U.S. Geological Survey
Fact Sheet 039-03

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

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.

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 USGS Open-File Report 03-214. 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.

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.

The 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. Map above 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 1868 Hayward and 1989 Loma Prieta magnitude 6.9 earthquakes areshown in yellow.

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| Earthquake Probabilities 2003–2032 | From Probabilities to Hazard | A Call to Action |

| Earthquake Odds—A Balancing Act |

UNDERSTANDING EARTHQUAKE HAZARDS IN THE SAN FRANCISCO BAY REGION

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URL of this page: https://pubs.usgs.gov/fs/2003/fs039-03/
Maintained by: Michael Diggles
Created: April 17, 2003
Last modified: May 17, 2005 (mfd)