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OCTOBER 18, 1989 (M=7.1) [c6, p165-166]

In the late afternoon of October 17, 1989, the San Andreas fault ruptured in its first major earthquake since 1906 at 5:04 p.m. P.d.t. (0004 G.m.t. on Oct. 18). Centered along a remote segment of the fault in the southern Santa Cruz Mountains, the Loma Prieta earthquake reruptured the southernmost 40 km of the 1906 fault break, producing the Nation's most costly natural disaster. The earthquake claimed 62 lives and injured an additional 3,757 people. It destroyed 963 homes and damaged more than 18,000 others, displacing 12,000 people from their residences. The combined dollar loss to the private and public sectors exceeded $6 billion (Plafker and Galloway, 1989).

Damage in the epicentral region was most severe where the earthquake shaking was compounded by local ground failures, commonly involving landslide movement but also including some fractures of probable tectonic origin; the shaking clearly reactivated some fissures observed in 1906. Primary fault displacement, however, did not reach the surface. In the hard-hit communities of Santa Cruz, Watsonville, and Los Gatos, unreinforced masonry buildings bore the brunt to the damage, and ground conditions played a significant role in the damage patterns.

The earthquake also caused grave damage and claimed the greatest number of lives far to the north, in San Francisco and Oakland, about 100 km from the epicenter. There, the earthquake selectively destroyed structures known to be at risk or located on poor ground (Plafker and Galloway, 1989). The root cause of the devastation in the Marina District of San Francisco ( Figure 6.10), as well as at most other sites along the margin of the San Francisco Bay, was liquefaction-induced ground failure. All of these localities sit on land reclaimed from the bay and are underlain by young, water-saturated sedimentary deposits. As we know from the clear lessons of history, provided by the earthquakes of 1865, 1868, and 1906 (Lawson, 1908), such materials perform poorly even under modest levels of earthquake shaking. The collapse of the double-decked section of California Interstate Highway 880 in Oakland ( Figure 6.10), where 41 people died, resulted principally from design defects. The section of the viaduct that collapsed was founded on soft estuarine sedimentary deposits that amplified the strong ground motion; the adjoining section, founded on alluvium, rode through the earthquake.

The earthquake broke the San Andreas fault where it makes a conspicuous leftward bend, connecting straighter subparallel segments to the north and south. The fault plane dips 700 SW., and movement in the earthquake involved comparable amounts of right-lateral strike slip and reverse slip, a kinematic response driven by the need to remove material from this compressional fault bend as the Pacific plate moves to the northwest around it. The rupture nucleated at the base of the seismic zone, at 18-km depth, and spread unilaterally upward and bilaterally along strike, filling a conspicuous void in the pre-event seismicity. Geodetic data collected immediately after the event suggest an average strike-slip displacement of 1.6 m and an average reverse-slip displacement of 1.2 m, rising from the hypocenter at 18 km to within 6 km of the surface.

The Loma Prieta earthquake fulfilled a long-term forecast for the rupture of this specific segment of the San Andreas fault (Lindh, 1983; Sykes and Nishenko, 1984; Working Group on California Earthquake Probabilities, 1988). The high earthquake potential assigned to this segment stemmed from its behavior in the 1906 earthquake, when the fault displacement, as measured at the surface, averaged about 1.5 m, far less than the average for the entire rupture. Estimates of the long-term slip rate along this segment of the San Andreas fault suggested that the strain released in the 1906 earthquake would be renewed in 75 to 136 years, implying that another earthquake was possible in the coming decades. With its occurrence, the Loma Prieta earthquake became the second event in 2 years to fill a recognized seismic gap along the San Andreas; the first was the 1987 Superstition Hills earthquake. The Loma Prieta earthquake also represents the third historical rupture of this segment of the San Andreas fault; the first was the October 8, 1865, earthquake, nominally assigned M=6-1/2, which also caused liquefaction-induced ground failure in San Francisco.