Determination of fault planes at Coalinga, California, by analysis of patterns in aftershock locations

Abstract

The May 2, 1983, Coalinga, California, earthquake was not anticipated because it took place along a previously unrecognized fault concealed beneath an anticline, in a region that had little historic seismicity. The main shock hypocenter was at 10 km depth, and faulting did not break the surface. There has been considerable controversy about which of the nodal planes from the main shock fault plane solution was the slip plane. Conflicting results from geodetic, geologic, and aftershock data imply that slip occurred along one or the other or both nodal planes. The aftershock zone is large; several planar features within the aftershock zone indicate that faulting was complex. To delineate the planes of slip, we applied the three‐point method, a statistical technique by which event planes can be determined from aftershock locations. We obtained several important results from our study. First, we identified a number of planes using locations of aftershocks, some of which have not been previously identified from visual inspection of the aftershock locations. Second, the method allows us to choose slip planes by associating three‐point planes with fault plane solutions based on proximity and similar orientation. Finally, three of the planes identified by the method intersect near the main shock hypocenter, making it difficult to determine which plane is the main shock slip plane. Nevertheless, aftershocks during the first 24 hours after the main shock clearly define the high‐angle NE dipping nodal plane, whereas few aftershocks were located along the conjugate plane. Assuming that the aftershocks took place along the rupture zone, we conclude that slip during the main shock occurred predominantly along the high‐angle NE dipping plane from the fault plane solution for this event.