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Earthquakes and associated tectonics in a part of coastal central California

DEPARTMENT OF GEOLOGY AND GEOPHYSICS THE UNIVERSITY OF CONNECTICUT, STORRS, CONNECTICUT 06268
SEISMOGRAPHIC STATION UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA 94720

Abstract

Seismicity along a corridor 65 km wide and 100 km long across the central Coast Ranges of California, southwest from the San Andreas fault zone, is used to determine tectonic properties of the three constituent provinces: a region of horizontal shear along the San Andreas fault zone; a moderately active belt of compressional faulting along the coastline; and an intermediate low seismicity zone. Fault-plane mechanisms of 33 earthquakes in the corridor indicate that the three provinces are distinguished by characteristic tectonic features: directions of P (compression) axes; fault types; and fault-slip directions. Each interprovince transition appears to be less than 12 km wide. Across the boundary between the San Andreas fault zone and the low seismicity region, just SW of the San Andreas zone, P axes are rotated about 45°, and the dominant right-lateral slip directions also change by about 45°. Across the boundary between the low seismicity and the compressive regions, near the Nacimiento fault, a small rotation occurs in P axes and slips change from dominantly horizontal shear to the NE to oblique reverse to the SW. A remarkable result is that almost all inferred faults in the corridor can be interpreted as having moderate to steeply NE dipping planes along which horizontal components of slip are right-lateral and vertical components have the NE side moving up. The base of the seismogenic zone is about 12 km deep throughout the corridor. It is suggested that the seismicity and stress patterns in each of the three provinces reflect the relative strengths of upper crustal rocks in the seismogenic zone.

The largest recorded earthquake (M_L = 6.0) in the corridor since 1941 is the Bryson earthquake of 1952. A reanalysis of the epicenter was made, based on the "calibration event" technique, in which readings of several newly collected seismograms were included; the revised location is just SW of the Nacimiento fault zone, about 6 km SW of Bryson. Displacement is right-lateral along a 55°NE dipping plane that strikes N55°W, in conformance with San Andreas-type faulting.

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