Loma Prieta earthquake

Abstract

The October 17, 1989, Loma Prieta earthquake of magnitude 7.1 provided a harsh reminder of the hazards associated with life on a major plate boundary. The scientific lessons of the earthquake are still being assessed as seismic, geologic, and geodetic data are analyzed and new data collected. Probably the most striking, and deadly, aspect of the earthquake was the intense damage at sites, such as San Francisco's Marina district and the 1‐880 overpass in Oakland, 100 km from the earthquake's epicenter. Similar damage occurred in Mexico City in 1985, 350 km from the epicenter of the M 8.1 Michoacan earthquake. Seismic recordings during the Loma Prieta earthquake and its aftershocks showed that pockets of poorly consolidated sediments and Bay muds with low seismic velocities greatly amplified the ground shaking. Sites on Bay mud experienced peak ground motions many times greater than sites only a few blocks away on bedrock. Site amplification during the mainshock was less severe (at frequencies of a few Hz) than during subsequent aftershocks, demonstrating significant non‐linearity in the site effects. The damage resulted from a combination of local site amplification and pervasive failure of artificial land fills. Both effects were predicted prior to the earthquake; published earthquake hazard maps had previously designated as high seismic risk zones the sites of intense damage.