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Preliminary isoseismal map for the Santa Cruz (Loma Prieta), California, earthquake of October 18, 1989 UTC

1990; United States Department of the Interior; Linguagem: Inglês

10.3133/ofr9018

2332-4899

C.W. Stover, B.G. Reagor, F. W. Baldwin, L.R. Brewer,

Seismic Waves and Analysis

The University of California, Berkeley assigned the earthquake a local magnitude of 7.0ML.The earthquake caused at least 62 deaths, 3,757 injuries, and over $6 billion in property damage (Plafker and Galloway, 1989).The earthquake was the most damaging in the San Francisco Bay area since April 18,1906.A major arterial traffic link, the double-decked San Francisco-Oakland Bay Bridge, was closed because a single fifty foot span of the upper deck collapsed onto the lower deck.In addition, the approaches to the bridge were damaged in Oakland and in San Francisco.Other severe earthquake damage was mapped at San Francisco, Oakland, Los Gatos, Santa Cruz, Hollister, Watsonville, Moss Landing, and in the smaller communities in the Santa Cruz Mountains.The Santa Cruz earthquake has been assigned a Modified Mercalli Intensity (MMI, Wood and Neumann, 1931) of VIII in the epicentral area; an MMI of IX was assigned to San Francisco's Marina District and to four areas that experienced damage to reinforced-concrete viaducts.These areas are: the Nimitz Freeway (Interstate 880, Cypress Section) in Oakland; the Embarcadero, Highway 101, and Interstate 280 in San Francisco (fig.2).Two previous large earthquakes with high MM intensities in the San Francisco Bay area have occurred along the San Andreas fault zone.The most widely known is the April 18, 1906, San Francisco earthquake that was located about 1.5 miles south of Daly City.This event had a magnitude of 8.3 (Richter, 1958), a MMI of XI, and an epicenter location at 37.67°N., 122.48°W.(Bolt, 1968).The other large earthquake occurred on October 8,1865; it was located near San Jose.This tremor had a magnitude of 6.3 (Toppozada and others, 1981), a MMI of VIE (Toppozada, personal communication), and an epicenter, inferred from an isoseismal map, at 37.2°N., 121.9°W.The effects produced by the 1865 tremor (Townley and Alien, 1939) show a damage pattern similar to the 1989 Santa Cruz earthquake.These data and the epicenter inferred by Toppozada and others (1981) suggests that the 1865 earthquake has a location near the epicenter of the 1989 earthquake.The effects caused by the 1865 earthquake indicate that the highest intensities occurred in San Francisco and in the Santa Cruz Mountains; a pattern that was observed for the 1989 earthquake.In 1865, the areas that received serious damage in San Francisco was south of Market Street and in the vicinity of Battery and Washington Streets.These two areas include some of the same neighborhoods that suffered damage in 1989.For example, the old Merchant Exchange Building (comer of Battery and Washington Streets) was completely ruined; in 1989, an older, masonry-walled, 8-story