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the source of a large (M about 6.8) earthquake in 1868. In 1990, the Working Group on California Earthquake Probabilities estimated that the chance of an M = 6.7 earthquake on the Hayward-Rogers Creek fault system before 2030 is approximately 36 percent. The same group concluded, in a major reassessment, that the chance of an earthquake larger than M 6.7 in the Bay area over the next 30 years was about 70 percent (37). However, it is uncertain how much of the slip along the northern portion of the Hayward fault is accommodated by aseismic creep, rather than by large earthquakes (38).
The S-curve in the San Andreas fault north and east of Los Angeles is an example of a restraining bend that results in compressive deformation taken up by auxiliary reverse faults. These have produced damaging thrust-type earthquakes, including the 1952 Kern County (M 7.5), 1971 San Fernando (M 6.7), and 1994 Northridge (M 6.7) events. The latter occurred on a blind thrust (i.e., a shallow-dipping reverse fault that does not crop out at the Earth’s surface). Blind thrusts are common in compressional regimes where thick sections of soft sediments cover active faulting in basement rocks. Rather than propagate to the surface, the faulting de-forms the overlying sediments into distinctive fold structures (Figure 3.11). Because they lack surface scarps, blind thrusts are more difficult to identify, and they cannot be studied with paleoseismic trenching techniques; rather, their geometry and slip rates must be inferred using structural and geomorphic methods supplemented with seismologic and geodetic data. From this type of neotectonic analysis, geologists have estimated that the blind thrust fault that produced the 1994 Northridge earthquake is slipping at about 1.5 millimeters per year and that the recurrence interval for such events is on the order of 1700 years (39). In 1995, the Southern California Earthquake Center (SCEC) published a major assessment of earthquake hazards that, for the first time, merged results from geodetic measurements, neotectonic slip rates, and historic seismicity into a probabilistic seismic hazard analysis for southern California (40). That report concludes that earthquakes of M 7.2 to 7.6 have occurred and will recur in the Los Angeles region to relieve the contractional strain accumulating across the “Big Bend” of the San Andreas.
The eastern Mojave shear zone, which splays off the San Andreas system just east of the Big Bend, accommodates a portion of the Pacific-North American plate motion (0.7 to 1.2 meters per century) (41). It has produced a series of major earthquakes during the last decade, including the 1992 Landers (M 7.3) and 1999 Hector Mine (M 7.1) earthquakes. Along the eastern side of the Sierra Nevada, the Mojave shear zone has also been a source of high seismicity throughout history, including the 1872 Owens Valley earthquake (M 7.6). Other fault systems that contribute to seismic hazards in California include the Mendocino fracture zone