STRESS TRANSFER DURING 3 SEQUENTIAL MODERATE EARTHQUAKES ALONG THE CENTRAL CALAVERAS FAULT, CALIFORNIA

Seismic and geodetic information on three sequential moderate earthquakes, the 1979 M5.9 Coyote Lake, the 1984 M6.2 Morgan Hill, and the 1988 M5.1 Alum Rock, along the central Calaveras fault, is used to examine how tractions on fault segments are transferred from one fault segment to the other. Geodetic data obtained before and after each earthquake are inverted for slip at depth. The perturbed stress field induced by each earthquake is calculated using 3-D elastic dislocation theory. Although only long-wavelength features of the slip distributions are well resolved, the model suggests that the distribution of slip is nonuniform. The largest slips tend to occur in regions having relatively few aftershocks or near the hypocenter. According to the model, the Coyote Lake earthquake induced 0.1-1 bar (0.01-0.1 MPa) shear stress across the northern part of the Morgan Hill segment and only about 0.1 bar at the hypocenter of the Morgan Hill mainshock. A shear stress of 0.5 bar was induced by the Morgan Hill earthquake at the hypocenter of the Alum Rock mainshock. The average stress drops for the Coyote Lake, Morgan Hill, and Alum Rock earthquakes are estimated to be 30, 25, and 15 bars, respectively. Thus the earthquake-induced shear stress of 0.1-0.5 bar at the hypocenter of the next earthquake is only a few percent of the total stress drop and cannot be solely responsible for the occurrence of the sequential earthquakes. However, the induced shear stress can trigger the adjacent earthquakes, in the sense that adjacent earthquakes would have occurred at a later time if there was no such coseismic stress transfer. It is inferred that fault geometry may influence the local stress field, thereby initiation and termination of the rupture, but not the regional stress accumulation.