Detailed Fault and Deformation Mapping along a Strike-Slip Fault in a Coastal Environment: The San Gregorio Fault near Half Moon Bay, California

The San Gregorio fault lies predominantly offshore in the coastal zone of the San Francisco Peninsula. Seismic reflection, swath bathymetric, and photographic data collected in 2007 between Año Nuevo Point and San Francisco combine with previously collected backscatter and seismic reflection data to provide a detailed view of the fault zone. The new data include 500 trackline-km of high-resolution seismic profiles, with a sub-bottom depth penetration of 50-100 m. The fault zone was also imaged with multibeam swath bathymetry and backscatter data, which was collected from the coastline offshore out to the 3-nautical mile limit of state waters. In addition, video and high-resolution 35-mm photographic surveys were made on three lines that crossed the fault zone.

Offshore of Half Moon Bay, the NNW-trending San Gregorio fault zone consists of two primary strands. The eastern strand, which extends along the trend of the Seal Cove fault, is a continuous and very linear scarp from Pillar Point at least 15 km southward. The scarp has 1 – 1.5 m of vertical offset, with the west side upthrown. Numerous folds are truncated by this strand. Its sharp surface expression suggests that the eastern strand may currently be the primary active strand. Fold orientations indicate at least three episodes of deformation; the youngest folds trend ESE-WNW and deform older, more north-trending folds as revealed in the seismic profiles and bathymetry. The western strand of the fault zone is not well imaged in the bathymetry but appears in the seismic-reflection data as the edge of a zone of deformation. A minor third strand of the fault appears to separate the two apparently older fold domains.