GEOLOGIC MAPPING IN SAN LUIS OBISPO COUNTY ON THE CENTRAL COAST OF CALIFORNIA

The California Geological Survey (CGS) is engaged in a multi-year project to prepare a seamless onshore/offshore geologic map and geologic database of the San Luis Obispo 30 x 60-minute quadrangle in the Central Coast region of California. So far, four 7.5-minute quadrangle geologic maps have been completed (Morro Bay South, San Luis Obispo, Pismo Beach and Atascadero). Two quadrangles are in preparation and will be released in June, 2013 (Arroyo Grande NE and Oceano). Two more quadrangles are scheduled for release in 2014 (Nipomo and Santa Margarita). This project is funded by the STATEMAP component of the National Cooperative Geologic Mapping Program.

Coastal San Luis Obispo County is a region of active crustal deformation and high seismicity caused by transpressive movement between the Pacific and North America Plates. Deformation is accommodated by a number of active and potentially active faults that extend through the area, including the Rinconada, Oceano, Los Osos, and West Huasna Faults and the Southwest Boundary Fault Zone of the San Luis Range. The active Hosgri and Shoreline Faults lie just offshore. In addition to earthquakes, parts of the region are underlain by weak bedrock subject to landslides and alluvial and estuarine deposits subject to liquefaction. CGS will use the new geologic maps to prepare Seismic Hazard Zone Maps delineating Zones of Required Investigation for Liquefaction and Earthquake-induced Landslides. The maps will also be useful for evaluating aggregate and groundwater resources and geologic conditions for City and County planning and public works projects.

In addition to CGS’s mapping activities, western San Luis Obispo County is currently the focus of extensive seismic hazard studies by Pacific Gas & Electric (PG&E) for a twenty year renewal of their license to operate Diablo Canyon Nuclear Power Plant, about 8 miles southwest of San Luis Obispo. Following detailed guidelines adopted by the Nuclear Regulatory Commission, extensive new onshore and offshore data are being generated by PG&Es geosciences staff, their consultants, the USGS and the Seafloor Mapping Lab at CSU Monterey Bay. These data will be incorporated into the San Luis Obispo 30 x 60-minute quadrangle geologic database as they become available.