DISCUSSION [c5, p144]

The Pacific plate moved northwestward with respect to the North American plate by 300 to 400 mm during the 7-yr interval 1980-86. Earthquakes occurring along the San Andreas fault system during the same interval, however, accommodated only a small fraction of this relative plate motion. Only four earthquakes of M>5 occurred along branches of the San Andreas fault system during 1980-86: the pair of M=5.9-5.3 Livermore earthquakes (events 29, 30, Figure 5.10A) on the Greenville fault (Jan. 24-27, 1980), the M=6.2 Morgan Hill earthquake (event 33) on the Calaveras fault (Apr. 24, 1984), and the M=5.6 North Palm Springs earthquake (event 80) on the Banning segment of the San Andreas fault (July 8, 1986). Each of these moderate San Andreas earthquakes ruptured fault segments limited to 20 to 30 km in length, with average displacements over the respective rupture surfaces of 100 to 200 mm (see Hartzell and Heaton, 1986). As is typical of earthquakes along the San Andreas fault system, each of these events involved nearly pure right-lateral strike-slip displacement coincident with the local strike of the fault. As is also typical of San Andreas earthquakes, slip on the first three events occurred on near-vertical fault planes with a northwestward to north-northwestward strike. The North Palm Springs earthquake, which ruptured a section of the east-west-striking Banning fault in the structurally complex San Gorgonio bend in the fault system at the southern margin of the Transverse Ranges, represents an important deviation from typical San Andreas earthquakes. Although its displacement was dominantly right-lateral strike slip, it occurred along a plane that dips 45° N. (Jones and others, 1986) and included a small but significant component of reverse slip (Mendoza and Hartzell, 1988). With the arguable exception of the North Palm Springs earthquake (arguable because of the complex section of the fault system in which it occurred), however, none of these M>5 earthquakes ruptured the main trace of the San Andreas fault. Indeed, the two most recent M>5 earthquakes to clearly do so were the M=6 Parkfield earthquake of 1966 (Bakun and McEvilly, 1984) and the M=7.1 Loma Prieta earthquake of 1989 (see chap. 6).

Thus, aside from the displacement accommodated by steady aseismic slip at a rate of 32 to 37 mm/yr along the creeping section of the fault in central California, most relative plate motion across the San Andreas transform boundary during this 7-yr interval accumulated as elastic shear strain. Accordingly, the earthquakes plotted in figures 5.3 through 5.9 are symptomatic of accumulating strain along the San Andreas fault system rather than of effective strain release. The latter requires rupture with a major earthquake along one of the locked stretches of the San Andreas fault.