Shallow Structures In and Near an Active Transcurrent Fault Zone – The Hosgri Deformation Zone

Nishenko, Stuart P.; Ward, Hans Abramson; Angell, Michael; Pearce, Justin; Gray, Brian; Greene, H. Gary; and Hogan, Phil
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The discovery of the Shoreline fault zone offshore of Point Buchon, central California, in close proximity to the Diablo Canyon Power Plant, has stimulated further investigations of the regional nearshore geology. Low-energy (<2 kJ) marine 3D seismic-reflection profile surveys undertaken by the Pacific Gas and Electric Company to image the shallow subsurface structures in the region have recently been completed. These surveys focused on areas where bathymetric expression of the Shoreline fault zone is absent, but localized micro-seismicity at depth indicates a fault could be present. Seismic reflection profile data collected during these surveys used the same seismic source, a triple-plate boomer, and different receiver arrays. One survey concentrated on the northern segment of the Shoreline fault zone, and the other on the southern extension, both using a four-streamer (receiver) array. Additional data were later collected in the south using a 14-streamer P-Cable arrangement with the same seismic source. Interpretation of the 3D data volumes indicate that although shallow structural deformation in the form of fault splays and associated folds exists, no well-defined continuous shallow fault is observed that directly overlies the zone of seismicity and extends north into the Hosgri Fault Zone. However, a fairly continuous and linear fault, the Point Buchon fault zone, may be associated with a fault at depth that is characterized by the zone of seismicity. Structures that indicate a zone deformation alternating from transtension to transpression, such as grabens and a pressure ridge, have been mapped along this strike-slip regime. In the south, the Shoreline fault zone is well expressed in a 3D coherency time-slice and seen to cut across bedrock-incised paleo-stream channels that appear useful for the determination of offset along fault traces. We will report upon the interpretation and use of these channels as piercing points to constrain estimated fault slip-rates.

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