FIGURE 4.27 Southwest-looking perspective of the Puente Hills blind thrust fault (contoured in kilometers), which ruptured to produce the M 6 Whittier Narrows earthquake of 1987 (white dot). Relocated aftershocks (red dots) delineated a north-dipping plane, and seismic reflection surveys (colored contours) showed that this fault extends toward the surface beneath the active Puente Hills anticline. SOURCE: J.H. Shaw and P. Shearer, Southern California Earthquake Center.

Fault Rupture Many important active faults ruptured most recently in prehistoric or pre-instrumental time. In the case of southern California’s great 1857 earthquake, the approximate length of the rupture could be deduced from written accounts of shaking, but the actual slip as a function of distance along the San Andreas fault was determined only by measurement of offset landforms more than a century after the event (97); these measurements implied a moment magnitude of about 7.9. Incrementally larger offsets suggested that prior events were similar in size (98). Other examples include ruptures along faults in New Zealand and Alaska, the North Anatolian fault, and the Fuyun fault (99). The erosion of dated scarps in extensional regimes, such as the Basin and Range Province of the western United States (100), permits determination of the sequence of ruptures along normal faults. Numerous paleoseismic investigations have used colluvial wedges as evidence of prehistoric ruptures. These features are preserved along dip-slip faults.

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