Fault intersections along the Hosgri Fault Zone, Central California
Abstract
It is well-established that stresses concentrate at fault intersections or bends when subjected to tectonic loading, making focused studies of these areas particularly important for seismic hazard analysis. In addition, detailed fault models can be used to investigate how slip on one fault might transfer to another during an earthquake. We combine potential-field, high-resolution seismic-reflection, and multibeam bathymetry data with existing geologic and seismicity data to investigate the fault geometry and connectivity of the Hosgri, Los Osos, and Shoreline faults offshore of San Luis Obispo, California. The intersection of the Hosgri and Los Osos faults in Estero Bay is complex. The offshore extension of the Los Osos fault, as imaged with multibeam and high-resolution seismic data, is characterized by a west-northwest-trending zone (1-3 km wide) of near vertical faulting. Three distinct strands (northern, central, and southern) are visible on shallow seismic reflection profiles. The steep dip combined with dramatic changes in reflection character across mapped faults within this zone suggests horizontal offset of rock units and argues for predominantly strike-slip motion, however, the present orientation of the fault zone suggests oblique slip. As the Los Osos fault zone approaches the Hosgri fault, the northern and central strands become progressively more northwest-trending in line with the Hosgri fault. The northern strand runs subparallel to the Hosgri fault along the edge of a long-wavelength magnetic anomaly, intersecting the Hosgri fault southwest of Point Estero. Geophysical modeling suggests the northern strand dips 70° to the northeast, which is in agreement with earthquake focal mechanisms that parallel this strand. The central strand bends northward and intersects the Hosgri fault directly west of Morro Rock, corresponding to an area of compressional deformation visible in shallow seismic-reflection profiles. The southern strand of the Los Osos fault zone does not appear to reach the Hosgri fault. The Shoreline fault, first recognized as a northwest trending, vertical linear trend of earthquake hypocenters running parallel to the coastline (Hardebeck, 2010), is expressed at the seafloor as a series of discontinuous, en echelon lineations and scarps visible in multibeam bathymetry. Detailed marine and helicopter magnetic data reveal a linear trend of magnetic anomalies, the edges of which are coincident with or parallel to the surface expression of the Shoreline fault, suggesting the magnetic source rocks are cut by the Shoreline fault. Geophysical modeling indicates a steeply-dipping Shoreline fault to depths of at least 8 km, which is in agreement with earthquake focal mechanisms that extend this interpretation to 12 km depth. Seismicity clearly shows the Shoreline fault merging with the Hosgri fault at about a 30° angle approximately 6 km northwest of Pt. Buchon. While multibeam bathymetry and shallow seismic-reflection show no evidence of faulting in the near surface at the intersection, the seismicity is on trend with the edge of a linear magnetic anomaly within Franciscan basement rocks, suggesting a fault connection at depth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFMGP41A0977W
- Keywords:
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- 1517 GEOMAGNETISM AND PALEOMAGNETISM / Magnetic anomalies: modeling and interpretation;
- 3025 MARINE GEOLOGY AND GEOPHYSICS / Marine seismics;
- 3045 MARINE GEOLOGY AND GEOPHYSICS / Seafloor morphology;
- geology;
- and geophysics;
- 4302 NATURAL HAZARDS / Geological