Foreshock, Main Shock, and Larger Aftershocks of the Borrego Mountain Earthquake

Abstract

The Borrego Mountain earthquake, magnitude 6.4, occurred at 02:28:59.1 G.m.t. on April 9, 1968 and has been assigned a hypocenter at 33°11.4' N., 116°07.7' W., h=11.1 km. The focal-mechanism solution indicates right-lateral slip on a fault striking N. 48° W. and dipping 83° NE., which is consistent with the field observations of faulting and the regional tectonic framework. A single foreshock of magnitude 3.7 preceded the main shock by one minute, but no other precursory activity has been identified. During the year following the event, 135 aftershocks of magnitude 3.0 and greater have been identified and located, outlining a broad zone of activity centered on but displaced 2-3 km northeast of the 33-km-long surface rupture on the Coyote Creek fault. Fracturing at depth during the aftershock period evidently occurred throughout the width of the San Jacinto fault zone, but initial surface faulting was localized along the Coyote Creek fault at the zone's southwestern margin. The area of aftershock activity enlarged progressively with time, and the region of the original epicenter became relatively inactive late in the aftershock period, leading to a doughnut-shaped epicentral distribution of late aftershocks. Inasmuch as the epicenter of the main shock was roughly midway along the zone of aftershock activity, the faulting presumably was bilateral. This kind of faulting is unusual in California.