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[c6, p154-155]

Motion between the North American and Pacific plates at the latitude of the San Andreas fault produces a broad zone of large-magnitude earthquake activity extending more than 500 km into the continental interior. The San Andreas fault system defines the western limits of plate interaction and dominates the overall pattern of seismic strain release. Few of the M>=6 earthquakes that have occurred in the past 2 centuries were located on the San Andreas fault proper, an observation emphasizing the importance of secondary faults for both seismic-hazard assessment and tectonic processes.

Between Punta Gorda on the northern California coast and the head of the Gulf of California, 1,350 km to the southeast, lies the active transform boundary that forms the modern San Andreas fault system ( Figure 6.1). Dextral motion between the North American and Pacific plates along this system is accommodated within an elongate zone, broadening from about 100 km at its north end to about 300 km in southern California. The San Andreas fault proper hugs the east side of this zone at its south terminus and gradually migrates across the zone, lying on the west edge of the zone at its north terminus. The San Andreas fault system transmits about three-fourths of the relative motion across the plate boundary, as shown by various geologic and geodetic evidence. Much of this motion is stored elastically in the upper crust along the major faults in the system, ultimately to be released in large plate-boundary earthquakes. These large earthquakes and their implications for the mechanics of North American-Pacific plate interactions are the subject of this chapter.

Earthquake activity in California and Nevada at the latitude of the San Andreas fault extends well beyond the confines of the San Andreas system ( Figure 6.2). In the past century alone, only about half of the M>=6 activity has fallen within the San Andreas system; of the rest, half is associated with the western Basin and Range province, and the other half with the Mendocino triple junction and the Gorda plate. Although activity in the latter region reflects the tectonics of the triple junction and the collision of the Gorda plate with the North American plate, seismicity east of the San Andreas system along the east flank of the Sierra Nevada and in the Basin and Range province reflects the incomplete accommodation of plate motion along the San Andreas fault system. A significant proportion of this "missing" motion occurs in the Basin and Range, the seismicity of which plays an integral role in the tectonics of the plate boundary.