Five moderate (magnitude 6) earthquakes with similar features have occurred on the Parkfield section of the San Andreas fault in central California since 1857. The next moderate Parkfield earthquake is expected to occur before 1993. The Parkfield prediction experiment is designed to monitor the details of the final stages of the earthquake preparation process; observations and reports of seismicity and aseismic slip associated with the last moderate Parkfield earthquake in 1966 constitute much of the basis of the design of the experiment.
Examination of main shock and microearthquake data from the Calaveras fault during the last 20 years reveals that main shock hypocenters occur at depths of 8-9 km near the base of the zone of microearthquakes. The spatial pattern of pre-main shock microseismicity surrounding the Coyote Lake and Morgan Hill hypocenters is similar to the pattern of the post-main shock microseismicity. Microseismicity extends between depths of 4 and 10 km and defines zones of concentrated microseismicity and aseismic zones. Estimates of the fault regions which slipped during the Coyote Lake and Morgan Hill earthquakes as derived from seismic radiation coincide with zones which are otherwise aseismic. We propose that these persistent aseismic zones represent stuck patches which slip only during moderate earthquakes. From the pattern of microearthquake locations we recognize six aseismic zones where we expect future main shocks will rupture the Calaveras fault. From an analysis of historic seismic data we establish the main shock rupture history for each aseismic zone and identify two zones that are the most likely sites fo_.r the next M > 5 earthquakes. The first zone is located near Gilroy and was last ruptured by a M5.2 earthquake in 1949. The second zone is located south of Calaveras Reservoir and north of the 1988 M5.1 Alum Rock earthquake. It has not slipped seismically since at least 1903, and the size of the aseismic region is sufficiently large to sustain a M5.5 earthquake.This paper is not subject to U.S. copyright. Published in 1990 by the American Geophysical Union.
Paper number 90JB00266.central California (Figure 1). Because these concepts were developed for large earthquakes on readily defined segments with large aspect ratios (that is, the segments are much longer than they are wide), they may not necessarily describe the behavior of "heterogeneous" faults such as the Calaveras which apparently accommodate slip by aseismic slip (creep), seismic slip in large numbers of microearthquakes (M < 3), and seismic slip in infrequent moderateto-large events. Thus in assessing the likelihood of future earthquakes occurring on a given segment of a heterogeneous fault, we must determine how the segment slips, for it is only those fault patches which slip in infrequent moderateto-large shocks to which •e can reasonably hope to apply the concept of seismic gaps with well-defined recurrence intervals.Our assessment of the seismic potential of the Calaveras fault considers four aspects of the faulting process: (1) the relationship between microearthquake locations and main shock slip zones, (2) the stability of microearthquake locations over time, (3) the recurrence history of main shocks on the Calaveras fault, and (4) triggering of subsequent main shocks by post-main shock slip processes. We believe that our observations of seismicity along the Calaveras fault provide the basis for identifying two sections of the fault which are the most likely sites for the next M > 5 earthquakes.
MAIN SHOCK LOCATIONSThe location and date of occurr...
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