Aftershocks of the Coyote Lake, California, earthquake of August 6, 1979: A detailed study

Aftershock hypocenters and focal mechanism solutions for the Coyote Lake, California, earthquake reveal a geometrically complex fault structure, consisting of multiple slip surfaces. The faulting surface principally consists of two right stepping en echelon, northwest trending, partially overlapping, nearly vertical sheets and is similar in geometry to a slip surface inferred for the 1966 Parkfield, California, earthquake. The overlap occurs near a prominent bend in the surface trace of the Calaveras fault at San Felipe Lake. Slip during the main rupture, as inferred from the distribution of early aftershocks, appears to have been confined to a 14-km portion of the northeastern sheet between 4- and 10-km depth. Focal mechanisms and the hypocentral distribution of aftershocks suggest that the main rupture surface itself is geometrically complex, with left stepping imbricate structure. Seismic shear displacement on the southwestern slip surface commenced some 5 hours after the mainshock. Aftershocks in this zone define a single vertical plane 8 km long between 3- and 7-km depth. Within the overlap zone between the two main slip surfaces, the average strike of aftershock nodal planes is significantly rotated clockwise relative to the strike of the fault zone, in close agreement with the stress perturbations predicted by crack interaction models. Aftershock activity in the overlap zone is not associated with a simple dislocation surface. Space and time clustering within the entire aftershock set suggest an alternation of seismic displacement between the component parts of the fault zone. This alternation is consistent with local stress perturbations predicted by crack interaction models. We conclude that the fault structure is geometrically complex and that the displacements that occur on its component surfaces during the aftershock process dynamically interact by generating perturbations in the local stress field which, in turn, control the displacements. Table 5 is available with entire article on microfiche. Order from American Geophysical Union, 2000 Florida Avenue, N.W., Washington, D.C. 20009. Document J82-006; $1.00. Payment must accompany order.