FOCAL MECHANISMS AND
TRANSFORM-BOUNDARY KINEMATICS [c5, p136]

Focal mechanisms of selected earthquakes recorded in California from 1933 through 1988 are shown in figure 5.11A, and the corresponding source parameters are listed in Table 5.2. and Table 5.3. Primary considerations in the selection of these events were (1) size-larger events were chosen where available, because they represent large-scale processes along major boundaries; (2) date of occurrence-the quality of data for focal-mechanism determinations improved significantly during the mid-1970's; and (3) location-some larger events were omitted because they were redundant in terms of mechanism and location, and some smaller events were included because they occurred in regions of significant seismicity where no larger events were available. Most focal mechanisms were determined from first arrivals at stations in the northern and southern California seismic networks. The evolving capability of these networks for such studies is reflected in the number of stations in the networks, summarized in Table 5.1. Fault-plane solutions for the few large earthquakes on the list before the mid-1970's were supported by observations from stations outside the California networks.

Since the mid-1970's, focal mechanisms have been determined for only a fraction of the events for which adequate local first-motion data were available. Therefore, in addition to the three considerations listed above, there was a fourth, the interests of the investigators who analyzed the data. These interests included topical studies of large earthquakes and aftershock sequences, analyses of regional travel times on the basis of M>=4 earthquakes, and a special study of the focal mechanisms of earthquakes on or near the San Andreas fault in southern California (Jones, 1988).

Focal mechanisms discussed in the first two subsections below are for earthquakes in the contiguous Coast Ranges-Transverse/ Peninsular Ranges-Mojave Desert region associated with the principal seismic expression of the San Andreas fault system, where the seismic networks are best developed. Outside that region, except for the Cape Mendocino area and the vicinity of Long Valley caldera, the few well-determined focal mechanisms that are available provide only limited information on tectonic processes.