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On October 17, 1989, at 5:04 P.M. (PDT), a 7.1 magnitude earthquake occurred in the Santa Cruz mountains. Movement occurred along a 40-km segment of the San Andreas fault from southwest of Los Gatos to north of San Juan Bautista. Measurements along the surface of the Earth after the earthquake show that the Pacific plate moved 1.9 m to the northwest and 1.3 m upward over the North American plate. The upward motion resulted from deformation of the plate boundary at the bend in the San Andreas fault. At the surface the fault motion was evident as a complex series of cracks and fractures.

Even though the earthquake occurred in the remote Santa Cruz Mountains, it caused severe damage in San Francisco and Oakland 80 km to the north. This is somewhat unusual for an earthquake of this magnitude. Some of the statistical results of the disaster were: more than $7 billion in property damage (2.5 billion in San Francisco alone), 414 single-family homes destroyed, 104 mobile homes destroyed, 18,306 homes damaged, 97 businesses and 3 public buildings destroyed, 2,575 businesses damaged, 12,000 people displaced from homes and housed in shelters, 3,757 injuries, and 67 deaths.

One of the areas in San Francisco that sustained major damage was the Marina District. This area was a lagoon at the time of the 1906 San Francisco earthquake. During that earthquake the margins of the lagoon shook violently.

However, after the 1906 earthquake, the lagoon was filled with sand and rubble of destroyed buildings to make a fairground for the 1915 Panama Pacific International Exposition. This fill area later became the site of an expensive real-estate development known as the Marina District. The unconsolidated soils amplified the shaking and became liquefied (behaved like a dense fluid) causing permanent deformation of the ground. This was one of the causes of the increased damage in the area.

Construction practices also contributed to the damage. Some four-story buildings built above garages had inadequate lateral bracing. Thirty five of the buildings in the Marina District were eventually torn down and 150 others were structurally damaged. The two major causes of structural failures were poor soil conditions and inadequate structural design.

South of Market Street, several buildings between 5 and 10 stories high were damaged. Old masonry buildings were badly damaged, including a warehouse where collapse of fourth floor exterior walls killed five people parked in cars along the street. There were also severely damaged buildings in the Mission District. In Oakland, severe damage occurred to several mid-rise buildings and many old brick buildings in the downtown area.

Primarily due to the effects of liquefaction, buried underground utilities such as gas pipelines, water lines, and sewer lines were heavily damaged. This left about one thousand homes without gas or water. As in the 1906 earthquake, fires in the Marina District could not be fought with city water because water mains had failed.

One of the sources for concern produced by the earthquake is the damage or failure of transportation systems at comparatively large distances from the epicenter. The most deadly structural failure of the earthquake occurred when the upper deck of the Interstate 880 (Nimitz Freeway) in Oakland fell onto the lower roadway causing an official death toll of 41. Another spectacular failure occurred on the Oakland Bay Bridge. Interstate 280, the Embarcadero Freeway, and Highway 101 at Fell Street were also damaged.

In Oakland, Highway 980 and the MacArthur Maze developed cracks in the support columns. It will cost about $1.5 billion to rebuild state and local roads damaged in the earthquake. In addition to damage to roads, the Oakland International Airport, naval port, and the Alameda Naval Air Station runways were damaged by liquefaction.

It is interesting to compare this earthquake with the slightly smaller (magnitude 6.9) Armenian earthquake of December 1988 that killed 25,000 people and destroyed entire towns. Good construction and engineering practices in the Loma Prieta area obviously contributed to the preservation of property and human lives.