SEISMICITY AND CRUSTAL STRUCTURE STUDIES OF SOUTHERN CALIFORNIA: TECTONIC IMPLICATIONS FROM IMPROVED EARTHQUAKE LOCATIONS (FAULTS, CONTINENTAL BORDERLAND)

The 5.1 M(,L) Santa Barbara earthquake of 13 August 1978 was located 3 km southeast of Santa Barbara at a focal depth of 12.7 km. The temporal-spatial development of the aftershock zone may indicate that the initial rupture plane was considerably smaller than that of the eventual aftershock zone. The aftershock hypocenters outline a nearly horizontal plane (dipping 15(DEGREES) or less) at 13-km depth and the preferred focal mechanism indicates north-over-south thrusting. These observations are consistent with a tectonic model in which much of the slip occurred on a nearly horizontal plane, and this earthquake may be taken as evidence for mid-crustal horizontal shearing in the western Transverse Ranges.;Data from a large quarry explosion on Catalina Island were utilized to derive a 3-layer Continental Borderland velocity structure to improve the locations of the 1981 Santa Barbara Island earthquakes. The Santa Barbara Island earthquake (5.3 M(,L)) occurred on September 4, 1981. Aftershocks exhibited a clear northwest-southeast alignment that coincides with the submarine escarpment of the Santa Cruz-Catalina fault and was consistent with focal mechanisms. The aftershock zone was initially small, but grew bilaterally with time. This seismic activity suggests strike-slip motion on the Santa Cruz-Catalina fault, with the Santa Monica basin and Catalina Island behaving as a coherent block being displaced southeastward from the Transverse Ranges.;To further test the decollement hypothesis, Caltech catalog locations were reviewed to determine the depth distribution of earthquakes in the Transverse Ranges. The seismogenic zone is thickest along the southern front of the Transverse Ranges and is thinnest in the southern Mojave Desert and at the east end of the Transverse Ranges. The seismicity of the western Transverse Ranges is typified by north-dipping planar structures and the eastern Transverse Ranges are typified by pervasive seismicity extending down to the floor of the seismogenic zone. The San Bernardino Mountains are underlain by a well-defined bottom of the seismogenic zone that dips southward from 5-km depth under the Mojave Desert to 15-km depth where it intersects the San Andreas fault.