High-Resolution Seismic-Reflection Data from Offshore of Northern California—Bolinas to Sea Ranch

Introduction

In September 2009, the U.S. Geological Survey (USGS) collected high-resolution seismic-reflection data offshore of northern California between Bolinas and Sea Ranch (fig. 1). The survey area spans ~125 km of the coastline, extending offshore beyond the 3-nautical-mile limit of California’s State Waters. The southern part of the survey area stretches around the Point Reyes National Seashore and extends in to the Greater Farallones National Marine Sanctuary. Data were collected aboard the USGS R/V Parke Snavely. Cumulatively, ~1,150 km of seismic-reflection lines were acquired.

The study area is cut by the northern San Andreas Fault (Clark and Brabb, 1997; Prentice and Kelson, 2006), which is the primary structure in the widely distributed transform plate boundary along much of the west coast of North America (for example, Argus and Gordon, 2001). In this area, the San Andreas Fault lies offshore between Bodega Bay and Tomales Bay, and between Salmon Creek and Fort Ross. Outside of these areas, the onshore fault zone forms a prominent topographic lineament that coincides with Bolinas Lagoon and Tomales Bay. This section of the San Andreas Fault has an estimated slip rate of 17–24 mm per year (U.S. Geological Survey, 2010). The devastating Great 1906 California earthquake (M 7.8) is thought to have nucleated on the San Andreas Fault a few kilometers south of this survey area, offshore of San Francisco (for example, Bolt, 1968; Lomax, 2005).

The study area has particular importance for understanding geologic hazards and seafloor environments, which is especially relevant in this region for effective stewardship of California’s State Waters and several marine protected areas, the Greater Farallones National Marine Sanctuary, and the Point Reyes National Seashore. To that end, the seismic-reflection profiles released in this report have been incorporated in the comprehensive map publications of the California Seafloor Mapping Program (CSMP) (available at http://walrus.wr.usgs.gov/mapping/csmp/). Seven CSMP publications from this area (Cochrane and others, 2015; Johnson and others, 2015a,b,c,d; Watt and others, 2015a,b) include map sheets displaying interpreted seismic profiles that show faults, folds, and sediment thickness. These interpretations are incorporated into seamless offshore–onshore geologic maps and maps of unconsolidated sediment distribution and thickness. Specific applications that rely on seismic-reflection profiling include earthquake hazard assessments; tsunami hazard assessments; planning offshore infrastructure; providing baselines for monitoring change; input to models of sediment transport, coastal erosion, and coastal flooding; regional sediment management; understanding coastal aquifers; and marine spatial planning.

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