The 24 November 1987 Superstition Hills earthquakes occurred on the conjugate northwest-striking right-lateral Superstition Hills fault and a previously unknown northeast-striking left-lateral structure defined by a lineation of hypocenters extending from the Superstition Hills fault to the Brawley seismic zone. Master event locations of the earthquakes using Caltech-USGS seismic network data reveal the following. The first main shock (33° 4.9′ N, 115° 47.7′, h = 10.6 km, MS = 6.2, 0154 GMT), on the northeast striking structure and its foreshocks co-locate. Events on the northeast structure cluster in space and time, and some aftershocks occur in the Brawley seismic zone. The second main shock (33° 0.9′ N, 115° 50.9′ W, h = 1.9 km, MS = 6.6, 1315 GMT), on the Superstition Hills fault, was 12 hr after the first main shock and initiated at shallow depth where the two trends join. Aftershocks of the second main shock lie in a northwest trend between the Superstition Hills and Superstition Mountain faults and have a sharply defined western edge. The extent of the northwest-trend aftershocks is not coincident with surface rupture on the Superstition Hills fault. In general, the earthquakes in the northeast trend are deep and the earthquakes in the northwest trend are shallow. We compare the earthquake distribution to the distribution of crystalline basement rocks defined from a refraction study of Fuis et al. (1982). The earthquake locations and extent of aftershock activity appear to be controlled by the presence of crystalline basement rocks.

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