Fault plane solutions for the 1984 Morgan Hill, California, Earthquake Sequence: Evidence for the state of stress on the Calaveras Fault

Abstract

Fault plane solutions for 946 aftershocks of the April 24, 1984, Morgan Hill, California, M6.2 earthquake reveal a pattern of complex faulting within a 10‐km‐wide zone surrounding the Calaveras fault. The fault plane solutions fall into three groups: strike‐slip mechanisms located along the Calaveras fault with north‐northwest striking dextral slip planes nearly parallel to the fault, strike‐slip mechanisms located northeast of the Calaveras fault with north striking dextral slip planes, and reverse mechanisms located southwest of the Calaveras fault with northeast or southwest dipping slip planes. The average azimuth of P axes for aftershocks located on the Calaveras fault is N10°±1°E. In contrast, the average azimuths of P axes for aftershocks northeast and southwest of the Calaveras fault are N49°±7°E and N37°±3°E, respectively. By assuming that the earthquakes occur on preexisting cracks in response to a uniformly oriented regional stress field, we are able to infer from the observed average slip directions an orientation for σ1, the axis of maximum horizontal compression, 63°–80° from the local strike of the Calaveras fault. Such a high‐angle orientation of σ1 is incompatible with the classical Andersonian model of strike‐slip faulting but is consistent with an alternate model in which a weak Calaveras fault is driven by plate motion slightly convergent with respect to the San Andreas fault system in central California. If this inferred high‐angle uniform stress field is combined with the changes in the static stress field calculated with elastic dislocation theory for the 1984 Morgan Hill main shock, the resulting stress field predicts most of the observed spatial pattern of the aftershocks and their fault plane solutions. The orientations of the inferred aftershock slip planes are generally consstent with the fracture orientations predicted by the Coulomb failure criterion. Areas with predicted increases in the Coulomb failure function of only a few tenths of a megapascal correspond to areas of intense aftershock activity. The invariance of focal mechanisms in the vicinity of the south end of the main shock rupture before and after the main shock suggests that the magnitude of compressive stress in this area is at least 10 MPa. In addition to explaining a preponderance of the seismic observations of the aftershock sequence, the presence of a predominantly fault‐normal compressive regional stress field accounts for the contemporary development of folds, reverse faults, and topographic relief observed throughout the central California Coast Ranges.