Aftershocks and source characteristics of the 1969 Coyote Mountain earthquake, San Jacinto Fault zone, California

abstract The Coyote Mountain earthquake (ML = 5.8) occurred as an abrupt northwesterly extension of the aftershock zone of the ML = 6.4 Borrego Mountain event of a year earlier. Locations of 127 aftershocks in the Coyote Mountain area outline a zone of scattered activity about 10 km across, with most hypocenters clustered at depths between 10 and 13 km. Aftershock activity was unusually low for an earthquake of this magnitude. Nine of the larger aftershocks showed first-motion patterns consistent with right-lateral strike-slip movement on northwest-trending faults. The mainshock focal mechanism is similar, but with some dip-slip motion also indicated. Teleseismic P-wave spectrum measurements suggest a relatively small source dimension and large stress drop for the mainshock. The unusual characteristics of this sequence may have been the result of a de-stressing of the upper 10 km of this section of the San Jacinto Fault zone by two earlier events greater than magnitude 6 and a concentration of stresses below that depth in a small area at the northwest end of the Borrego Mountain zone.