SHORELINE FAULT ZONE, SOUTH-CENTRAL COASTAL CALIFORNIA

High-resolution multibeam echo sounding, seismic-reflection, and aeromagnetic data collected in 2009 have improved the resolution and definition of the Shoreline fault zone (SFZ) in south-central coastal California. The SFZ was identified by Hardebeck (2009) as a lineation of relocated seismicity about 0.5 km offshore of the Irish Hills between Point Buchon and Point San Luis, east of the Hosgri fault zone. Subsequent investigations indicate that the seismicity lineament is coincident, in part, with a series of prominent bathymetric and magnetic lineaments that cut across Cretaceous and Miocene rocks. The surface expression of the SFZ consists of three distinct segments: 1] a 6 to 9 km Northern segment that is defined by a distinct N40°W trending discontinuous scarp that truncates bedding and structures of the Obispo and Monterey formations; 2] an 8 km Central segment expressed as a distinct bathymetric and magnetic lineament that juxtaposes different bedrock lithologies, truncates bedding and structures, and has scarps, gas-related pock marks and mud extrusions; and 3] a 6 km Southern segment expressed as a poor to moderate bathymetric lineament with local scarps and juxtaposed bedding. The Central and Southern segments form a right-stepping en echelon pattern with a strike of N60° to 70°W, consistent with the right-lateral strike-slip focal mechanisms and the vertical plane of the microseismicity lineament that extends to a depth of ~10 km. The northern part of the seismicity lineament is more diffuse, extends to a depth of 12 km and diverges west from the surface expression of the Northern segment. High-resolution seismic-reflection profiles across the northern portion of the seismicity lineament indicate the lineament is not associated with near-surface faulting. It may be that this portion of the seismicity lineament is associated with a fault that does not reach the surface or is associated with the western trace of the Hosgri fault zone that dips east toward the SFZ.