COMPLEXITIES OF THE FAULT SYSTEM
SEGMENTATION [c1, p8-12]

Different behavior patterns along different parts of the fault began to be recognized when Steinbrugge and Zacher (1960) found that continuous slip or "creep" occurred along the fault in central California. Allen (1968) delineated five different regions along the San Andreas fault, three displaying seismic activity and two displaying little or no current activity. Wallace (1970) described in more detail major differences in behavior along different segments.

Large segments of the fault system that are believed to produce damaging earthquakes are illustrated in Figure 1.6a and Figure 1.6b, which also show an evaluation of the probabilities of earthquakes of different magnitude along these major segments of the San Andreas fault and three of its branches, the Hayward, San Jacinto, and Imperial faults. Both historical seismicity and paleoseismic evidence of large earthquakes and slip rates that characterize these different segments have been used in this assessment (Working Group on California Earthquake Probabilities, 1988).

Segmentation at a scale of a few to several kilometers is shown in Figure 1.7, and a cumulative plot of the segment lengths in Figure 1.8. The maximum length of these segments is about 18 km, but a more significant upper range appears to be near 10 km. These mappable segments are based on relatively fresh geomorphic features considered to be "young," that is, probably less than several thousand years old.

Both left (Figure 1.7)- and right-stepping echelon patterns are displayed, and combinations or transition zones are also present. Complex patterns are common.