeMaps”—Instant Maps of Earthquake Shaking (USGS Fact Sheet 

103-00), When Will the Next Great Quake Strike Northern California? (USGS Fact Sheet 094-96), and Hazard Maps Help Save 

Lives and Property (USGS Fact Sheet 183-96)

This fact sheet and any updates to it are available online at:

http://geopubs.wr.usgs.gov/fact-sheet/fs039-03

SIMPLE STEPS TO EARTHQUAKE PREPAREDNESS

Although quakes cannot be prevented, the 

damage they do can be greatly reduced 

through prudent planning and preparedness. 

Much preparation has already been done, 

but because a large quake is likely and could 

happen at any moment, further preparations 

should not be delayed. 

WG02’s reassessment of earthquake probabilities in the Bay region reaffirms that the 

earthquake hazard is both high and widely 

spread. This reassessment will help business, 

government, and the public make informed 

choices as they continue their earthquake 

preparations. Using the new knowledge on 

the level of earthquake hazard and the degree 

of exposure and vulnerability in different 

parts of the Bay region, USGS and other 

scientists are working with decisionmakers 

to help plan and prioritize mitigation efforts 

and strategies. The ongoing work of USGS 

and other scientists in evaluating earthquake 

probabilities for the San Francisco Bay region is part of the National Earthquake Hazard Reduction Program’s efforts to safeguard 

lives and property from the future quakes 

that are certain to strike in northern California and elsewhere in the United States.

The final layer in the assessment of risk 

involves combining the level of hazard with 

the quantity and vulnerability of structures 

exposed to the hazard. The earthquake hazard has been shown to extend throughout 

the entire Bay region, but the local risk and 

potential loss depend on the exposure. A 

densely built urban area with an expectation 

of high shaking intensity is clearly at greater 

risk than an agricultural area with the same 

expected level of shaking. An analysis of 

potential earthquake losses in the Bay region 

conducted by the USGS and the California 

Geological Survey indicates that areas along 

the Hayward Fault have the highest likely 

proportional loss of any parts of the region. 

This high potential loss is the result of dense 

development directly along and adjacent to 

the Hayward Fault and the fact that earthquakes on the Hayward, though smaller than 

those on the San Andreas Fault, occur more 

frequently.

Large earthquakes in the San Francisco 

Bay region can produce sudden and tremendous loss of life and property, threatening 

the region’s social and economic fabric. 

tribution of shaking intensities. By using such 

information from all anticipated scenario ruptures in the region, an overall probability can 

be calculated that a location will experience at 

least a given level of shaking within a specific 

time period, say the expected lifetime of a 

building—a probability of considerable interest 

to building designers and structural engineers.

P a c i f i c O c e a

n

Monterey

Bay

San Francisco Bay

Nov

YOLO

NAPA

SOLANO

CONTRA COSTA

ALAMEDA

STANISLAUS

MERCED

SAN JOAQUIN

SONOMA

SAN

MATEO

SANTA CLARA

SANTA CRUZ

MONTEREY

SAN BENITO

LAKE

MARIN

S F

SACRAMENTO

I II III IV V VI

MODIFIED MERCALLI INTENSITY

VII VIII IX X

San Francisco

Half Moon Bay

Oakland

Modesto

Stockton

Sacramento

Antioch

Menlo

Park

San

Mateo

Walnut

Creek

Livermore

Hayward

Fremont

Tracy

Santa Cruz

Gilroy

Monterey Salinas

San Jose

Santa

Rosa

Petaluma

Novato

San

Rafael

Napa

Vallejo

Region considered in

WG02 assessment

SAN ANDREAS FAULT SAN

ANDREAS FAULT

RODGERS CREEK FAULT

HAYWARD FAULT

GREENVILLE

CALAVERAS FAULT

FAULT CON

CORD–GREEN VALLEY MT. DIABLO

THRUST FAULT

SAN GREGORIO FAULT

San Jose

ALAMEDA Counties

Cities and towns

Faults

EXPLANATION

0

0 30 MILES

30 KILOMETERS

N

This map, derived by combining the predicted 

frequencies and intensities of shaking from all likely 

earthquake scenarios in the San Francisco Bay 

region, shows levels of shaking that have a 50% 

chance of being exceeded over the next 30 years. 

Most populated areas of the region have even odds 

of experiencing shaking of intensity VII or greater on 

the Modified Mercalli Intensity (MMI) scale, suffi -

cient to cause moderate damage. Areas on soft soil, 

such as on the edges of the Bay, have even odds of 

experiencing MMI intensity VIII or greater, capable 

of causing serious structural damage.

SIMPLE STEPS TO EARTHQUAKE PREPAREDNESS

SHAKING INTENSITIES WITH 50% CHANCE OF BEING EXCEEDED IN THE NEXT 30 YEARS

Some good places to get preparedness information are the 

front of telephone directories, 

libraries, Red Cross (http://

www.bayarea-redcross.org/),

California Governor’s Offi ce of

Emergency Services (510-286-

0873; http://www.oes.ca.gov/), 

Association of Bay Area Governments (http//www.abag.ca.gov/), 

and U.S Geological Survey (http:

//earthquake.usgs.gov).

•What to do during a 

quake,

•What supplies to have on 

hand, and

•How to make sure your 

home, offi ce, and 

schools are safe.

Before the next quake, learn: