SEISMICITY ALONG THE SAN ANDREAS FAULT SYSTEM [c5, p120]

Sections of the San Andreas fault system stand out on seismicity maps as a network of northward-branching alignments of epicenters through the central and northern Coast Ranges and as subparallel lineations of clusters of epicenters that branch northward from the south end of the Imperial fault toward the Transverse Ranges In southern California (Figure 5.4). One of the most remarkable aspects of the seismicity pattern associated with the fault system, however, is the nearly complete absence of earthquake activity down to even the smallest magnitudes (M 1.5) along those sections of the fault that have ruptured with the largest historical earthquakes, the great (M8) 1857 and 1906 earthquakes (see Figure. 5.3 and Figure 5.4). The southernmost section of the San Andreas fault, from Indio to the Salton Sea, also lacks microseismicity, although no large earthquake has ruptured this section In the past 200 yr. These quiescent ("locked") segments of the fault stand in sharp contrast to the segments marked by persistent linear concentrations of small to moderate earthquakes.

This dual expression of the fault system evident on current seismicity maps apparently reflects fundamental differences in the long-term behavior of the respective segments. In particular, seismic activity along the "locked" segments of the main trace of the San Andreas fault may be limited to the recurrence of major earthquakes at intervals of 100 to 300 yr accompanied by immediate foreshock and aftershock sequences, and these segments may remain quiescent for most of the interevent time associated with the cycle between great earthquakes (Ellsworth and others, 1981). In contrast, those segments with persistent microearthquake activity probably seldom, if ever, rupture with great earthquakes, although they may be capable of generating earthquakes as large as M6.

Aseismic creep also characterizes and is largely confined to those fault segments along the San Andreas fault system that show persistent microearthquake activity (Wesson and others, 1973; Schulz and others, 1982). Creep is most pronounced along the central California segments of the fault system, where average creep rates match the long-term displacement rates of 32 to 34 mm/yr. Louie and others (1985) documented creep along sections of the seismically active fault segments in the Salton Trough, and Astiz and Allen (1983) documented creep along a section of the Garlock fault that is marked by microearthquake activity. The creep rates in these two areas, however, are more than an order of magnitude less than the long-term deformation rates.

In the following subsections, we consider the 1980-86 seismicity along and adjacent to the major sections of the San Andreas fault system in more detail. We begin with the Mendocino triple junction in the north and move southward, generalizing slightly Allen's (1968) subdivision of the fault system into four major sections of contrasting seismic behavior: (1) the quiescent 1906 break and subparallel branches, (2) branches forming the central California active (creeping) section, (3) the quiescent 1857 break, and (4) branches forming the southern California active section south of the Transverse Ranges.