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

The Borrego Mountain earthquake, magnitude 6.4, occurred \nat 02:28:59.1 G.m.t. on April 9, 1968 and has been assigned \na hypocenter at 33°11.4' N., 116°07.7' W., h=11.1 km. The \nfocal-mechanism solution indicates right-lateral slip on a fault \nstriking N. 48° W. and dipping 83° NE., which is consistent \nwith the field observations of faulting and the regional tectonic \nframework. A single foreshock of magnitude 3.7 preceded the \nmain shock by one minute, but no other precursory activity has \nbeen identified. During the year following the event, 135 aftershocks \nof magnitude 3.0 and greater have been identified and \nlocated, outlining a broad zone of activity centered on but displaced \n2-3 km northeast of the 33-km-long surface rupture on \nthe Coyote Creek fault. Fracturing at depth during the aftershock \nperiod evidently occurred throughout the width of the \nSan Jacinto fault zone, but initial surface faulting was localized \nalong the Coyote Creek fault at the zone's southwestern \nmargin. The area of aftershock activity enlarged progressively \nwith time, and the region of the original epicenter became relatively \ninactive late in the aftershock period, leading to a \ndoughnut-shaped epicentral distribution of late aftershocks. \nInasmuch as the epicenter of the main shock was roughly midway \nalong the zone of aftershock activity, the faulting presumably \nwas bilateral. This kind of faulting is unusual in \nCalifornia.