An analysis of 745 aftershocks of the M7.1 Loma Prieta earthquake of 17 October, 1989 reveals a wide variety of focal mechansims. At the northwestern end of the aftershock zone earthquakes that apparently occurred off the main rupture plane exhibit mechanisms with predominantly reverse slip on planes nearly parallel to the San Andreas fault. At the southeastern end the mechanisms exhibit right-lateral motion on near-vertical planes, suggesting that these aftershocks involve slip on the San Andreas fault. Few of the aftershock mechanisms in the central zone resemble the main shock mechanism (strike N130°E, dip 70°SW, rake 140°), but instead exhibit reverse, right-lateral, left-lateral, and normal motion on planes subparallel to the main shock rupture plane. The dip of the aftershock zone is parallel to the main shock slip plane and includes the main shock hypocenter. However, the lack of agreement between of the main shock and the aftershock mechanisms suggests that few of the aftershocks occurred on the main shock slip plane. This behavior is consistent with observations of aftershock sequences for other dip-slip events and also with studies indicating that main shock rupture zones are at all other times mostly aseismic. If the stress drop for the main shock relieved most of the tectonic stress, the mechanisms could reflect the heterogeneity of the near-field stress redistribution. Alternatively the variety of the aftershock mechanisms may reflect deformation by block motion within a narrow zone adjacent to the main shock rupture plane.
