ties, directed at improving the applicability of these maps as risk mitigation tools. This effort has paid off in greater utilization (see Section 2.7). Utilization has also been enhanced by the availability of the data and data products that went into the map calculations. An important data product for specialized applications is the disaggregation of the hazard at a map point into the individual earthquake sources (36). (Examples are shown for Knoxville, Tennessee, in Figure 3.10.) Such disaggregations identify which potential earthquakes dominate the hazard for a given site. These sources can then be used as scenarios for constructing ground-motion time histories needed to design critical facilities, conduct emergency management exercises, and estimate earthquake losses.
California has the highest levels of seismic hazard in the lower 48 states because more than 75 percent of the relative motion between the Pacific and North American plates occurs as active faulting within its borders. The cumulative fault slip at each latitude from the Mexican border to Cape Mendocino averages more than 3.5 meters each century. California seismic hazard is dominated by the San Andreas system. This “master fault” of the plate boundary has a documented history of earthquakes as large as M 8. Two of its four major segments have broken in large, historic events: a 420-kilometer segment (Cape Mendocino to San Juan Bautista) during the 1906 San Francisco earthquake and a 350-kilometer segment (Parkfield to Wrightwood) during the 1857 Fort Tejon earthquake. The frequency of large events on the 1906 segment remains poorly constrained, but paleoseismic investigations suggest a mean recurrence interval of about 250 to 300 years. The frequency of large events along the 1857 segment is better documented, varying by locality from about 50 to 300 years. These segments are separated by the “creeping section” of the San Andreas (San Juan Bautista to Parkfield), where most of the strain is taken up as aseismic creep and large earthquakes appear to be absent. According to paleoseismic data, the 200-kilometer-long southernmost segment (Wrightwood to Bombay Beach) has broken in at least four major earthquakes since A.D. 1000. The last rupture occurred circa 1680, so this segment appears to be overdue for another.
Auxiliary strike-slip faults of the San Andreas system are also hazardous. Over the past century alone, the Imperial, San Jacinto, Elsinore, and Newport-Inglewood faults of southern California have together produced more than a dozen earthquakes larger than M 6. In the San Francisco region, the San Andreas fault splays into several branches. One of the branches on the east side of San Francisco Bay, the Hayward fault, was