Loma Prieta earthquake

The Event A 6.9 magnitude earthquake

Date October 17, 1989

Place The Santa Cruz Mountains, near Loma Prieta Peak, southeast of San Francisco

After decades of relative seismic quiet, the Loma Prieta earthquake was a wakeup call for Californians, warning them to prepare for potentially stronger future earthquakes.

At 5:04 p.m., on the evening of October 17, 1989, an earthquake occurred in the Santa Cruz Mountains southeast of San Francisco, California. The epicenter was near Loma Prieta Peak, in Nisene Marks State Park, at a distance of about ten miles from the town of Santa Cruz. Sometimes called the World Series earthquake because it happened just before the third game of the 1989 World Series between the Oakland As and the San Francisco Giants, the Loma Prieta earthquake was the first major earthquake to shake the area since the Great San Francisco Earthquake of 1906.

Although its epicenter was located in the mountains about sixty miles away, the earthquake caused extensive damage and loss of life in the densely populated areas of San Francisco, Monterey, and Oakland. This quake, which was felt by millions of people scattered over an area of about 400,000 square miles, occurred along the San Andreas Fault, which marks the boundary between the North American and Pacific tectonic plates, but geologists believe that the rupture actually occurred ten or eleven miles below the fault itself.

Casualties and Destruction

Although it lasted only fifteen seconds, the earthquake caused 65 deaths, 3,757 injuries, and $5.9 billion of property damage. Approximately 90 percent of the reported injuries were in Alameda, San Francisco, Santa Cruz, Santa Clara, and Monterey Counties, the five counties closest to the epicenter. Many human-made structures collapsed, particularly old, unreinforced masonry buildings. In downtown San Francisco, the exterior walls of old buildings fell on cars and caused five deaths. In Watsonville, flying debris from an outside wall struck and killed a passerby, and in Santa Cruz, three people were killed when a roof collapsed along Pacific Avenue, where twenty-nine buildings were destroyed.

Many other deaths were caused by the damage done to transportation arteries. The Bay Bridge, which links San Francisco to Oakland, sustained a steel truss failure, causing two fifty-foot spans of the upper deck to collapse onto the lower deck. Two interstate highways, Highway 280 and Highway 880 (also known as the Nimitz Freeway), suffered major failures in their reinforced concrete structure. The upper deck of the freeway collapsed at the Cypress Street viaduct, in the western suburbs of Oakland. Forty-two people died in the collapse, crushed by tons of concrete, burned by gas tank explosions, or killed when their vehicles were thrown from the pitching roadway. Casualties continued for some time after the initial event. Thirty minutes after the collapse, a twenty-three-year-old woman died when she failed to notice the gap in the upper deck of the Bay Bridge and drove over the precipice. More than seven hours after the earthquake, a driver was killed when he hit three horses running loose on the Santa Cruz freeway, and a civilian was gunned down as he was directing traffic in San Francisco.

Liquefaction

Most of the loss of life caused by the earthquake was associated with the failure of human-made structures, much of which was the result of a geologic process called liquefaction. Underneath the area where the earthquake struck sit loosely compacted alluvial deposits in riverbeds and soft mud around the bay. There is also a considerable amount of artificial fill, used by builders to extend the developable portion of the Bay Area. Although the 1906 San Francisco earthquake had already demonstrated that these soils are not suited to withstand ground motion triggered by an earthquake, the practice of using fill continued. Moreover, the Bay Area has a very shallow water table, which saturates the underground materials. Soft mud, alluvial deposits, and loosely compacted fill amplify the shaking of an earthquake, and, when combined with a shallow water table, they can produce severe ground failure. During liquefaction, the strength of the soils decreases and the ability of these materials to support bridge and building foundations is so diminished that buildings tilt, freeway overpasses collapse, and gas pipelines and water mains break.

Geologic studies were conducted after the Loma Prieta earthquake to examine prehistoric earthquakes, not only on the San Andreas Fault but also on the Hayward and Calavera faults. Integrating the information gained in these studies with its recent observations, the U.S. Geological Survey (USGS) reported in 1990 that there was a 60 percent probability that one or more destructive earthquakes (magnitude 7.0 or larger) would occur in the San Francisco Bay Area between 1990 and 2020. Other studies undertaken by the California Division of Mines and Geology and the USGS have led to significant changes in building codes related to the design and construction of bridges, highways, and buildings. At the federal level, the Federal Response Plan was created to better organize the activation, mobilization, and deployment of personnel and resources and the assessment of damages.

Impact

The highly televised earthquake was an instant reminder of the danger of living along a tectonic plate boundary, and it stimulated much research to assess both the probability and the possible impact of more major earthquakes striking the San Francisco Bay Area. Scientists thus attempted to provide people with better tools to build a more secure life on unstable land.

Bibliography

Hough, Susan Elizabeth. Earthshaking Science: What We Know (and Don’t Know) About Earthquakes. Portland, Oreg.: Book News, 2004. Overview of seismology designed to provide a basic introduction for a lay audience.

‗‗‗‗‗‗‗. Finding Fault in California: An Earthquake Tourist’s Guide. Missoula, Mont.: Mountain Press, 2004. Details the locations and history of significant faultlines within California.

Reti, Irene, ed. The Loma Prieta Earthquake of October 17, 1989: A UCSC Student Oral History Documentary Project. Santa Cruz, Calif.: University of California, Santa Cruz, University Library, 2006. Extensive oral history of the earthquake and its effects, assembled by students at the university nearest its epicenter.

Yeats, Robert S, Living with Earthquakes in California: A Survivor’s Guide. Corvallis: Oregon State University Press, 2001. Guide to the steps one should take in advance of an earthquake in order to increase the likelihood of survival.