INTRODUCTION [c5, p115-116]

Tens of thousands of small earthquakes occur in California each year, reflecting brittle deformation of the margins of the Pacific and North American plates as they grind inexorably past one another along the San Andreas fault system. The deformational patterns revealed by this ongoing earthquake activity provide a wealth of information on the tectonic processes along this major transform boundary that, every few hundred years, culminate in rupture of the San Andreas fault in a great (M of about 8) earthquake.

Earthquake activity along the San Andreas fault system ( Figure 5.1) reflects brittle accommodation of the crust to the relative motion along the dextral transform boundary between the Pacific and North American plates (see chap. 3). Great (M 8) earthquakes along the main branch of the San Andreas fault accommodate most of this relative plate motion. These plate-boundary earthquakes rupture the entire 15- to 20-km thickness of the brittle crust with right-lateral offsets of as much as 10 m extending for several hundred Kilometers along the fault trace, and they recur along a given section of the San Andreas fault at intervals of several hundred years (Sieh, 1981; Sieh and Jahns, 1984). The 1857 Fort Tejon earthquake in southern California and the 1906 San Francisco earthquake in northern California are only the two most recent such great events along the San Andreas fault (see chap. 6).

In this chapter, we focus on the smaller, more frequent earthquakes that dominate the seismicity within the San Andreas fault system between recurrences of great, plate-boundary events. Although this interevent seismicity contributes only marginally to relative plate motion, it is symptomatic of processes underlying the earthquake cycle. In particular, the wealth of data generated by frequent smaller events provides important clues to the seismotectonic fabric, kinematics, and state of stress within the brittle crust and, ultimately, to the seismogenic processes common to earthquakes of all sizes within the San Andreas fault system. This persistent, inter-event seismicity also captures widespread popular interest because it includes most of the felt earthquakes in California (earthquakes of M3 can be locally felt), and the larger of these interevent earthquakes (M=6-7) can cause extensive damage and loss of life when they strike near major population centers.

We examine here the detailed patterns of earthquake occurrence along the San Andreas fault system recorded by the combined northern and southern California seismograph networks for the 7-yr interval 1980-86. These networks, which had evolved to much their present configuration ( Figure 5.2) by mid-1979, enable uniform detection and location of all earthquakes of M1.5 throughout the San Andreas fault system and of M2 throughout most of California. The telemetered seismic stations in the combined networks approach 550 in number. Signals from the 300 central and northern California stations are recorded and processed at the U.S. Geological Survey's (USGS) offices in Menlo Park; signals from the 250 southern California stations are recorded and processed under a joint USGS-California Institute of Technology (Caltech) effort on the Caltech campus in Pasadena. These dense telemetered networks overlie regional seismic networks operated by the University of California, Berkeley, and Caltech that provide records of M3 earthquakes in northern and southern California, respectively, from the early 1930's onward (Table 5.1; Hileman and others, 1973; Bolt and Miller, 1975).

After a brief overview of the San Andreas fault system in the context of a broad transform boundary, we focus on the three-dimensional distribution of earthquakes along the San Andreas fault system itself on the basis of a series of detailed seismicity maps and cross sections for the years 1980-86. We then review selected focal mechanisms for the larger of these earthquakes as a guide to the kinematics of seismogenic deformation along the fault system. Finally, we discuss the implications of these seismicity patterns in terms of current tectonic processes along the transform boundary.

The seismicity maps and cross sections in this chapter, which form the core of our presentation, are largely self-explanatory. The following points, however, deserve special emphasis:

1. The reliability of hypocentral locations correlates closely with the local density of the seismograph network ( Figure 5.2 ). Relative epicentral locations are better than +/- 0.5 km for earthquakes within the densest sections of the network (corresponding focal depths are better than +/- 1.0 km). Relative locations may be uncertain by several kilometers or more, however, for earthquakes occurring beyond the margins of the network.

2. All cross sections have a 2 x vertical exaggeration as a means of illustrating patterns in the depth distribution of earthquakes beneath profiles that are many times longer than the limited range of focal depths (less than 15 km along most of the fault system).

3. Hypocentral locations are plotted using small circles that scale only weakly with magnitude, to better emphasize detailed spatial structures within the seismicity patterns.

4. The locations of the most commonly used place names and faults in this chapter are shown in Figure 5.3 (see front of book for a more complete map).