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Intellectual Merit
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Active faults dipping from offshore to beneath the mainland are hazardous to coastal cities and facilities. Two fault systems were investigated, the right-lateral Hosgri fault in south-central California, and the North Channel-Pitas Point Red Mountain fault system from Ventura through Santa Barbara and UCSB, to Pt. Conception. M7.8 to M8.1 earthquakes have been proposed for this fault system, including its onshore fault strands (Hubbard et al., 2014). Based on widely-applied fault-propagation fold models, huge sea floor ruptures and tsunamis were modeled on a relatively short offshore part of the fault (Ryan et al., 2015). However, these faults and associated folds are imaged by abundant 2D and 3D seismic reflection data. There is no evidence of faults near the model fault rupturing to the sea floor during the last half million years. High resolution seismic refection data show no fault offset of the Last Glacial Maximum unconformity, and kink folding of it decreases to zero beyond 10 km west of Pitas Point. Instead, deep fault slip has been absorbed by broad tilting of an anticline forelimb for the entire 120 km length of the offshore fault system. Parts of certain fault strands have not propagated upward for the last ½ to ¾ of a million years. Slip in earthquakes must die out over several km below the fault tip.
Newly-formed strike-slip faults are expected to be vertical. However, continental faults are commonly reactivated, and moderately-dipping strike-slip faults are known globally, and in California. The gentle to moderate dip of the southern Hosgri fault below about 1 km depth (imaged to about 3 km depth) suggest that strike-slip fault rupture at depth during earthquakes will be significantly farther east, closer to or beneath the coast, than suggested by the position of the sea floor fault trace. A component of left-lateral strike-slip can be inferred on strands of the Red Mountain fault west of UCSB, and this left-lateral component may be present gently-dipping blind faults west of the Red Mountain fault strands. |
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Broader Impacts
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Graduate students have been involved in the larger project over the last decade (Hopkins, 2006, Marshall, 2012, Doris et al., 2014, others for the stratigraphy). The main Broader Impacts for the 2015 work is related to Earthquake hazard. Digital fault surfaces are provided to the SCEC Community Fault Model as they are prepared. Faulting cannot be properly understood without incorporating associated deformation due to folding. Information on folding through time is contained in digital grids of dated stratigraphic horizons. These grids are released as supplemental data at the time of publication (Sorlien et al., 2013, 2015).
Information in advance of publication has been provided to those research scientists willing to properly reference theses, technical reports, and abstracts. These scientists include Samuel Johnson of U.S. Geological Survey, Patricia Persaud of Cal Tech, Kaj Johnson of Indiana University, and Thomas Rockwell of San Diego State University. The new results have been freely shared with members of the group for the larger project, including Craig Nicholson. Information shared includes interpreted depth-converted seismic reflection profiles, fault maps, and rough drafts of a manuscript. The Kingdom Suite project for offshore central California, and another one for the western half of Santa Barbara Channel, was provided to Sam Johnson during June 2015, with permission of Richard Behl and Craig Nicholson. This sharing provides guidance for those researchers for their modeling and manuscripts. |
