High-Resolution Seismic-Reflection and Marine Magnetic Data Along the Hosgri Fault Zone, Central California

Introduction

In June 2008, the U.S. Geological Survey (USGS) collected seismic-reflection and marine magnetic data on the central California Continental Shelf between Cayucos and Pismo Beach along the Hosgri Fault Zone and other offshore faults (cruise S-6-08-SC, Figure 1). This research was supported by both the USGS Coastal and Marine Geology Program and the Cooperative Research and Development Agreement (CRADA) between the Pacific Gas & Electric Co. and the U.S. Geological Survey. These new high-resolution data, which allow detailed characterization of the Continental Shelf along the active Hosgri Fault Zone, will be incorporated into a regional tectonic model for the central California coast region used for earthquake- and tsunami-hazard analysis.

The Hosgri Fault Zone represents the southernmost section of a complex system of right-slip faults along the California coastline, including the San Gregorio, Sur, and San Simeon Faults. The 110-km-long Hosgri Fault Zone extends from Estero Bay in the north to Point Pedernales in the south. Recent studies have characterized the modern Hosgri Fault as a right-lateral transpressional feature (McLaren and Savage, 2001) with a Quaternary slip rate of 1 to 3 mm/yr (Hanson et al., 2004). However, uncertainties still remain as to the detailed slip history, downdip orientation, and fault connectivity of the Hosgri and other offshore faults in the survey area.

High-resolution seismic-reflection (with subbottom penetration of tens to hundreds of meters) and marine magnetic data can provide detailed information on shallow geologic structure and recent sedimentary deposits and their deformation. Nearshore studies elsewhere (Dehler and Potter, 2002) indicate that marine magnetic data in combination with aeromagnetic and high-resolution seismic data can be effective in identifying offsets in sedimentary strata and basement rocks to characterize important fault properties. Magnetic data are especially useful in characterizing vertical and near-vertical fault planes, which are commonly difficult to image with seismic-reflection data. These new geophysical data, in combination with industry aeromagnetic deep-penetrating seismic data, multibeam bathymetry, and other geologic information, will be used to refine fault location, length, segmentation, shallow geometry, and structure and to identify sampling targets for constraining fault-slip rates and tsunami-hazard potential.

For more information, contact Ray Sliter.