Aftershocks and Preearthquake Seismicity

Abstract

Although primary surface faulting was mapped for nearly 30 km, aftershocks extended in a complex pattern more than 100 km along the trend of the Imperial fault. A first-motion focal mechanism for the main shock is consistent with right-lateral motion on a vertical fault striking N. 42° W., in agreement with the strike of the Imperial fault within the limits of resolution. There is evidence that conjugate faulting on a buried complementary northeast-trending structure occurred at the north limit of displacement on the Imperial fault near Brawley, Calif. This faulting was apparently initiated at the time of a magnitude 5.8 aftershock 8 hours after the main shock. A line of epicenters extending along the trend of the San Andreas fault nearly 100 km into the eastern Imperial Valley was noted during the aftershock sequence, in an area recognized as notably aseismic during the preceding 5 years. The main shock was preceded by a 3-month period of significantly reduced seismicity affecting the central Imperial Valley. Although three small events near the incipient epicenter during this interval may be deemed foreshocks, no distinct foreshocks immediately before the main shock were observed.