Recent seismicity in the San Fernando region and tectonics in the west-central transverse ranges, California

Abstract

Since the San Fernando earthquake, February 1971, the density of the southern California seismic array has increased by an order of magnitude. The enhanced coverage provides an ideal setting for the study of the long-term seismicity of the San Fernando aftershock zone and adjacent regions. Most of the recent activity within the San Fernando zone has been thrust faulting at depths shallower than and south of the main shock. One event located slightly deeper than and several kilometers north of the main event suggests shear along a flat plane. Transport of the upper block is south. This event is very similar to another deep, M_L = 4.5, earthquake 30 km west of San Fernando. If these events are typical of midcrustal deformation, the west-central Transverse Ranges may be a form of decollement. A rapid increase in seismicity (M_L ≧ 3.0) in the region south of San Fernando suggests an increase in regional strain that either was contemporaneous with or immediately followed the San Fernando earthquake.