U.S. Geological Survey
Fact Sheet 152-99
Online Version 1.0
UNDERSTANDING EARTHQUAKE HAZARDS IN THE SAN FRANCISCO BAY REGION
| On the basis of research conducted since the 1989 Loma Prieta earthquake, U.S. Geological Survey (USGS) and other scientists conclude that there is a 70% probability of at least one magnitude 6.7 or greater quake, capable of causing widespread damage, striking the San Francisco Bay region before 2030. Major quakes may occur in any part of this rapidly growing region. This emphasizes the urgency for all communities in the Bay region to continue preparing for earthquakes. |
Just before dawn, residents of a bayside urban area, thought to be well prepared for earthquakes, were jolted from their beds by a magnitude 6.9 quake. This 1995 temblor killed more than 6,000 people and caused $100 billion in damage. The quake struck Kobe, Japan, but similar losses could have occurred in the San Francisco Bay region in 1989 had the magnitude 6.9 Loma Prieta earthquake been centered in an urbanized area.
Damaging earthquakes are inevitable in the Bay region, but taking actions based on the odds 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 quake odds. In 1997, the USGS working group, now known as WG99, was expanded to include more than 100 scientists from Federal and California State governments, consulting firms, industry, and universities.
Earthquake probabilities are based on balancing the continual motions of the plates that make up the Earth's outer shell with the slip on faults, which occurs primarily during earthquakes. To determine Bay region earthquake probabilities, WG99 gathered new data, developed analytical tools, and debated a wide variety of interpretations about how future temblors may occur.
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| The threat of earthquakes extends across the entire San Francisco Bay region, and a major quake is likely before 2030. Knowing this will help people make informed decisions as they continue to prepare for future quakes. (more detailed version) |
WG99 determined that there is a 70% chance (±10%) of at least one magnitude 6.7 or greater earthquake striking the San Francisco Bay region between 2000 and 2030. This result is the most important outcome of WG99's work, because any major quake can cause damage throughout the region. This was dramatically demonstrated when the 1989 Loma Prieta earthquake caused severe damage in Oakland and San Francisco, more than 50 miles from the fault rupture. Although earthquakes can inflict damage at a considerable distance, shaking will be very intense near the fault rupture. Therefore, temblors located in urbanized areas of the region have the potential to cause much more damage than the 1989 quake.
In the Bay region's rapidly growing eastern valleys, four faults slice through Contra Costa, Alameda, Solano, Santa Clara, San Benito, and Napa Counties. WG99 calculated the odds of major quakes on these faults for the first time. They determined that there is a 30% chance of one or more magnitude 6.7 or greater quakes occurring somewhere on the Calaveras, Concord-Green Valley, Mount Diablo Thrust, and Greenville Faults before 2030.
Residents living near the Pacific coast in burgeoning San Mateo, Santa Cruz, and Monterey Counties are sandwiched between the San Andreas and San Gregorio Faults. New data have allowed WG99 to calculate the first earthquake probabilities for the San Gregorio Fault and to better estimate probabilities for the San Andreas Fault. Combined, these two faults have a 25% chance of producing one or more magnitude 6.7 or greater quakes in these coastal areas before 2030.
When the 1990 USGS probability report was released, earthquake odds could only be estimated for the San Andreas Fault and the Hayward-Rodgers Creek Fault, although the danger posed by other faults was recognized. WG99 found that, of all the faults in the Bay region, these two and the Calaveras pose the greatest threat, because they have high quake odds and run through the region's urban core.
Comparison to Previous Studies
There are important differences between the 1990 and WG99 studies. WG99 analyzed five additional faults, which would be expected to increase the estimated regional probability of major quakes. This expected increase was largely compensated for, however, by two effects not included in the 1990 report: (1) slip on faults in the absence of earthquakes and (2) the effect of the 1906 earthquake in reducing quake activity throughout the region.
Additionally, the 1990 study considered only earthquakes of about magnitude 7 in determining there was a 67% chance of major quakes in the Bay region between 1990 and 2020. WG99 decided to focus on earthquakes of magnitude 6.7 and greater in their calculations, 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.
Magnitude 6.7 or greater quakes can cause damage throughout the Bay region, but even smaller quakes could be serious if centered in an urbanized area. WG99 found an 80% chance of one or more magnitude 6 to 6.6 quakes occurring in the Bay region before 2030.
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| The rate of large earthquakes in the San Francisco Bay region abruptly dropped 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. |
Call to Action
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WG99's conclusions from their 2-year effort are to appear in USGS Circular 1189, "Earthquake Probabilities in the San Francisco Bay Region: 2000 to 2030." Their finding that a major temblor is more likely than not emphasizes the ongoing need for the Bay region to prepare for earthquakes. Large earthquakes in the San Francisco Bay region can produce sudden and tremendous loss of life and property, threatening the social and economic fabric of this region. 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. WG99's results will help business, government, and the public make informed decisions as they continue their preparations. The work of USGS and other scientists in evaluating earthquake probabilities for the San Francisco Bay region is an ongoing part of the National Earthquake Hazard Reduction Program's efforts. These efforts help to safeguard lives and property from the earthquakes that will inevitably strike in northern California and elsewhere in the United States. |
By
Andrew J. Michael, Stephanie L. Ross, David P. Schwartz, James W. Hendley II, and Peter H. Stauffer
Graphics by Susan Mayfield, Sara Boore, and Michael F. Diggles
Web design by Carolyn Donlin and Scott Haefner
COOPERATING ORGANIZATIONS
Association of Bay Area Governments
California Division of Mines and Geology
California Governor's Office of Emergency Services
Federal Emergency Management Agency
Lawrence Livermore National Laboratory
Geomatrix Consultants Inc.
Pacific Gas and Electric Company
University of California at Berkeley
William Lettis & Associates
Many other institutions, organizations, and firms
For
more information contact:
Earthquake Information Hotline (650) 329-4085
U.S. Geological Survey, Mail Stop 977
345 Middlefield Road, Menlo Park, CA 94025
There is a USGS website where you can learn more about earthquakes and how to prepare for them.