1/1000 1/100 1/10 1 10 100

30-year M≥6.7 likelihood 

(percent)

San Francisco

region

Los Angeles

region

State

boundary

UCERF3: A New Earthquake Forecast for California’s Complex Fault System

With innovations, fresh data, and lessons learned from recent 

earthquakes, scientists have developed a new earthquake forecast 

model for California, a region under constant threat from potentially damaging events. The new model, referred to as the third Uniform California 

Earthquake Rupture Forecast, or “UCERF3” (http://www.WGCEP.org/

UCERF3), provides authoritative estimates of the magnitude, location, 

and likelihood of earthquake fault rupture throughout the state. Overall 

the results confirm previous findings, but with some significant changes 

because of model improvements. For example, compared to the previous 

forecast (UCERF2), the likelihood of moderate-sized earthquakes (magnitude 6.5 to 7.5) is lower, whereas that of larger events is higher. This is 

because of the inclusion of multifault ruptures, where earthquakes are 

no longer confined to separate, individual faults, but can occasionally 

rupture multiple faults simultaneously. The public-safety implications of 

this and other model improvements depend on several factors, including site location and type of structure (for example, family dwelling 

compared to a long-span bridge). Building codes, earthquake insurance 

products, emergency plans, and other risk-mitigation efforts will be 

updated accordingly. This model also serves as a reminder that damaging earthquakes are inevitable for California. Fortunately, there are many 

simple steps residents can take to protect lives and property.

Uniform California 

Earthquake Rupture 

Forecast (Version 3) 

(UCERF3)

Figure 1. Three-dimensional perspective view of the likelihood that each region of California will experience a 

magnitude 6.7 or larger (M≥6.7) earthquake in the 

next 30 years (6.7 matches the magnitude of 

the 1994 Northridge earthquake, and 

30 years is the typical duration 

of a homeowner mortgage). 

What is UCERF3?

California is sandwiched between the Pacific and North 

American tectonic plates, with the former migrating northwest 

about two inches per year compared to the latter. The plate boundary is far from smooth, reflecting more of a fragmented zone 

locked in a tectonic battle over which areas will give way, producing some of the steepest mountain ranges in the world. The sliding 

between plates is also not steady, but rather plays out in fits and 

starts with periods of rest interrupted by sudden slip along cracks in 

the Earth. These “fault ruptures” in turn cause the ground to shake, 

much like the ripples that radiate from a pebble tossed in a pond, 

and it is this shaking that causes the most damage in earthquakes.

Two kinds of scientific models are used to help safeguard 

against earthquake losses: an Earthquake Rupture Forecast, which 

tells us where and when the Earth might slip along the state’s many 

faults, and a Ground Motion Prediction model, which estimates 

the subsequent shaking given one of the fault ruptures. UCERF3 is 

the first type of model, representing the latest earthquake-rupture 

forecast for California. It was developed and reviewed by dozens 

of leading scientific experts from the fields of seismology, geology, 

geodesy, paleoseismology, earthquake physics, and earthquake 

engineering. As such, it represents the best available science with 

respect to authoritative estimates of the magnitude, location, and 

likelihood of potentially damaging earthquakes throughout the 

state (further background on these models, especially with respect 

to ingredients, can be found in U.S. Geological Survey Fact 

Sheet 2008–3027, http://pubs.usgs.gov/fs/2008/3027/).

Faults are shown by the rectangles outlined in black. The entire colored area represents greater 

California, and the white line across the middle defines northern versus southern California. Results 

do not include earthquakes on the Cascadia Subduction Zone, a 750-mile offshore fault that extends 

about 150 miles into California from Oregon and Washington to the north.

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2015–3009

March 2015

ISSN 2327-6916 (print) ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20153009
Figure 2. Changes with time of the inventory of faults used in California 

earthquake forecast models (WGCEP, Working Group on California 

Earthquake Probabilities).

Why a New Earthquake Forecast Model?

All scientific models, including earthquake rupture forecasts, are an approximation of the physical system they represent, in the same way that “the map is not the actual territory” 

(Korzbski, 1931). UCERF3 represents the latest model from 

the Working Group on California Earthquake Probabilities 

(WGCEP) (WGCEP, 2014), which also released forecasts in 

1988, 1990, 1995, 2003, and 2007. This historical progression 

of models reflects increasingly accurate, detailed, and sophisticated representations of a particularly complex natural system.

A puzzling feature of previous models has been a forecasted 

rate of moderate-sized earthquakes (between magnitude 6.5 

and 7.0) that is up to a factor of two higher than that observed 

historically. The first discovery of this discrepancy, by the 

1995 WGCEP, was particularly disturbing in that one such 

event, the magnitude 6.7 1994 Northridge earthquake, had 

just surprised many as the costliest earthquake in U.S. history. 

In fact, the prospect of such events becoming more frequent 

contributed to an ensuing homeowner-insurance-availability 

crisis, as most insurance providers opted to pull out of the 

market altogether, rather than comply with a state law requiring 

they offer an earthquake option with each policy. This insurance availability crisis was ultimately solved in 1996 with the 

legislative creation of the California Earthquake Authority 

(http://www.earthquakeauthority.com), which has since become 

the largest earthquake insurance provider in the state. However, 

the discrepancy between the forecast rate and the observed 

rate at moderate magnitudes has remained through the most 

recent previous study (WGCEP, 2007), and scientists have hotly 

debated whether this is real or a result of some model limitation.

Recent earthquakes have fortunately provided clues. For 

example, the Northridge earthquake occurred on a previously 

unrecognized fault, which motivated scientists to search for 

other faults and quantify those that might be capable of producing damaging earthquakes. The effort has paid off. Whereas 

the 1988 WGCEP considered only 16 different faults, albeit the 

main ones, by the time of the WGCEP 2007 effort there were 

about 200. With UCERF3, there are now more than 350 fault 

sections in the model, thanks in part to using space-based geodesy where geologic data are limited. This historical progression 

is shown in the fault model evolution figure at left.

Another clue with respect to the moderate-magnitude rate 

discrepancy is that many recent earthquakes have plowed past 

previously inferred fault-rupture boundaries. That is, past models have generally assumed that earthquakes are either confined 

to separate faults, or that long faults like the San Andreas can 

be divided into different segments that only rupture separately. 

However, all three of the most-recent, largest earthquakes in 

California ruptured right past such boundaries, jumping from 

one fault to another as multifault ruptures. These were the 1992 

magnitude 7.3 Landers, the 1999 magnitude 7.2 Hector Mine, 

and the 2010 magnitude 7.2 El Mayor–Cucapah earthquakes. 

The 2011 magnitude 9.0 Tohoku, Japan earthquake also violated previously defined fault-segment boundaries, resulting in 

a much larger fault-rupture area and magnitude than expected, 

and contributing to the deadly tsunami and Fukushima 

nuclear disaster.

Given these observations, the possibility of multifault ruptures clearly needed to be considered in our new model. In fact, 

as the inventory of California faults has grown over the years, it 

Slip rate (mm/year)

0 10 20 30 40

WGCEP 1988

WGCEP 2007

WGCEP 2014

Fault Model Evolution
Readiness of Faults

(probability gain for M≥6.7 earthquakes)

Very low Equal 2 times greater

Current earthquake likelihood relative to long-term likelihood

Recent earthquakes (less ready):

 

1983 Coalinga

1952 Kern County

1992 Landers

1999 Hector Mine

1906 San Francisco

Particularly ready faults:

Hayward

Calaveras

Southern San Andreas

has become increasingly apparent that we 

are not dealing with a few well-separate 

faults, but with a vast interconnected fault 

system. In fact, it has become difficult to 

identify where some faults end and others 

begin, implying many more opportunities 

for multifault ruptures. As a consequence, 

UCERF3 now considers more than 

250,000 different fault-based earthquakes, 

including multifault ruptures, whereas 

UCERF2 had about 10,000, and previous 

models had far fewer. Because we still lack 

a complete inventory of faults, UCERF3 

(and UCERF2 before it) also includes the 

possibility of earthquakes on unrecognized 

faults elsewhere in the region.

Solving for the rate of all possible 

ruptures in the interconnected fault 

system represented a significant challenge. The UCERF3 methodological 

breakthrough, referred to as the “grand 

inversion,” allowed us to not only solve 

for the rate of each earthquake rupture, 

but to also draw upon a broader range 

of observations in doing so. For example, 

the previous rate discrepancy at moderate-magnitudes was turned into part of 

the solution. That is, because the total 

plate-tectonic deformation is generally 

well known, any increase in the rate of 

larger, multifault ruptures must come 

with a consequent reduction in rates at 

lower magnitudes. The grand inversion 

manages the overall plate-tectonic, faultsystem budget mathematically, adding 

whatever multifault ruptures are needed 

to eliminate the rate discrepancy at 

moderate magnitudes. So, not only does 

UCERF3 include the types of multifault 

ruptures seen in nature, but doing so 

has also eliminated the overprediction 

of moderate-sized events, implying the 

latter was simply a manifestation of the 

isolation and segmentation of faults in the 

previous models.

UCERF3 also includes the notion 

of fault “readiness,” where earthquake 

likelihoods go down on faults that have 

recently ruptured, and build back up with 

time as tectonic stresses reaccumulate. 

Although this concept, known formally as 

Reid’s elastic rebound theory (Reid, 1911), 

has been around for more than a century, 

applying it in a model that includes multifault ruptures also proved challenging. A 

new methodology was therefore developed, which also relaxes the requirement 

that the date-of-last event be known where 

applied. That is, we may not know when 

the most recent event occurred on many 

California faults, but we do know that it 

had to have been prior to 1875 (the year 

when reliable recordkeeping began). Being 

able to account for this “historic open interval” for events that precede 1875 allowed 

us to quantify fault readiness throughout 

the entire fault system (fig. 3), rather than 

being limited to only a subset of faults as 

in previous studies.

There are many uncertainties in both 

the data and scientific theories that go into 

UCERF3, and alternative values for each 

element can lead to a different forecast. 

Consequently, UCERF3 is not a single 

model, but rather a collection of 5,760 different viable models. The results presented in 

the next section represent an average of these 

forecasts. Calculating grand-inversion results 

for all the models required the use of super 

computers, as they would have taken more 

than 8 years on a single desktop computer. 

What Are the Results, and 

How Do They Differ from 

Previous Estimates?

UCERF3 results for various regions 

and faults of interest are shown in the 

figures and tables here. How have expected 

earthquake rates changed from the previous 

model? Overall, the results confirm earlier 

findings (California is earthquake country), 

but with some important refinements in 

certain areas. Considering the entire region, 

the average time between magnitude 6.7 

and larger earthquakes has gone from 1 

every 4.8 years in UCERF2, to 1 about 

every 6.3 years in UCERF3, representing a 

30 percent decrease in the new forecasted 

Figure 3. California earthquake likelihood in UCERF3 

incorporates the concept that earthquake probabilities 

change with time according to elastic-rebound theory. 

Faults are less likely to rupture (less ready) when and 

where there has been a recent earthquake, and are 

more likely to rupture (more ready) where tectonic forces 

have built up during many years without an earthquake 

(although the event may still be several decades away) 

(M≥6.7, magnitude 6.7 or larger).
Greater California region

Magnitude

(greater than 

or equal to)

Average

repeat time

(years)

30-year

likelihood of

one or more 

events

Readiness

5 0.12 (0.7) 100% (1.0) 1.0

6 1.2 (0.9) 100% (1.0) 1.0

6.7 6.3 (1.3) >99% (1.0) 1.0

7 13 (1.3) 93% (1.0) 1.0

7.5 52 (1.0) 48% (1.0) 1.1

8 494 (0.8) 7% (1.5) 1.2

Northern California region

Magnitude

(greater than 

or equal to)

Average

repeat time

(years)

30-year

likelihood of

one or more 

events

Readiness

5 0.24 (0.7) 100% (1.0) 1.0

6 2.4 (0.9) 100% (1.0) 1.0

6.7 12 (1.2) 95% (1.0) 1.0

7 25 (1.2) 76% (1.0) 1.1

7.5 92 (0.9) 28% (1.1) 1.0

8 645 (0.8) 5% (1.4) 1.1

Southern California region

Magnitude

(greater than 

or equal to)

Average

repeat time

(years)

30-year

likelihood of 

one or more 

events

Readiness

5 0.24 (0.7) 100% (1.0) 1.0

6 2.3 (0.9) 100% (1.0) 1.0

6.7 12 (1.5) 93% (1.0) 1.0

7 25 (1.4) 75% (0.9) 1.1

7.5 87 (1.2) 36% (0.9) 1.2

8 522 (0.4) 7% (2.5) 1.3

San Francisco region

Magnitude

(greater than 

or equal to)

Average

repeat time

(years)

30-year

likelihood of

one or more 

events

Readiness

5 1.3 (0.7) 100% (1.0) 1.0

6 8.9 (1.0) 98% (1.0) 1.0

6.7 29 (1.1) 72% (1.1) 1.1

7 48 (0.9) 51% (1.3) 1.1

7.5 124 (0.7) 20% (1.6) 0.9

8 825 (0.7) 4% (1.9) 1.0

Los Angeles region

Magnitude

(greater than 

or equal to)

Average

repeat time

(years)

30-year

likelihood of

one or more 

events

Readiness

5 1.4 (0.6) 100% (1.0) 1.0

6 10 (1.1) 96% (1.0) 1.0

6.7 40 (2.1) 60% (0.8) 1.1

7 61 (2.0) 46% (0.7) 1.2

7.5 109 (1.3) 31% (0.9) 1.3

8 532 (0.4) 7% (2.5) 1.3

rate (and note that most of these events 

occur in remote areas of the state). For 

magnitude 8 and larger, on the other hand, 

the rate has increased by 20 percent in 

UCERF3, with an expected repeat time of 

494 years for UCERF3, down from 1 every 

617 years in UCERF2. These changes are a 

direct and expected manifestation of including multifault ruptures in UCERF3. A more 

careful analysis of historical seismicity has 

also produced an increased rate for magnitude 5 and greater earthquakes, going from 

about 5.8 per year in UCERF2 to 8.3 per 

year in UCERF3. All of these trends are 

similar to those seen in various subregions 

of the state, with differences being slightly 

more dramatic for the Los Angeles area 

because that region has a large number of 

faults that can now host multifault ruptures.

Results are also expressed in terms 

of the likelihood of experiencing one or 

more earthquakes in the next 30 years, 

the duration of a typical home mortgage, 

and these values also take fault readiness into consideration (how long it has 

been since the most recent event). As in 

UCERF2, the likelihood for magnitude 

6.7 and larger earthquakes somewhere in 

the entire region remains near certainty 

(greater than 99 percent). The likelihood 

is 7 percent for magnitude 8 and greater, 

a 50 percent increase over UCERF2, 

resulting from both the inclusion of multifault ruptures and the particular readiness of some large faults.

One particularly ready fault is the 

Southern San Andreas, which contributes to 

its continued status of being the most likely 

to host a large earthquake. Specifically, it 

has a 19 percent chance of having one or 

more events larger than magnitude 6.7 in 

the next 30 years near Mojave, Calif. The 

comparably low values for the Northern 

San Andreas, such as 6.4 percent near 

San Francisco, are partly because of the 

relatively recent 1906 earthquake on that 

fault. In fact, probabilities on two other Bay 

Area faults, the Hayward–Rodgers Creek 

and the Calaveras, currently rival or exceed 

those on the Northern San Andreas, in part 

because they are both relatively ready.

Compared to the previous model, 

UCERF2, the San Jacinto fault has a 

three-fold decrease in the likelihood of 

magnitude 6.7 or larger earthquakes. Much 

of this decrease is because of the inclusion 

of more multifault ruptures, as indicated by 

the factor of 57 increase in the likelihood 

of magnitude 8 and larger earthquakes. 

In other words, the fault has traded some 

moderate-sized events for rare larger ones.

Table 1. Average time between earthquakes in the various regions together with 

the likelihood of having one or more such 

earthquakes in the next 30 years (starting

from 2014). Values listed in parentheses indicate the factor by which the rates and likelihoods have increased, or decreased, since 

the previous model (UCERF2). “Readiness” 

indicates the factor by which likelihoods are 

currently elevated, or lower, because of the 

length of time since the most recent large 

earthquakes (see text). These values include 

aftershocks. It is important to note that 

actual repeat times will exhibit a high degree 

of variability, and will almost never exactly 

equal the average listed here.

The Calveras fault, on the other hand, 

has a three-fold increase in the likelihood 

of magnitude 6.7 or larger earthquakes. 

In UCERF2 most Calaveras events were 

well below magnitude 6.7, so the inclusion of multifault ruptures in UCERF3 has 

increased the frequency of earthquakes 

above magnitude 6.7.

We have only touched on a few of the 

more important changes between UCERF2 

and UCERF3, and have highlighted only 

some of the influential factors. Many more 

are currently understood, and scientists 

will be further analyzing results and testing 

assumptions for years to come.

So what do these changes imply with 

respect to seismic hazard, the likelihood 

of ground shaking, as well as for seismic 

risk, the threat to the built environment 

with respect to fatalities and economic 

losses? The answer turns out to be 

entirely dependent on what you are 

concerned about. For example, increasing 

the likelihood of large multifault earthquakes, which consequently reduces the 

likelihood of moderate-sized events, may 

increase the risk to tall buildings or large 

bridges, but actually lower the risk to 

residential homes.

As a consequence, it is difficult to 

make generalizations about the hazard 

or risk implications of UCERF3 without 

first specifying both asset types and their 

locations. Conclusions will vary depending on whether you are designing a single 

family dwelling in Sacramento, retrofitting 

the San Francisco–Oakland Bay Bridge, 

considering the location of a nuclear 

power plant, laying pipeline across the 

San Andreas Fault, or considering aggregate losses over a large insurance portfolio. 

The practical implications will need to be 

considered on a case-by-case basis.

What Next?

UCERF3 can now be used to evaluate seismic hazard and risk in California. 

In fact, it has already been used for the 

2014 update of the U.S. Geological 

Survey National Seismic Hazard Maps 

(http://earthquake.usgs.gov/hazards/), 

which in turn are used in building 

codes. The California Earthquake 

Authority, which is required by law to 

use the best available science, will use 

UCERF3 to evaluate insurance premiums 

charged to customers, as well as their 

own level of reinsurance. UCERF3 will 

be used in many other risk mitigation 
30-year M≥6.7 likelihood (percent)

1/100 1/10 1 10 100

Map data: GoogleTM Earth 

Data SIO, NOAA, U.S. Navy, NGA, GEBCO

Image Landsat

Data LDEO-Columbia, NSF, NOAA

Data MBARI

San Francisco

region

Tabulated values represent the likelihood of having one or more earthquakes in the next 30 years (starting from 2014).

[At the points on the fault indicated by white circles. M≥6.7 means magnitude greater than or equal to 6.7, and likewise for the other two magnitude thresholds. %, percent. 

Values listed in parentheses indicate the factor by which the likelihoods have increased, or decreased, relative to the previous model (UCERF2), where “--” means the previous 

value was zero. “Readiness” indicates the factor by which probabilities are currently elevated, or lower, because of the length of time since the previous large earthquake]

Figure 4. Likelihood of magnitude 6.7 or greater earthquakes in the next 30 years, from 2014, on the faults near San Francisco, Calif.

Northern San Andreas

M≥6.7: 6.4% (0.8)

M≥7.5: 5.7% (1.1)

M≥8.0: 2.1% (1.4)

Readiness: 0.6

Hayward

M≥6.7: 14.3% (1.2)

M≥7.5: 3.6% (93.7)

M≥8.0: <0.1% (--)

Readiness: 1.6

Calaveras 

M≥6.7: 7.4% (1.1)

M≥7.5: 0.5% (--)

M≥8.0: 0.1% (--)

Readiness: 1.4

efforts in the years to come, including 

engineering design of buildings and 

lifelines, loss estimation for catastrophic 

bonds and other risk-linked securities, and 

emergency preparedness, all of which have 

the ultimate goal of increasing public safety 

and community resilience.

UCERF3 should also serve as a 

reminder that California is earthquake 

country, and residents should always be prepared. Simple safeguards include practicing 

“drop, cover, and hold on,” securing items 

in your home and workplace that could fall 

during an earthquake, and storing sevendays worth of food and water. Homeowners 

can also consider structural retrofits, such 

as bolting the house to its foundation, as 

well as earthquake insurance options. For 

further guidance on how to prepare for, 

survive, and recover after big earthquakes, 

follow the Seven Steps to Earthquake 

Safety (http://www.earthquakecountry.org/

sevensteps).

Although UCERF3 is a clear 

improvement over the previous model 

(UCERF2), it is still an approximation 

of the natural system. For example, 

it does not model the earthquaketriggering process that produces 

aftershocks, even though we know 

such events can be large and damaging. Through the National Earthquake 

Hazard Reduction Program (http://

www.nehrp.gov), the U.S. Geological 

Survey and its partners will continue 

to conduct research aimed at improving our understanding of fault behavior and estimates of earthquake hazard 

in the future.
30-year M≥6.7 likelihood (percent)

1/100 1/10 1 10 100

Map data: GoogleTM Earth 

Data SIO, NOAA, U.S. Navy, NGA, GEBCO

Image Landsat

Data LDEO-Columbia, NSF, NOAA

Data MBARI

Los Angeles

region

References

Korzybski, A. 1931, A non-Aristotelian System and its necessity 

for rigour in mathematics and physics: presention at the 

American Mathematical Society, December 28, 1931, New 

Orleans, Louisiana, meeting of the American Association 

for the Advancement of Science. Reprinted in Science and 

Sanity, 1933, p. 747–61.

Reid, H.F., 1911, The elastic-rebound theory of earthquakes: 

University of California Publications, Bulletin of the Department of Geological Sciences v. 6, p. 413–444.

Working Group on California Earthquake Probabilities, 2007, 

Published as: Field, E.H., Dawson, T.E., Felzer, K.R., 

Frankel, A.D., Gupta, V., Jordan, T.H., Parsons, T., Petersen, 

M.D., Stein, R.S. Weldon, II, R.J., and Wills, C.J., 2009, 

Uniform California earthquake rupture forecast, Version 

2 (UCERF 2): Bulletin of the Seismological Society of 

America, v. 99, p. 2053–2107, doi:10.1785/0120080049.

Working Group on California Earthquake Probabilities, in 

press, Published as: Field, E.H., Biasi, G.P., Bird, P., 

Dawson, T.E., Felzer, K.R. Jackson, D.D., Johnson, K.M., 

Jordan, T.H., Madden, C. Michael, A.J., Milner, K.R., Page, 

M.T., Parsons, T., Powers, P.M., Shaw, B.E., Thatcher, 

W.R., Weldon, R.J. II, , and Zeng, Y., 2015, Long-term, 

time-dependent probabilities for the third uniform California earthquake rupture forecast (UCERF3), Bulletin of the 

Seismological Society ofAmerica.

—Authors: Edward H. Field and members 

of the 2014 WGCEP

Cooperating organizations:

Southern California Earthquake Center (SCEC)

California Geological Survey (CGS)

California Earthquake Authority

U.S. Geological Survey

Additional Resources:

For general earthquake information contact:

1-888-ASK-USGS (1-888-275-8747)

http://earthquake.usgs.gov/

http://ask.usgs.gov

or 

SCEC Education and Outreach: 213-740-3262

For UCERF3 information see:

http://www.WGCEP.org/UCERF3

For technical questions contact:

Edward (Ned) Field: field@usgs.gov

Figure 5. Likelihood of magnitude 6.7 or greater earthquakes in the next 30 years, from 2014, on the faults near Los Angeles, Calif.

Southern San Andreas

M≥6.7: 19.0% (0.9)

M≥7.5: 17.3% (1.0)

M≥8.0: 6.8% (2.5)

Readiness: 1.5

Elsinore

M≥6.7: 3.8% (0.9)

M≥7.5: 1.0% (1.0)

M≥8.0: <0.1% (0.3)

Readiness: 1.0

San Jacinto

M≥6.7: 5.0% (0.4)

M≥7.5: 4.9% (1.3)

M≥8.0: 2.7% (56.7)

Readiness: 1.1

Tabulated values represent the likelihood of having one or more earthquakes in the next 30 years (starting from 2014).

[At the points on the fault indicated by white circles. M≥6.7 means magnitude greater than or equal to 6.7, and likewise for the other two magnitude thresholds. %, percent. 

Values listed in parentheses indicate the factor by which the likelihoods have increased, or decreased, relative to the previous model (UCERF2), where “--” means the previous 

value was zero. “Readiness” indicates the factor by which probabilities are currently elevated, or lower, because of the length of time since the previous large earthquake]