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

Introduction

In June of 2008 and July of 2009 the U.S. Geological Survey (USGS) collected seismic-reflection and marine magnetic data on the Central California continental shelf between Piedras Blancas and Point Sal along the Hosgri Fault Zone (cruises S-6-08-SC and S-6-09-SC, Figure 1). This work was supported by the USGS Coastal and Marine Geology Program, the USGS Earth Surface Processes Program, and the Cooperative Research and Development (CRADA) between Pacific Gas and Electric Company 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 coast region used for earthquake and tsunami hazard analysis.

The Hosgri Fault Zone represents the southernmost part of a complex system of right-slip faults along the California coastline including the San Gregorio, Sur, and San Simeon faults. 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 and others, 2004). However, uncertainties still remain as to the detailed slip history, down-dip orientation, and fault connectivity of the Hosgri and other offshore faults in the area.

High-resolution seismic-reflection (with penetration of tens to hundreds of meters) and marine magnetic data can provide detailed information on shallow geologic structure and recent sediment deposits and 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 that are often difficult to image with seismic-reflection data. These new geophysical data, combined with aeromagnetic, industry 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.