MAGISTRALE, HAROLD, Department of Geological Sciences, San Diego State University, San Diego, CA

ABSTRACT: Seismotectonics of the South Coast Region, California

Sixty years of instrumental recording and analysis provide the following observations. The Los Angeles basin has right-lateral strike-slip earthquakes on the major northwest-striking faults south of the Transverse Ranges associated with ordinary San Andreas-type motion; these faults terminate at the south side of the Transverse Ranges to produce left-lateral strike-slip earthquakes on east to northeast-striking faults associated with motion of material around the Transverse Ranges. Buried faults produce north over south thrust earthquakes; segmentation of the thrust faults is uncertain. Hypocentral parameters of earthquakes in the inner borderland area are poorly constrained. The epicenters form crude lineations along the predominantly right-lateral strike-slip offshore faults, but th two most significant larger recent earthquakes in the region were not right-lateral strike-slip events. Recent earthquakes near San Diego occurred mostly as swarms on northeast-striking faults where those faults intersect the Rose Canyon fault zone. East of San Diego much of the Elsinore fault is double stranded with the strands defined by earthquake clusters caused by geometric complications. The right-lateral northwest-striking faults of northwesternmost Baja California are well expressed in the seismicity. The connection of these faults to faults north of the border is not clear. The Agua Blanca fault zone has remarkably few earthquakes. The rate of earthquake activity around the San Diego region increased in 1984, and in 1986 in the Los Angeles basin, indicating a large length scale of the cause of the increase.

AAPG Search and Discovery Article #90992©1993 AAPG Pacific Section Meeting, Long Beach, California, May 5-7, 1993.