McNALLY, KAREN C., Univ. California Santa Cruz, Santa Cruz, CA; and DEBRA S. STAKES, MBARI, Moss Landing, CA

Abstract: Implications of the San Gregorio Fault Zone Seismicity for Monterey Bay Coastal Hazard Assessment

The relationship between the seismicity of the young San Gregorio (SGFZ) and older, perhaps reactivated Monterey Bay (MBFZ) fault zone of Central California is not well understood. In 1998, nine portable seismographs have supplemented five seafloor seismometers and the permanent USGS network in a cooperative, inter-institutional deployment. These investigations have been used to upgrade the seismicity catalog of events recorded since 1926 by the USGS using relocations and a new crustal velocity model to obtain well-constrained relocations and fault mechanisms. Both the fault mechanisms and the depth distribution of seismicity through the west and central portion of the bay indicate that the San Gregorio fault dips at 50-70° downward to the east. Thrust faulting mechanisms are found for deeper (h=8-12 km) easterly events along the SGFZ and oblique right-slip with strikes of ~N20° to 40°W on shallower westerly events. Earthquakes along the MBFZ indicate vertical faults with right lateral strike-slip motion at N50°W. No events were located on the southern SGFZ along Carmel Canyon. These results are particularly important for assessing earthquake hazards on the Monterey area in light of the reclassification of the San Gregorio fault as a Class A fault with a magnitude potential of 7.3. Furthermore, the dip of a major fault necessitates special design consideration according to the updated Uniform Building Code (UBC) requirements. Most importantly, these new results allow reconsideration of the role of the MBFZ in the early tectonic history of the Monterey Bay through slivering of the Salinian granitic basement.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California