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High-Resolution Seismic-Reflection Data Offshore of Dana Point, Southern California Borderland

Introduction

In September 2006, the U.S. Geological Survey (USGS) collected seismic-reflection data from offshore Laguna Beach, Orange County, Calif., to Oceanside, San Diego County, Calif., (cruise S-2-06-SC, Figure 1). Reflection profiles were located to provide information on the geometry and location of offshore faults and recency of fault activity. The profiles were primarily oriented perpendicular to the shore (NE-SW) in waters depths from mid-shelf to the base of the slope. Data acquisition focused on the imaging of blind thrust faults and folds that had been proposed to lie beneath much of offshore Orange and San Diego Counties (the Oceanside fault) and the offshore Newport-Inglewood fault zone.

The Oceanside fault is a northeast-dipping, low-angle detachment fault that underlies the inner southern California borderland offshore of Oceanside, California (Crouch and Suppe, 1993; Bohannon and Geist, 1998). The fault has been proposed to have been reactivated as a thrust fault from offshore Laguna Beach to the Mexican border (Rivero and others, 2000). Fault-related folds imaged in the hanging wall of the Oceanside fault (Crouch and Bachman, 1989; Fischer and Mills, 1991) suggest that the detachment surface has been reactivated as a blind thrust, at least locally, along the continental slope off of San Mateo Point (the San Mateo fold and thrust belt of Fischer and Mills, 1991).

Reflection profiles also were collected across the Dana Point section of the Newport-Inglewood fault zone. South of Dana Point, this section of the fault zone follows the shelf break and generally is composed of multiple subparallel transpressional strands. North of Dana Point, the shelf is quite narrow and the fault zone is aligned with a prominent steep bathymetric scarp at the shelf break. This section of the fault lies immediately to the south of the section of the Newport-Inglewood fault zone that ruptured during the 1933 ML 6.3 earthquake (Hauksson and Gross, 1991).