The Hosgri Fault Zone, Central California: Collection and Preliminary Analysis of Marine Magnetic and Seismic Reflection Data

Newly acquired high-resolution marine magnetic and seismic-reflection data collected offshore Point Buchon, California, are being combined with existing regional magnetic, oil industry multichannel seismic (MCS), onshore geology, and seismicity data to investigate the tectonics and earthquake hazards associated with the Hosgri and other nearshore fault zones. This research is part of a broader study of earthquake hazards in the central coastal California region that was most recently illustrated by the 2003 M6.6 San Simeon earthquake. High-resolution marine magnetic and single-channel mini-sparker seismic reflection data were collected in June 2008 aboard the USGS R/V Parke Snavely. These data were collected in the offshore areas between Cayucos and Pismo Beach from the nearshore (6 m depth) to just west of the Hosgri Fault Zone. The seismic reflection data were collected using a mini-sparker source and a 15-meter single-channel hydrophone streamer at a 500 joule power level and shot interval of 1/2 second, generating a signal between 200 and 1500 Hz. The G-882 cesium marine magnetometer was mounted on the opposite side of the stern as the seismic source and streamer and was towed approximately 30 meters behind the vessel. Mini-sparker and marine magnetic data were collected simultaneously along shore-perpendicular tracklines spaced 800 meters apart. Along the Hosgri Fault Zone, marine magnetic data were collected with 400 meter trackline spacing. Marine magnetic data reveal two distinctive magnetic anomaly patterns north and south of Point Buchon. The transition between these anomaly patterns corresponds to the boundary between the Los Osos Valley and the San Luis/Pismo structural block to the south. Within the northern offshore Los Osos Valley block a series of broad magnetic highs extends northwestward from Morro Rock. These anomalies may signify the offshore extension of the Oligocene Morro Rock-Islay Hill igneous complex that appears to have been displaced along previously identified right-lateral strike-slip faults. Short wavelength magnetic anomalies along the offshore extension of the northern Cambria Fault indicate the presence of magnetic volcanic rocks and serpentinite within the Franciscan Complex, comparable to those mapped onshore. Similarly, the short wavelength anomalies in the new marine data along the coastline south of Point Buchon are likely caused by magnetic rocks within an offshore extension of the Franciscan Complex. Maximum horizontal gradient analysis of the marine magnetic data shows that gradients commonly correspond to locations of mapped faults.