The Golden Gate Bridge

1989 Loma Prieta Quake

TIME  October 17, 1989 / 5:04 pm PST

LOCATION    70 miles south of San Francisco, new Watsonville and Santa Cruz. This earthquake was the first major event to occur along the San Andreas fault zone since the1906 earthquake . The Loma Prieta earthquake ruptured the southernmost 40 km of the 1906 break. The average strike-slip displacement was 1.2 meters while the average reverse-slip displacement was 1.6 meters. This type of motion is not typical of the San Andreas fault and suggests that the earthquake occurred on a sub-parallel fault and not on the San Andreas itself.

MAGNITUDE  6.9

Buildings in the Marina District

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)

HUMAN/PHYSICAL TOLL 63 persons died and 3,757 were injured. Property damage exceeded $6 billion. At the time, this was the most costly natural disaster in the United States. Damage occurred to 18,306 houses and 2,575 businesses. Approximately 12,053 persons were displaced. In total, in all areas, 22,000 homes were damaged and 1,500 homes were destroyed or rendered uninhabitable.

LESSONS LEARNED Several life-lines were damaged, notably the Nimitz Freeway and the Bay Bridge. The most intense damage was confined to areas where buildings and other structures were situated on loosely consolidated, water-saturated soils, such as in the Marina District of San Francisco. Loosely consolidated soils tend to amplify shaking and increase structural damage. Water saturated soils compound the problem due to their susceptibility to liquefaction and corresponding loss of bearing strength.

Although damage to the two highways and to structures in the Marina district of San Francisco was widely reported, most of San Francisco fared well during this earthquake with an epicenter about 56 miles away. Most damage was located in isolated pockets, and was severe only where it affected older buildings constructed on fill material or on unconsolidated strata. In cities closer to the epicenter, such as Santa Cruz, Watsonville, and Los Gatos, a much higher percentage of structures was affected.

"Soft story" buildings, typically with parking on ground floor, like the one pictured here, are common throughout the Bay Area and were particularly at risk during the Loma Prieta Quake. (USGS photo) Many of the damaged structures were 50 to 100 years old and built with unreinforced masonry. Unreinforced masonry is brittle and weak and therefore crumbles easily during the intense shaking of an earthquake.