U.S. Department of the Interior

U.S. Geological Survey USGS Fact Sheet 2008-3027

2008

Forecasting California’s Earthquakes—What Can We Expect 

in the Next 30 Years?

I

n a new comprehensive study, 

scientists have determined that the 

chance of having one or more magnitude 6.7 or larger earthquakes in the 

California area over the next 30 years is 

greater than 99%. Such quakes can be 

deadly, as shown by the 1989 magnitude 

6.9 Loma Prieta and the 1994 magnitude 

6.7 Northridge earthquakes. The likelihood of at least one even more powerful 

quake of magnitude 7.5 or greater in the 

next 30 years is 46%—such a quake is 

most likely to occur in the southern half 

of the State. Building codes, earthquake 

insurance, and emergency planning 

will be affected by these new results, 

which highlight the urgency to prepare 

now for the powerful quakes that are 

inevitable in California’s future.

What Is an Earthquake 

Rupture Forecast?

Californians know that their State is 

subject to frequent—and sometimes very 

destructive—earthquakes. Accurate forecasts 

of the likelihood of quakes can help people 

prepare for these inevitable events. Because 

scientists cannot yet make precise predictions 

of the date, time, and place of future quakes, 

forecasts are in the form of the probabilities 

that quakes of certain sizes will occur during 

specified periods of time.

In our daily lives, we are used to making 

decisions based on probabilities—from weather 

forecasts (such as a 30% chance of rain) to 

the annual chance of being killed by lightning 

(about 0.0003%). Similarly, earthquake probabilities derived by scientists can help us plan 

and prepare for future quakes.

Earthquake forecasts for California have 

been developed in the past by multidisciplinary 

groups of scientists and engineers, each known 

as a “Working Group on California Earthquake 

Probabilities” (WGCEP 1988, 1990, 1995, 

2003). However, those forecasts were limited 

to particular regions of California. Because 

of this, WGCEP 2007 was commissioned to 

develop an updated, statewide forecast, the 

latest result of which is the Uniform California 

Earthquake Rupture Forecast, Version 2, or 

“UCERF” (U.S. Geological Survey (USGS) 

Open-File Report 2007-1437, http://pubs.usgs.

gov/of/2007/1437/). Organizations sponsoring 

WGCEP 2007 include the USGS, California 

Geological Survey, and the Southern California 

Earthquake Center. The comprehensive new 

forecast builds on previous studies and also 

incorporates abundant new data and improved 

scientific understanding of earthquakes.

When an earthquake occurs, two things 

happen—a fault ruptures (a crack in the Earth’s 

crust gives way and slips under tectonic pressure) and seismic waves, caused by this sudden 

fault motion, radiate out like ripples from a 

pebble tossed into a pond. The shaking that 

occurs as seismic waves pass by causes most 

quake damage. The strength of the waves 

depends partly on the quake’s magnitude, 

which is a function of the size of the fault that 

moves and the amount of slip.

The UCERF study’s goal was to determine 

probabilities for different parts of California 

of earthquake ruptures of various magnitudes, 

but not to estimate the likelihood of shaking 

(“seismic hazard”) that will be caused by these 

quakes. This distinction is important, because 

even areas in the State with a low probability of 

fault rupture can experience shaking and damage from distant, powerful quakes. 

CALIFORNIA AREA

EARTHQUAKE PROBABILITY

More than 99%

Regional 

30-year

earthquake 

probabilities

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0.01% 0.1% 1% 10%

30-Year Earthquake Probability

probability in the next 30 years for one 

or more magnitude 6.7 or greater quake 

capable of causing extensive damage 

and loss of life. The map shows the 

distribution throughout the State of the 

likelihood of having a nearby earthquake rupture (within 3 or 4 miles).

Boundary used in this study 

 between northern and 

 southern California

Cascadia Subduction Zone

Magnitude San Francisco 

region*

Los Angeles 

region

6.7 63% 67%

Magnitude Northern

 California**

Southern

California

6.7 93% 97%

7 68% 82%

7.5 15% 37%

8 2% 3%

*Probabilities from UCERF for the San Francisco region are nearly 

identical to the previous results from WGCEP 2003.

**These probabilities do not include the Cascadia Subduction Zone
How Did Scientists Make This Forecast?

Geology

FAULTS

Seismology

Geodesy

Global positioning system (GPS) 

observations by satellite document how fast 

various points in California are moving (arrows) in 

response to the steady motion of the Pacific and North 

American tectonic plates.

TECTONIC MOVEMENT

Geologic field mapping and aerial 

photos trace out California’s many faults 

and document the accumulated slip in earthquakes over thousands of years. Color spectrum 

shows rates of slip, from fast (purple and red) to very 

slow (dark blue).

Monitoring instruments provide a 

record of California earthquakes 

during recent historical times—where 

and when they occur and how strong they are

Seismograph

California sits on the boundary between two of the Earth’s major tectonic plates—the Pacific and North 

American Plates—which move inexorably past each other at a rate of about 2 inches per year. Much of this 

motion is accommodated from time to time by sudden slip on faults, producing earthquakes. Although the 

San Andreas Fault is the main locus of slip, hundreds, if not thousands, of other faults splay out from the 

plate boundary, spreading the threat of large earthquake ruptures through most of the State.

The new Uniform California Earthquake Rupture Forecast (UCERF) combines information from geodesy (precise data on the slow relative movement of 

the Earth’s tectonic plates), geology (mapped locations of faults and documented offsets on them), seismology (occurrence patterns of past earthquakes), and paleoseismology (data from trenches across faults documenting 

the dates and offsets of past earthquakes on them). The first three kinds of data 

are shown here as layers in the diagram. All four kinds of data are combined 

mathematically to produce the final probability values for future ruptures in the 

California area, in regions of the State, and on individual faults.

 Building on several previous studies and decades of data collection, UCERF 

was developed by a multidisciplinary group of scientists and engineers, known 

as the 2007 Working Group on California Earthquake Probabilities. Advice and 

comment was sought regularly from the broader community of earthquake scientists and engineers through open meetings and workshops. Where experts 

disagreed on aspects of the forecast, alternative options were accounted for in 

calculations to reflect these uncertainties. The final forecast is a sophisticated 

integration of scientific fact and expert opinion.

North American Plate

Pacific Plate
0.01% 0.1% 1% 10%

30-Year Earthquake Probability

The Composite 

Forecast—UCERF

e.

EARTHQUAKE

 EPICENTERS

Paleoseismology

By analyzing the evidence for dates 

and amounts of slip of past earthquakes in the walls of a trench dug 

across a fault, scientists can extend 

the fault’s earthquake record into 

prehistoric time.

M

Fault Plane Ruptured

Length

(miles)

Depth

(miles)

Average slip

(feet)

Area

(square miles)

5.0 1.8 1.8 0.5

5.5 3.1 3.1 0.8

6.0 5.6 5.6 1.5

6.5 13 7.5 2.7

7.0 42 7.5 4.8

7.5 133 7.5 8.5

8.0 420 7.5 15

The San Andreas Fault 

passes through the

Carrizo Plain

Trenching across the

Hayward Fault in Fremont

The final forecast results from 

evaluating and integrating several 

types of scientific data.

Earthquake Magnitudes and the Areas of Fault Rupture

The magnitude of an earthquake (M), which is a measure of 

the energy released in the quake, is dependent on the area 

of the fault plane that ruptures (length times depth) and the 

distance the fault slips during the quake. 

0 1,000 2,000 3,000 square miles
Printed on recycled paper

San Jacinto Fault

Elsinore Fault

Hayward-Rodgers

Creek Fault

Calaveras Fault

Garlock Fault

Southern San Andreas Fault

Northern San Andreas Fault

Equal

Current Earthquake Probabilities

Relative to Long-Term Probabilities

1.6 times

greater

1.6 times

smaller

How Likely is a Damaging 

Quake in the Next 30 Years?

California straddles the boundary between 

two of the Earth’s tectonic plates—as a result, 

it is broken by numerous earthquake faults. 

Taking into account the earthquake histories and 

relative rates of motion on these many faults, 

the UCERF study concludes that there is a 

probability of more than 99% that in the next 30 

years Californians will experience one or more 

magnitude 6.7 or greater quakes, potentially 

capable of causing extensive damage and loss 

of life. For powerful quakes of magnitude 7.5 

or greater, there is a 46% chance of one or more 

in the next 30 years—such a quake is twice as 

likely to occur (37%) in the southern half of the 

State than in the northern half (15%).

Smaller magnitude earthquakes are more 

frequent than larger quakes. According to the 

new forecast, about 3 magnitude 5 or greater 

quakes will occur in the California region per 

year, and a magnitude 6 or greater quake about 

every 1.5 years. These numbers do not include 

aftershocks that follow larger quakes—including them would roughly double the expected 

number of magnitude 5 or greater quakes.

The UCERF forecast was evaluated by an 

independent scientific review panel, as well as 

by both the California and National Earthquake Prediction Evaluation Councils, making 

it one of the most extensively reviewed earthquake forecasts ever produced. Uncertainties 

remain because the new quake probabilities 

are the result of evaluating and accommodating several earthquake theories. As scientific 

understanding of quakes improves, the probabilities will change.

The results of the UCERF study are a 

reminder that all Californians live in earthquake 

country and should therefore be prepared (see 

Putting Down Roots in Earthquake Country at 

http://www.earthquakecountry.info/roots/). The 

Which Faults Currently Have Elevated Probabilities?

USGS has already used the UCERF to estimate 

California’s seismic hazard, which in turn 

will be used to update building codes. Other 

subsequent studies will add information on the 

vulnerability of manmade structures to estimate 

expected losses (“seismic risk”). In these ways, 

UCERF will help to increase public safety and 

community resilience to earthquake hazards.

Earthquakes cannot be prevented, but the 

damage they do can be greatly reduced through 

prudent planning and preparedness. The ongoing work of USGS, California Geological 

Survey, Southern California Earthquake Center, 

and other scientists in evaluating quake probabilities 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 California and elsewhere 

in our Nation.

Edward H. Field, Kevin R. Milner, 

and the 2007 Working Group on 

California Earthquake Probabilities

Edited by Peter H. Stauffer and James W. Hendley II 

Layout by David R. Jones

COOPERATING ORGANIZATIONS

Southern California Earthquake Center (SCEC)

 California Geological Survey (CGS)

 California Earthquake Authority

For more information contact:

Earthquake Information Hotline (650) 329-4085

U.S. Geological Survey, Mail Stop 977

345 Middlefield Road, Menlo Park, CA 94025

http://earthquake.usgs.gov/

http://www.scec.org

This Fact Sheet and any updates to it are available 

online at http://pubs.usgs.gov/fs/2008/3027/

The new California earthquake forecast incorporates the concept that earthquake probabilities change with time. For example, a large quake may be less likely in the near future on a fault 

that has recently had one—but more likely on a fault on which 

the tectonic stresses have had much time to build back up. The 

forecast also incorporates adjustments for areas that have 

recently shown a change in the level of earthquake activity. 

Faults that have elevated current probabilities include the southern San Andreas and Hayward-Rodgers Creek Faults, though 

major quakes on these faults may still be decades away.

Magnitude

30-year probability of 

one or more events 

greater than or equal 

to the magnitude

Average 

repeat 

time 

(years)

6.7 >99% 5

7 94% 11

7.5 46% 48

8 4% 650

*Not including Cascadia Subduction Zone

STATEWIDE EARTHQUAKE PROBABILITIES

The numbers represent current best estimates. 

As earthquake science progresses, these probabilities will change. Actual repeat times vary 

considerably and only rarely will be exactly as 

listed in the table. 

For the entire California region, the fault 

with the highest probability of generating at 

least one magnitude 6.7 or larger earthquake is 

the southern San Andreas (59% in the next 30 

years). For northern California, the most likely 

source of such a quake is the Hayward-Rodgers 

Creek Fault (31% in next 30 years)—see 

USGS Fact Sheet 2008-3019. Quake probabilities for many parts of the State are similar to 

those in previous studies, but the new probabilities for the Elsinore and San Jacinto Faults in 

southern California are about half those previously determined. For the far northwestern part 

of the State, a major source of quakes is the 

offshore 750-mile-long “Cascadia Subduction 

Zone,” which extends south about 150 miles 

into California. For the next 30 years there is 

a 10% probability of a magnitude 8 to 9 quake 

somewhere along the zone—such quakes occur 

about every 500 years.

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INDIVIDUAL FAULT PROBABILITIES

The UCERF report assigns individual probabilities to specific known major faults. Below are 

30-year probabilities for seven of the faults for 

which scientists have the most data. Many other 

faults also have significant probabilities—in fact, 

the next big quake in California is just as likely to 

occur on one of the other faults in the State. 

Fault

Probability of one or 

more magnitude 6.7 

or greater quake

Southern San Andreas 59%

Hayward-Rodgers Creek 31%

San Jacinto 31%

Northern San Andreas 21%

Elsinore 11%

Calaveras 7%

Garlock 6%

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