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DISCUSSION
[c6, p169-170]

The spatial distribution of large earthquakes during the past 2 centuries defines the San Andreas fault system as a 100-to 300-km-wide zone containing numerous active faults in addition to the San Andreas fault proper ( Figure 6.1). Except for the two largest events, the great 1857 and 1906 earthquakes that together ruptured two-thirds of the total length of the San Andreas fault, large earthquakes are conspicuously absent along the master fault itself. Although these two great earthquakes account for half of the seismic-strain release since 1769, most of the rest occurs on other, smaller elements of the fault system. Major historical events on these secondary faults, such as the 1927 Lompoc and 1952 Kern County earthquakes, serve to define the boundaries of the San Andreas system. Their mechanisms differ significantly from right-lateral strike slip parallel to the plate-motion vector and illustrate the diversity and complexity of seismic-strain release within the plate-boundary zone.

Over the timespan of the historical catalog, the most enduring characteristic of the earthquake distribution may be the spatial clustering of activity at specific localities along the plate boundary. Notable hotspots include the Cerro Prieto, Imperial, San Jacinto, and Calaveras faults, all of which are major branches of the San Andreas fault, and the Parkfield segment of the San Andreas fault in the transition zone between the 1857 rupture and the 150-km-long central, creeping segment of the fault. In each of these areas, the seismic activity coincides with these high-slip-rate faults (1-3.5 cm/yr), and in some places it clearly represents recurrent rupture of the same segment of the fault. At greater distances from the San Andreas fault, the historical events (or sequences) tend to represent isolated occurrences on slower moving faults. Thus, although the overall seismicity spans the broad plate-boundary zone, seismic-strain release over the past 2 centuries correlates with the local rate of fault movement.

In general, the locations of historical earthquakes resemble the overall distribution of microearthquake activity, despite more than six orders of magnitude difference in average seismic moment (Figure 6.13; see chap. 5). One important difference between the distribution of large and small earthquakes is the virtual absence of smaller events along the San Andreas fault segments that ruptured in 1857 and 1906. Similarly, seismic activity is distinctly absent on the potentially dangerous segment between the 1857 break and the Imperial Valley. Except for the central, creeping segment, where numerous small events occur, the San Andreas fault is almost completely aseismic during the long intervals between its rupture in major earthquakes (see Figure 5.6) and Figure 5.9).

This inverse correlation between the source regions of large earthquakes and the distribution of smaller events can also be observed for smaller main shocks. Recent studies of the rupture dynamics of M 6 events occurring within seismically active regions indicate that the rupture zones of these events are similarly aseismic, with smaller events occurring predominantly outside the slip surface, even during the aftershock sequence (Reasenberg and Ellsworth, 1982; Hartzell and Heaton, 1986; Mendoza and Hartzell, 1988). Thus, the sites of future large earthquakes cannot be identified on the basis of minor seismicity alone.