REGIONAL SEISMICITY AND THE SAN ANDREAS TRANSFORM BOUNDARY [c5, p116-120]

Dickinson (1981), among others, emphasized that the San Andreas fault system and the San Andreas transform boundary are not strictly equivalent structures. The latter is more general, incorporating, for example, the concept of temporal evolution of Pacific-North American plate interaction and the recognition that the faults accommodating most of the plate motion have changed over time. In this section, we emphasize that, although great earthquakes along the San Andreas fault system currently accommodate most of the relative plate motion, plate interactions along the transform boundary influence deformation of the brittle crust over a much broader region.

The breadth of the seismicity pattern in California and western Nevada (Figure 5.4) suggests the lateral extent of deformation associated with the San Andreas transform boundary. Indeed, it corresponds closely to the zone of distributed shear between those plates as interpreted by Ward (1988) from more than 5 years of very long baseline interferometry (VLBI) observations at 20 Western United States stations from 1982 through 1987. Figure 5.4 includes all M>=1.5 events recorded by the telemetered seismic networks in Figure 5.2 during the 7-yr interval 1980-86, as well as events recorded by adjacent telemetered networks in Nevada (see Rogers and others, in press). Although details within this seismicity pattern fluctuate from year to year, the broad aspects of the pattern have remained stable for the entire historical record of earthquake occurrence in California (see chap. 6; Ellsworth and others, 1981; Hill and others, in press; Hutton and others, in press).

In outline, the seismicity pattern for California and western Nevada forms a hollow ellipse with its long axis nearly coincident with the transform boundary. This pattern is widest across central California, where it approaches nearly a third of the 1,100-km length of the transform boundary, from the Mendocino triple junction in the north to the head of the Gulf of California at the south. Extended alignments of epicenters within this pattern suggest a coarse structural fabric linking the broad distribution of earthquakes to the transform boundary. Seismicity along the San Andreas fault system itself stands out as a series of subparallel, northwest-trending lineations extending the length of coastal California. The alignment of epicenter clusters along the east side of the Sierra Nevada in eastern California branches northward from the south end of the San Andreas fault system in the Salton Trough only to bend back toward the north terminus of the San Andreas fault system at the Mendocino triple junction in northern California. The Sierra Nevada-Great Valley and western Mojave Desert blocks form a broad quiescent region between the San Andreas and eastern California seismicity bands. In southern California, pronounced transverse seismicity belts coincident with the southern margin of the Sierra Nevada and Transverse Ranges, respectively, span this otherwise-quiescent region. A weaker, more diffuse seismicity belt near lat 37° N. spans the Sierra Nevada-Great Valley block in central California, forming a visual, if not structural, link between the San Andreas fault system and the dense cluster of epicenters in eastern California. This major cluster in the eastern Sierra Nevada represents an episode of intense earthquake activity in Long Valley caldera and vicinity that began In 1978 and has persisted to the present (Van Wormer and Ryall, 1980; Hill and others, 1985b).

The displacement pattern associated with earthquakes throughout California and western Nevada is broadly consistent with deformation under a coherent, regional stress field dominated by plate-boundary interaction along a northwest-striking, dextral transform boundary (Hill, 1982). Strike-slip focal mechanisms with right-lateral slip on northwest- to north-northwest-striking planes, for example, are common through most of the region. Regional variations within this pattern include a tendency toward normal slip on northerly-striking planes along the western margin of the extensional Basin and Range province in eastern California, and toward reverse slip on easterly-striking planes in the Transverse Ranges of southern California. Compressional deformation perpendicular to the San Andreas fault within the Coast Ranges, however, represents an important deviation from this regional pattern.