Aftershocks and Preearthquake Seismicity

Although primary surface faulting was mapped for nearly 30 km, \naftershocks extended in a complex pattern more than 100 km along \nthe trend of the Imperial fault. A first-motion focal mechanism for the \nmain shock is consistent with right-lateral motion on a vertical fault \nstriking N. 42° W., in agreement with the strike of the Imperial fault \nwithin the limits of resolution. There is evidence that conjugate faulting \non a buried complementary northeast-trending structure occurred \nat the north limit of displacement on the Imperial fault near Brawley, \nCalif. This faulting was apparently initiated at the time of a magnitude \n5.8 aftershock 8 hours after the main shock. A line of epicenters \nextending along the trend of the San Andreas fault nearly 100 km into \nthe eastern Imperial Valley was noted during the aftershock sequence, \nin an area recognized as notably aseismic during the preceding \n5 years. The main shock was preceded by a 3-month period of \nsignificantly reduced seismicity affecting the central Imperial Valley. \nAlthough three small events near the incipient epicenter during this \ninterval may be deemed foreshocks, no distinct foreshocks immediately \nbefore the main shock were observed.