Preliminary Surface Geologic Materials
Seismic waves may be amplified by near-surface materials. Loose or soft soils typically amplify shaking more than hard rock. The soils are classified based on 1:250,000 scale geologic maps (CGS's Geologic Atlas Series Maps) and on shear wave velocities measured in the upper 100 feet of the earth's surface at about 600 sites, primarily in the Los Angeles and San Francisco Bay areas. Shear wave velocity is used in calculating the earthquake shaking. Click on the image to obtain a more detailed version of the map.
Soft soils can amplify shaking and lead to increased damage, as in the Marina District of San Francisco during the 1989 Loma Prieta earthquake. Photo by J. Nakata, USGS.