Geometry and Active Tectonics of the Los Osos-Hosgri Fault Intersection

Watt, J.T.; Johnson, S.Y.; and Hartwell, S.R.
[email protected]

The ~400 km-long San Gregorio – Hosgri Fault System forms the western boundary of the tectonically active central California Coast Ranges. The geometry and connectivity of active faults within the Coast Ranges remain poorly understood. The ability of recent large earthquakes to rupture through fault intersections has underscored the need to re-examine the seismic hazard of these areas in California. We combine high-resolution seismic-reflection, potential-field, and multibeam bathymetry data with existing geologic and seismicity data to investigate the fault geometry and active tectonics of the Los Osos – Hosgri fault intersection.

The Hosgri fault is a northwest-trending, steeply-dipping dextral strike-slip fault where it converges with the Los Osos fault zone in Estero Bay. 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 active faulting. Three distinct strands (northern, central, and southern) are visible on shallow seismic reflection profiles. The fault zone is interpreted as predominantly strike-slip based on variable amounts of contractional and extensional deformation along strike and the prevalence of associated strike-slip first motion focal mechanisms within Estero Bay. The onshore Los Osos fault connects to the offshore fault structures through a ~4 km-wide releasing stepover. The central and northern strands become progressively more northwest-trending as they converge with the Hosgri fault, while the southern strand dies out within a few kilometers of the Hosgri fault. The northern strand runs subparallel to the Hosgri fault along the edge of a magnetic anomaly. Geophysical modeling of this anomaly suggests the northern strand is steeply northeast dipping. The central strand bends northward and intersects the Hosgri fault directly west of Morro Rock, corresponding to an area of compressional deformation and uplift of Tertiary strata along the Hosgri fault to the south and subsidence to the north. The geometry of the Los Osos – Hosgri fault intersection can be used in dynamic models of fault branching to determine whether or not a joint Los Osos – Hosgri fault rupture is likely.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013