Offshore South-Central 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 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 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-, 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: 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, 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 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.

2. Data and Methods The SCEC Community Fault Model (CFM) version 5.0 for offshore south-central California and the western half of Santa Barbara Channel is mainly based on 2D fault traces published three decades ago [e.g., McCulloch 1987; McCulloch, 1989]. I used about 1000 2D seismic reflection profiles, eight 3D seismic reflection volumes, and 180 vertical slices from five more 3D volumes to interpret faults and folds to between ~3 km and 7 km depth over this region. A part of this work was funded by this SCEC project. Precisely-dated stratigraphy from scientific coring for the last 735 ka provides information on the evolution of faulting and folding through time [Nicholson et al., 2006; Marshall 2012; Dean et al., 2015]. Logs from numerous petroleum test wells provide velocity information, and paleontology provides age information for rock samples in these wells.

Figure 3: Western Geophysical profiles, located on Figure 1. Strands of the North Channel-Pitas Point fault are blind, with slip converted to progressive tilting (horizontal axis rotation) of the S-dipping homocline. C-C’ images greater deformation below the 1800 ka top Lower Pico horizon than above. Therefore, the fault-fold system was active there before Quaternary time rather than laterally propagating east to west during Quaternary time. Strands of the North Channel-Pitas Point fault system are not drawn on D-D’ because they are obvious to most readers.

Figure 4: Inline profile from the Dos Cuadras 3D seismic reflection survey, located on Figure 1. The shortening of the 1800 ka (yellow) horizon here is about 4 km, and its structural relief after accounting for erosion is more than 3 km [Sorlien and Kamerling, 2000]. The dip of the yellow top Lower Pico horizon is not imaged in the lower part of the S-dipping anticline forelimb: the dip is known to be steep to vertical from wells [Redin et al., 2005]. The upper tip of the cyan strand of the Pitas Point-Ventura fault (and of the Yellow fault) is at ~700 m depth.

Figure 5: U.S. Geological survey high-resolution seismic reflection profile (chirp) across the kink fold above the Pitas Point-Ventura fault strand. LGM is an angular unconformity formed during the Last Glacial Maximum (20 ka). Aggradation above the unconformity commenced later, when the ocean transgressed across this location. The black arrows mark an angular unconformity from the previous glacial time, Marine Isotopic Stage 6, which ended about 130 ka. The LGM unconformity vertical offset at the Pitas Point fold kink is only 2 m here, and decreases to zero very close to the west. Deformation above the 50 km of the Pitas Point – Ventura fault strand west of here has been by broad (4-5 km-wide) tilting, not kink folding or fault offset, for at least the last half million years.

3. Results and Discussion The offshore North Channel –Pitas Point fault system is 120 km-long, with the imaged upper 4 to 7 km of one strand, the Pitas Point-Ventura fault, being geometrically continuous for 75 km (Fig. 1). Strands of the fault system are blind, not reaching the sea floor (Figs. 3-5). The deep fault slip is absorbed by broad folding and tilting. Shortening accommodated by offshore faulting and folding on this fault system over the last ~1.8 Myr is at least 5 km east of Santa Barbara and at about 1 km near UCSB. The entire offshore fault-fold system was active long before 1.8 Ma.

Figure 6: Models for fault related folds from Seeber and Sorlien [2000]. (A), and Sorlien et al [2013], modified from Wickham [1995] (B). Only B predicts progressive tilting of a wide forelimb, and continuously variable rates of vertical motion, rather than the steps in relative uplift seen in A. Zero means no vertical motion in a quake, and larger uplifts are shown as longer red arrows. The red half arrows along the faults are a qualitative illustration of slip magnitude during an earthquake. All the slip reaches the tip of the fault in A; the slip is absorbed by tilting over a large part of the cross-sectional fault in B. The black arrows in A give the total displacement.

Ten interpreted horizons ranging in age from 1,800 ka to 110 ka illustrate folding by progressive tilting, requiring continuously variable vertical motion perpendicular to strike (Figs. 3-6). On scales of hundreds of kyr, or even during a single earthquake, the vertical motion will not be a step function across an active axial surface or an emergent fault (Fig. 7). It will gradually increase from zero in the basin to a peak at the anticline crest. Some of the blind fault strands do not appear to have propagated updip during the last few hundred kyr. Wherever there is forelimb progressive tilt, if the faults propagate at all, they have to do so more slowly than the slip rate. For non-propagating (oblique) thrust faults, in any one earthquake or hundreds of quakes, the slip is gradually absorbed updip by folding across several km of the hanging wall. Thus, this folding should be incorporated in tsunami and strong ground motion modeling. The localized 6-to-8 m Holocene uplift events at Pitas Point are thus unlikely to be representative of the seafloor offset along the principal N-dipping fault system that extends offshore.

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