future of significant bay area earthquakes

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The Past and Future of Significant Bay Area Quakes

Quakes can cause damage far from the fault rupture...

1969 -- Damage in Santa Rosa

1906 -- Damage in Healdsburg

1868 -- Damage in Hayward

1865 -- Damage in San Francisco

(Historical photos of shaking damage courtesy National Information Service for Earthquake Engineering (NISEE), University of California, Berkeley)

The threat of future quakes extends across the Bay Area...

There is a 62% probability that at least one earthquake of magnitude 6.7 or greater will occur on a known or unknown San Francisco Bay region fault before 2032. After a century of study by geologists, many faults have been mapped in the region, but not all faults are apparent at the surface -- some quakes occur on previously unknown faults.

An example of a damaging quake on a previously unknown fault is the September 3, 2000, Yountville (Napa) earthquake. This magnitude 5.2 quake struck at 1:36 a.m., 10 miles northwest of Napa, rudely awakening many residents of the Bay Area. It injured 25 people, caused at least $10 million in damage, and forced 70 people to seek shelter at Red Cross facilities.

Most Earthquake Damage Is Caused by Shaking...

Damage in earthquakes is mainly from shaking. The intensity of shaking that a structure will experience during an earthquake is a function of three main factors:

The magnitude of the earthquake -- the larger the quake, the stronger the shaking.
The distance from the fault that ruptured -- the nearer you are to the fault, the greater the shaking.
The type of ground materials beneath the structure -- soft soils amplify the shaking; hard bedrock does not.

Buildings in the Marina District of San Francisco were badly damaged in the 1989 Loma Prieta earthquake. "Soft story" buildings, typically with parking on ground floor, like the one pictured here, are common throughout the Bay Area and are particularly at risk when exposed to strong shaking. (USGS photo)

Expected Levels of Shaking from Future Earthquakes

For more information:
http://www.consrv.ca.gov/cgs/rghm/psha/index.htm

On this map, bands of highest expected shaking generally follow major faults.

Shaking levels are also influenced by the type of materials underlying an area -- soft soils tend to amplify and prolong shaking, even at great distances from a quake.

The worst soft soils in the Bay Area are the loose clays and filled areas bordering San Francisco Bay and the Sacramento-San Joaquin Delta.

Deep soils in valleys shake more than bedrock in the hills -- most urban development is in the valleys.

Intense shaking can damage even strong, modern buildings and their contents.

Soft Soils Amplify Earthquake Shaking

The Cypress freeway structure in Oakland was built in the 1950s, before the use of modern seismic-safety standards. Part of the structure standing on soft mud (dashed red line) collapsed in the 1989 magnitude 6.9 Loma Prieta earthquake, whose epicenter was nearly 60 miles (100 km) to the south. Adjacent parts of the structure (solid red) that were built on firmer ground remained standing. Seismograms (upper right) show that the shaking was especially severe in the soft mud. (Photo by Lloyd S. Cluff, Pacific Gas & Electric)