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Multinational Partnership for Research in Earthquake System Science Offshore South-Central California for the Community Fault Model Report for SCEC Award #15098 Submitted March 28, 2015 Investigators: Christopher Sorlien I. Project Overview Offshore South-Central California for the Community Fault Model A. Abstract The SCEC Community Fault Model in offshore central California and western Santa Barbara Channel is based on 2D fault traces published in the 1980s. There are abundant multichannel seismic reflection (MCS) data, including 3D data, which image the 3D faults. Notably, the right- lateral Hosgri fault is imaged by 3D MCS data to be gently to moderately E-dipping between about 1 and 3 km depth. Much of the effort was focused on northwest Santa Barbara Channel, because of publications proposing M~8 earthquakes on the North Channel – Pitas Point (Ventura) –San Cayetano fault system, and a publication modeling huge sea floor uplifts and tsunamis. This fault system con- tinues 120 km west of Ventura, to west of Pt. Conception where it interacts with the southern termination of the Hosgri fault. The upper 4 km to 7 km of many strands of this fault system are imaged. There are only two geometric segment boundaries in the offshore faults; one located 10 km west of UCSB, and the other being near Gaviota. One lower strand of the system, the Pitas Point-Ventura fault, is continuous for 75 km. There is no evidence for sea floor rupture of the off- shore 60 km of this fault in the last half million years, including since formation of the Last Gla- cial Maximum unconformity. Instead, deep fault slip has been absorbed by a tilting anticline forelimb. Forelimb tilting is continuous to west of Pt. Conception. Shortening is slower in the west than the east for any interval of time since 1.8 Ma. B. SCEC Annual Science Highlights 1: Central California Seismic Project (CCSP) 2: Unified Structural Representation (USR) 3: Earthquake Geology C. Exemplary Figure 1 Figure 1. Faults and locations of seismic profiles displayed in this report. The color scale gives the two-way travel time to gridded fault surfaces. The shallow part of the Red Mountain fault system ends 10 km west of UCSB, and the lower blind N-dipping fault strands also are truncated there by faults striking 25°+ more west, oblique to the regional shortening direction. The dashed E-W thick gray line in the east is the seafloor trace of the model fault used by Ryan et al (2015) and the dashed WNW-ESE violet line in the same area is the seafloor/surface trace of the Ventura fault of Hubbard et al. (2014). Credit: Sorlien, C. C., C. Nicholson, M. J. Kamerling, and R. J. Behl, 2015, Strike-slip displacement on gently-dipping parts of the Hosgri fault and fold-related relief growth patterns above the blind oblique-slip North Channel-Pitas Point-Red Mountain fault system, poster USR-220, Proceedings Volume XXV, Southern California Earthquake Center Annual Meeting, September 12-16, Palm Springs, California. https://www.scec.org/sites/default/files/SCEC2015Proceedings.pdf D. SCEC Science Priorities 4c, 4b, 1a E. Intellectual Merit Active faults dipping from offshore to beneath the mainland are hazardous to coastal cities and facili- ties. 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, includ- ing 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. 2 F. Broader Impacts 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 supple- mental 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 Univer- sity, 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. Infor- mation 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 dur- ing June 2015, with permission of Richard Behl and Craig Nicholson. This sharing provides guidance for those researchers for their modeling and manuscripts. 3 1. Introduction Figure 1: Faults and locations of seismic profiles displayed in this report. The color scale gives the two- way travel time to gridded fault surfaces. The shallow part of the Red Mountain fault system ends 10 km west of UCSB, and the lower blind N-dipping fault strands also are truncated there by faults striking 25°+ more west, oblique to the regional shortening direction. The dashed E-W thick gray line in the east is the seafloor trace of the model fault used by Ryan et al. [2015] and the dashed WNW-ESE violet line in the same area is the seafloor/surface trace of the Ventura fault of Hubbard et al. [2014]. Black solid curves trace the upper edges of faults mapped in this project. A right-lateral fault system splits from the San Andreas fault near San Francisco and continues 400 km SSE, mostly just west of the current coastline. The southern part of this fault system is called the Hosgri fault. This fault has a minimum right-lateral slip rate from latest Pleistocene to present of 2.6 +/- 0.9 mm/yr [Johnson et al., 2014]. It dips steeply ENE, >70°, near the Diablo Canyon Nuclear Power Plant [Hardebeck, 2013]. It has been interpreted to dip steeply elsewhere based on 2D seismic reflection data [Willingham et al., 2013]. Farther south, associated faults beneath it accommodate oblique contraction [e.g., Sorlien et al., 1999a, b]. A 190 km-long oblique thrust fault system separates onshore and offshore Ventura basin from uplifting mountains and coastlines to the north. East of Santa Barbara, the fault system has accommodated 5 to 10 km of oblique contraction in the last million years [Huftile and Yeats, 1995; Sorlien and Kamerling, 2000], with ongoing rapid contraction indicated from GPS data [Marshall et al., 2013]. Yet, little data-based information has been published on its offshore 120 km, especially its westernmost 60 km. Earthquakes approaching Magnitude 8 have occurred globally on continental oblique thrust systems, rupturing multiple fault strands and segments. Six to eight meter late Holocene uplift events near Ventura have been used to infer M8 earthquakes and huge tsunamis on this fault system [Hubbard et al., 2014; McAuliffe et al., 2015; Ryan et al., 2015]. Figure 2: Profiles from 3D seismic reflection data volumes, located on Figure 1. ~No vertical exaggeration between 1 and 2 s time. A-A’: A moderately-dipping Hosgri fault is above a gently-dipping Hosgri between the red arrows. Stratal reflections terminate at reflections from the fault surfaces. B-B’: A NE- dipping blind fault strand bends in map view to become part of the N-dipping North Channel-Pitas Point fault system (Fig. 1). B-B’: The 0.8-0.88 Ma and the 1.07 Ma horizons have been correlated from our Santa Barbara Channel work [Nicholson et al. 2006; Marshall, 2012]. A southern part of the right-lateral Hosgri fault cuts above the North Channel fault. The North Channel fault is part of an oblique-left thrust system that is semi-continuous for 120 km along the north margin of Santa Barbara Channel.
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