Rolling and shaking in the Bay Area: probabilities and loss estimates

Rolling and shaking in the Bay Area: probabilities and loss estimates

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According to a new earthquake probabilities report released by the U.S. Geological Survey and partners, the Bay Area will experience a whole lot of rolling and shaking in the next 30 years. There’s a 62 percent chance that a major, damaging earthquake of magnitude 6.7 or higher will strike the San Francisco Bay Region (SFBR) in the next 30 years (2003-2032), and at least an 80 percent chance of one or more magnitude 6.0 to 6.6 quakes occurring in the same period. These earthquakes are most likely to occur on one of the seven main faults crossing the region. The probabilities were based on a comprehensive analysis by the USGS-led Working Group on California Earthquake Probabilities that included earthquake scientists from government, academia, and the private sector. The report updates and revises the 1999 probability estimate (70 percent
for the 30-year period 2000-2029). As part of the new report, the USGS is releasing, on the web, ShakeMaps of expected earthquake shaking intensities for all 34 potential earthquakes sources identified in the report.

Although the new earthquake probability for the SFBR appears slightly lower than 1999 estimates, the results of the new earthquake probability study should not be viewed as a message that the earthquake hazard in the region has somehow lessened, said David Schwartz, one of the report’s lead authors and chief of the USGS’s San Francisco Bay Area Earthquake Hazards Project. The earthquake sources identified by the Working Group form the basis for a new study on loss estimates for 10 of the most damaging earthquakes to strike the SFBR over the next 30 years. USGS, the California Office of
Emergency Services, California Geological Survey, and the Association of Bay Area Governments participated in this loss study, using the Federal Emergency Management Agency (FEMA’s) HAZUS earthquake loss model. Any of the ten quakes would result in losses as least as great as the 1989 M6.9 Loma Prieta quake, which caused about $6 billion in direct structural damage.

USGS scientist Mary Lou Zoback, lead author on the loss study, said the East Bay I-880 corridor along the Hayward fault would experience the most proportional property damage of the entire Bay area, a result of dense development directly along and next to the Hayward fault and the fact that earthquakes on this fault, while lower in magnitude than those on the San Andreas fault, have higher odds of occurring. Chris Wills, geologist with the California Geological Survey and a lead author on the report, noted that all ten of the most likely earthquakes predict far fewer fatalities than the 6,000 deaths resulting from the 1995 6.9 Kobe earthquake that occurred in the early morning directly beneath an
urban area in Japan with a population of 1.5 million. In the event of a strong quake, ShakeMaps of the worst shaking intensity and likely damage, created by USGS’ Advanced National Seismic System (ANSS), would be sent to first responders to speed aid to the most severely affected areas within minutes of the quake. ANSS, a national system when fully funded, uses networks of sophisticated ground-shaking stations to
send real-time information to disaster responders, utilities and the OES. USGS and its partners have developed ANSS to help save lives, improve public safety and reduce economic losses following an earthquake. The Bay Region experienced large, destructive earthquakes in 1838, 1868, 1906, and 1989. The report reveals that although earthquakes are most likely to occur on one of the region’s seven main fault systems, they may also occur on faults not characterized in the study. The earthquake likelihood is distributed
broadly across the SFBR, from the San Gregorio fault on the west to the Green Valley and Greenville faults on the east. While the urban core of the Bay Region remains at high risk, researchers identified significant additional earthquake hazards in three of the region’s most rapidly growing areas. The easternmost faults along the I-680 corridor in central and eastern Contra Costa and Alameda counties together have about a 19 percent
chance of a M6.7 earthquake. The combined probability for M6.7 earthquakes east of San Francisco Bay is 46 percent, west of the Bay, 34 percent for one or more 6.7 quakes occurring from 2003 to 2032. The USGS serves the nation by providing reliable scientific information to describe and understand the Earth; minimize loss of life and property from natural disasters; manage water, biological, energy, and mineral resources; and enhance and protect our quality of life.