FIGURE 6.5 Schematic diagram showing inferred motion on the San Andreas fault during the Loma Prieta earthquake. Along the southern Santa Cruz Mountains segment of the fault, the Pacific and North American plates meet along an inclined plane that dips approximately 70 degrees southwest. Plate motion is mostly accommodated by about 6.2 feet of slip along a strike of this plane and by 4.3 feet of reverse slip, in which the Pacific plate moves up the fault and overrides the North American plate. The amounts of fault slip and vertical surface deformation were determined from geodetic data. (Modified from a figure by M. J. Rymer. Reprinted from USGS, 1989, p. 6.)

deep underground (see Figure 6.5). The fault slip itself did not reach the surface to produce a trace, but surface waves achieved a magnitude of 7.1. As reported by the USGS, the ground shaking collapsed sections of the Bay Bridge and Interstate 880 (in Oakland); began fires in San Francisco's Marina district; ultimately caused 62 deaths and 3757 injuries; destroyed 963 homes; damaged another 18,000, leaving 12,000 temporarily homeless; and ultimately will have cost some $10 billion. These statistics rate it as one of America's most serious natural disasters. As such, it raises questions about how long-term forecasting fits into the American political infrastructure, since in the context of work done by Ellsworth, Agnew, Sieh, and many others, the USGS in the Science article classified Loma Prieta as "an anticipated event."

Californians have felt thousands of earthquakes over the decades,



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