Morphology, Structure, and Kinematics of the San Clemente and Catalina Faults Based on High-Resolution Marine Geophysical Data

Home , Gulf of Santa Catalina, Queen Charlotte Fault

Research Paper

GEOSPHERE Morphology, structure, and kinematics of the San Clemente and Catalina faults based on high-resolution marine geophysical data,

GEOSPHERE, v. 16, no. 5 southern California Inner Continental Borderland (USA) https://doi.org/10.1130/GES02187.1 Maureen A.L. Walton1, Daniel S. Brothers1, James E. Conrad1, Katherine L. Maier1,*, Emily C. Roland2, Jared W. Kluesner1, and Peter Dartnell1 1Pacific Coastal and Marine Science Center, U.S. Geological Survey, Santa Cruz, California 95060, USA 15 figures; 1 set of supplemental files 2School of Oceanography, University of Washington, 1501 NE Boat Street, Seattle, Washington 98195, USA

CORRESPONDENCE: [email protected] ABSTRACT fault of southeastern Alaska [USA], North Anatolian fault of Turkey, Alpine fault of New Zealand), and are capable of generating large (M6+) earthquakes (e.g., CITATION: Walton, M.A.L., Brothers, D.S., Conrad, J.E., Maier, K.L., Roland, E.C., Kluesner, J.W., and Catalina Basin, located within the southern California Inner Continental Stein et al., 1997; Hauksson et al., 2012; Howarth et al., 2012; Yue et al., 2013). Dartnell, P., 2020, Morphology, structure, and kine- Borderland (ICB), United States, is traversed by two active submerged fault Despite their proximity to large population centers, particularly near coasts, there matics of the San Clemente and Catalina faults based systems that are part of the broader North America–Pacific plate boundary: is much we do not yet understand about the way strike-slip systems form and on high-resolution marine geophysical data, southern California Inner Continental Borderland (USA): Geo- the San Clemente fault (along with a prominent splay, the Kimki fault) and the deform. For example, the recent 2016 Mw 7.8 Kaikoura earthquake of New Zea- sphere, v. 16, no. 5, p. 1312–1335, https://doi.org /10 Catalina fault. Previous studies have suggested that the San Clemente fault land (e.g., Hollingsworth et al., 2017) highlights the complexity involved during .1130/GES02187.1. (SCF) may be accommodating up to half of the ~8 mm/yr right-lateral slip a major strike-slip fault rupture and the need for improved understanding of distributed across the ICB between San Clemente Island and the mainland strike-slip systems. Ongoing research aims to better characterize how, why, and Science Editor: Andrea Hampel coast, and that the Catalina fault (CF) acts as a significant restraining bend in where strike-slip faults form, generate earthquakes, partition slip in oblique envi- Associate Editor: James A. Spotila the larger transform system. Here, we provide new high-resolution geophysical ronments, and behave at stepovers and endpoints, and ultimately, how to use

Received 6 August 2019 constraints on the seabed morphology, deformation history, and kinematics tectonic geomorphology to quantify deformation and potential geohazards. Revision received 1 April 2020 of the active faults in and on the margins of Catalina Basin. We significantly High-resolution constraints on fault geometry are particularly important for under- Accepted 27 May 2020 revise SCF mapping and describe a discrete releasing bend that corresponds standing Quaternary fault deformation history (e.g., Brothers et al., 2015) and with lows in gravity and magnetic anomalies, as well as a connection between for characterizing active fault systems, because even a subtle geometry change Published online 10 July 2020 the SCF and the Santa Cruz fault to the north. Subsurface seismic-reflection can inhibit or promote earthquake rupture propagation (e.g., Wesnousky, 2006). data show evidence for a vertical SCF with significant lateral offsets, while The California Inner Continental Borderland (ICB) offshore of southern the CF exhibits lesser cumulative deformation with a vertical component California (United States) and northern Baja California (Mexico) (Fig. 1) offers indicated by folding adjacent to the CF. Geodetic data are consistent with SCF an opportunity to examine a set of active strike-slip faults that accommodate right-lateral slip rates as high as ~3.6 mm/yr and transpressional convergence as much as 8 mm/yr of right-lateral shear, or ~15% of the total Pacific–North of <1.5 mm/yr accommodated along the CF. The Quaternary strands of the SCF America plate boundary slip budget of 48–50 mm/yr (Platt and Becker, 2010; and CF consistently cut across Miocene and Pliocene structures, suggesting DeMets and Merkouriev, 2016). Several significant earthquakes have occurred

generation of basin and ridge morphology in a previous tectonic environment along offshore faults in the ICB, including the 1981 Mw 6.0 Santa Barbara

that has been overprinted by Quaternary transpression. Some inherited crustal Island earthquake, the 1986 Mw 5.8 Oceanside sequence, the recent 2018 Mw

fabrics, especially thinned crust and localized, relatively hard crustal blocks, 5.3 Santa Cruz Island event, the 1951 Ms 5.9 San Clemente Island earthquake,

appear to have had a strong influence on the geometry of the main trace of and the largest recorded ICB earthquake to date, the Ms 6.2 offshore Ensenada the SCF, whereas inherited faults and other structures (e.g., the Catalina Ridge) earthquake of 1964 (Richter, 1958; Allen et al., 1960; Hauksson and Jones, 1988; appear to have minimal influence on the geometry of active faults in the ICB. Pacheco and Nábělek, 1988; Bent and Helmberger, 1991; Astiz and Shearer, 2000, Legg et al., 2015) (Fig. 1). Shaking from earthquake ruptures can also enhance the risk of local tsunamis via uplift at restraining bends or coseismic ■■ INTRODUCTION slope failure (e.g., Legg and Borrero, 2001; Legg et al., 2004a). Several large submarine landslides have been documented in the offshore California border- Strike-slip faults are characteristic of continental transform plate boundaries land (Bohannon and Gardner, 2004; Locat et al., 2004; Normark et al., 2004a; worldwide (e.g., San Andreas fault of southern California [USA], Queen Charlotte Lee et al., 2009; Legg and Kamerling, 2003; Brothers et al., 2018). This paper is published under the terms of the CC‑BY-NC license. *Now at National Institute of Water and Atmospheric Research (NIWA), 301 Evans Bay Parade, Hataitai, Wellington 6021, New Zealand

GEOSPHERE | Volume 16 | Number 5 Walton et al. | Morphology, structure, and kinematics of the San Clemente and Catalina faults Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/5/1312/5151191/1312.pdf 1312 by guest on 02 October 2021 Research Paper

Understanding how one structure can lead to the formation of another is N 120º W 119º W 118º W 117º W critical for accurate interpretation of tectonic geomorphology and geohazard 35º 0 30 60 N assessments. It is well understood that pre-existing crustal fabrics can influ- km ence strike-slip fault propagation or fault reactivation (e.g., Christie-Blick and SAF Biddle, 1985; Scholz, 2002; Cunningham and Mann, 2007). Numerous studies bathy elev. (m) have documented reactivation of pre-existing fault structures, including in our SBB WTR 0 study area offshore of southern California (e.g., Crouch and Suppe, 1993; Fisher N −2500 −5000 et al., 2009; Sorlien et al., 2015), and others have analyzed the effects of crustal 34º LA SMB structures on strike-slip fault geometry (e.g., Johnson and Watt, 2012; Johnson 2018 Fig. 6B et al., 2018). Christie-Blick and Biddle (1985) differentiated between “essential” SCB SPB 1981 and “incidental” pre-existing structures, i.e., structures that significantly influ- CF ence strike-slip fault geometry and propagation and those that are inherited and OCB

N SCFCB do not affect strike-slip deformation, respectively. The study of essential versus Fig. 14 Fig. 6A ~8 mm/yr1986 incidental pre-existing structures and their relative influence on the development 33º SNB ICB SDT of active fault systems is an important topic in crustal deformation research. SD Fig. 2, Fig. 13 1951 This study focuses on characterizing active structures and deformation U.S. in and on the margins of Catalina Basin, an understudied region of the ICB MEX (Figs. 1, 2A), which we interpret to contain two Holocene-active offshore fault zones—the San Clemente and Catalina fault zones. Four of the aforemen- N

tioned >M5 earthquakes in the ICB occurred near Catalina Basin (Astiz and 32º Fig. 12 Shearer, 2000), yet neither the San Clemente fault (SCF) nor the Catalina fault (CF) have been examined systematically with modern high-resolution marine Figure 1. Inner Continental Borderland (ICB) location map (southern California, geophysical data within Catalina Basin. Additionally, the SCF alone may be USA) showing the National Centers for Environmental Information Southern accommodating as much as 4–6 mm/yr of right-lateral slip based on geologic California Coastal Relief Model (version 2) bathymetry (Calsbeek et al., 2013), ESRI topography, approximate regional geologic boundaries (bold black dashed lines data and GPS models, about half of the total slip taken up within the ICB (Legg, with bold labels), and southern California faults (red lines) from the U.S. Geological 1985, 1991a, 2005; Larson, 1993; Bennett et al., 1996; Humphreys and Weldon, Survey and California Geological Survey Quaternary Fault and Fold Database for 1994; Goldfinger et al., 2000), and the CF has been thought to be convergent the United States (https://www.usgs.gov/natural -hazards /earthquake -hazards / faults). Approximate epicenters of the 1951 M 5.9 San Clemente Island earthquake, and thus tsunamigenic (e.g., Legg and Borrero, 2001). s 1981 Mw 6.0 Santa Barbara Island earthquake, 1986 Mw 5.8 Oceanside sequence,

Numerous important studies over the past few decades have described and 2018 Mw 5.3 Santa Cruz Island earthquake are highlighted with yellow stars. a first-order tectonic and geologic framework for the ICB and Catalina Basin; The 8 mm/yr value is GPS-modeled slip accommodated within the ICB from Platt and Becker (2010). Locations of Figures 2, 12, and 13 are outlined in dashed white. our study provides, for the first time, a systematic, comprehensive, high-res- Locations of crustal-scale two-dimensional seismic reflection profiles in Figures olution, detailed, and high-quality geophysical data set in Catalina Basin with 6A, 6B, and 14 are shown as solid white lines. WTR—Western Transverse Ranges; which to assess prior hypotheses and first-order results. We utilize a suite of OCB—Outer Continental Borderland; ICB—Inner Continental Borderland; LA—Los Angeles; SD—San Diego; CF—Catalina fault; SAF—San Andreas fault; SCF—San new high-resolution multichannel seismic (MCS) data, CHIRP (compressed Clemente fault; SBB—Santa Barbara Basin; SCB—Santa Cruz Basin; SMB—Santa high-intensity radar pulse) sub-bottom profiles, and high-resolution multibeam Monica Basin; SPB—San Pedro Basin; CB—Catalina Basin; SNB—San Nicolas bathymetry data in conjunction with legacy crustal-scale and other regional Basin; SDT—San Diego Trough; MEX—Mexico. data to examine the relationships between physiography, crustal fabric, and Quaternary and Holocene deformation in and on the margins of Catalina Basin. the Catalina Basin area, this study emphasizes the importance of integrated We revise the geometry and better define the kinematics of the Holocene-active high-resolution surface and subsurface imaging. SCF and CF and the Quaternary-active Kimki fault (KF) and discuss the implications for geohazards. We find that Quaternary faults commonly overprint pre-existing structures, and thus that modern physiography does not neces- ■■ TECTONIC AND SEDIMENTARY SETTING sarily indicate the presence of active faults. We also find that inherited crustal blocks (as defined by bulk physical properties of the deep crust), more so than Tectonic Evolution of the Inner Continental Borderland inherited faults, have likely affected Quaternary fault configuration, kinematics, and perhaps even fault formation. In addition to better defining post–late The ICB extends from the southern California mainland coast to ~100 km Miocene fault history, geometry, kinematics, and associated geohazards in offshore just past the SCF, where the basins of the Outer Continental Borderland

119º W 118.5º W 118º W 119º W 118.5º W 118º W SCrF

SMB A Fig. 6B NIF B

1 San Gabriel 1 PVF

Catalina Ridge Canyon 1981 Fig. 14

Fig.

Fig. 10 N N SPB −500 m 5º 5º San Gabriel SBI CRF −500 m 33 . 33 . Channel GoSC SCF −500 m KF

CI CF −500 m SDTF Kimki Basin Fig. 6A Kimki Ridge Fig. 4 San Clemente Fig. 8 Fig. 3 Catalina Ridge ? Basin Legend <5 <2 −1000 m Fig. 7 <10 2–3 N N Fig. 5 <15 3–4 SCF 1986 SNB (color) <25 4–5 Magnitude 33º 33º Depth (km) Emery Knoll 25+ 5+ (symbol size) Fig. 9 Legend MCS (this study) legacy MCS 0 N Quaternary faults (this study) (m) SCI −1500 pre-Quaternary −500 m 1951 bathy elev. faults (this study) N 0 10 20 30 USGS QFFD 0 10 20 30 100 m contours km km

Figure 2. (A) Map of Catalina Basin showing grayscale relief beneath a 75% transparent high-resolution color bathymetry compilation (Dartnell et al., 2017). Grayscale background is the National Centers for Environmental Information Southern California Coastal Relief Model (Calsbeek et al., 2013). Locations of Figures 3, 4, 5, and 9 are denoted by black dashed boxes. CI—Santa Catalina Island; SCI—San Clemente Island; SBI—Santa Barbara Island; SMB—Santa Mon- ica Basin; SPB—San Pedro Basin; GoSC—Gulf of Santa Catalina; SNB—San Nicolas Basin. (B) Map of Catalina Basin showing depth contours (100 m contour interval; gray lines) with seismicity (Hauksson et al., 2012; colored circles), new high-resolution seismic data coverage (pink lines), legacy seismic lines shown in this study (blue lines; see the National Archive of Marine Seismic Surveys at walrus.wr.usgs.gov/NAMSS for legacy data not shown here), new fault mapping (bold black solid and dashed lines; this study), and mapping from the U.S. Geological Survey and California Geological Survey Quaternary Fault and Fold Database (QFFD) for the United States (thin black lines; https://www.usgs.gov/natural-hazards/earthquake-hazards/faults, last accessed 16 April 2019). Lo-

cations of Figures 6A, 6B, 7, 8, 10, 11, and 14 are highlighted by bold colored lines. Approximate epicenters of the 1951 Ms 5.9 San Clemente Island earthquake,

1981 Mw 6.0 Santa Barbara Island earthquake, and 1986 Mw 5.8 Oceanside sequence are highlighted with yellow stars. SCrF—Santa Cruz fault; KF—Kimki fault; SCF—San Clemente fault; CF—Catalina fault; CRF—Catalina Ridge fault; SDTF—San Diego Trough fault; PVF—Palos Verdes fault; NIF—Newport-Inglewood fault; MCS—multichannel seismic.

neighbor the ICB to the west (Fig. 1). The Western Transverse Ranges province (Luyendyk et al., 1980; Ingersoll and Rumelhart, 1999; ten Brink et al., 2000). bounds the ICB to the north (Fig. 1). Morphologically, the ICB is characterized Miocene extension led to the exhumation of the Cretaceous Catalina Schist in by a network of submarine basins, faults, ridges, and islands, with bedrock the ICB, a metamorphic core associated with past subduction along the margin composed of thinned, extended continental crust, and outcrops featuring and correlatable with the Franciscan Complex (e.g., Crouch and Suppe, 1993; Miocene and older volcanic and metamorphic rocks (Barron, 1986; Vedder Bohannon and Geist, 1998; ten Brink et al., 2000; Miller, 2002). et al., 1986; Vedder, 1990; ten Brink et al., 2000). The ICB began to form in an The basin-and-ridge morphology and some faulting still present in the extensional tectonic regime beginning ca. 20 Ma (Crouch and Suppe, 1993; modern ICB evolved following the ca. 19 Ma rotation of the Western Trans- Nicholson et al., 1994; Bohannon and Geist, 1998; ten Brink et al., 2000; Miller, verse Ranges. The precise timing of subsequent ICB basin evolution is not 2002; Fisher et al., 2009). The rotation of the Western Transverse Ranges to well understood; Bohannon et al. (2004) interpreted Pliocene initiation of the the north caused rapid opening of the ICB at ca. 19 Ma, shortly after which Los Angeles Basin, attributing changes in seismic facies to a ca. 6 Ma reversal (ca. 17.5 Ma) subduction of the Arguello microplate ceased, leading to oblique in physiography that led to flooding of previously subaerial Miocene rocks. rifting of the Outer Continental Borderland (Lonsdale, 1991; Nicholson et al., The mechanism of local basin subsidence also has not been well defined, 1994; ten Brink et al., 2000). This event was followed by regional transrotation perhaps partly due to the complex stress environments in the ICB (Bohannon from ca. 18 Ma to ca. 12 Ma, and then transtension from ca. 12 Ma to ca. 6 Ma et al., 2004); Ingersoll and Rumelhart (1999) concluded that basin subsidence

may have even occurred independently of plate-motion changes. Proposed Catalina Basin models for ICB basin opening include nonuniform crustal thinning (Bohannon et al., 2004) and localized extension along strike-slip releasing bends (Namson Catalina Basin is bounded by Santa Catalina Island to the northeast, Santa and Davis, 1990; Legg et al., 1999, 2007; Legg and Borrero, 2001). Sometime Barbara Island to the northwest, and San Clemente Island to the southwest after ca. 6 Ma, the ICB underwent a tectonic transition from transtension to (Fig. 2A). These subaerial islands and submerged ridges (e.g., San Clemente regional transpression, which is ongoing today (e.g., Ingersoll and Rumelhart, Ridge, Catalina Ridge) generally feature exposed Miocene and older deposits 1999; Schindler, 2010). including volcanic, intrusive granitic, and metamorphic rocks like the Catalina Age constraints from basin sampling provide regional context for seismic Schist (Vedder et al., 1986; Vedder, 1990). Although these ridges and islands are stratigraphy in the ICB basins (e.g., Emery, 1960; Moore, 1969; Gorsline et al., composed of largely Miocene and older rocks, the linear, southwestern edge of 1984; Gorsline and Douglas, 1987; Teng and Gorsline, 1989; Bohannon et al., Catalina Ridge (Fig. 2A) has been previously interpreted as a Quaternary-active 2004; Brothers et al., 2018). Stratigraphic control in the ICB basins comes from fault (e.g., Chaytor et al., 2008; Legg et al., 2015). Emery Knoll, a large (~10-km-di- Ocean Drilling Program (ODP) sites in the Santa Monica Basin (ODP Leg 167, ameter) submerged circular uplift located in southern Catalina Basin (Fig. 2A), Site 1015; e.g., Normark and McGann, 2004; Normark et al., 2004b; Romans has been variably interpreted as a resurgent caldera (Legg et al., 2004b) and a et al., 2009) and in the San Nicolas Basin (ODP Leg 167, Site 1013; e.g., Ship- magmatic diapir (Junger and Sylvester, 1979; Ridgway and Zumberge, 2002). board Scientific Party, 1997; Janik et al., 2004; De Hoogh, 2012). Stratigraphic The SCF marks the southwestern boundary of Catalina Basin, and we now age constraints also come from industry wells in some nearshore areas (e.g., know it extends ~80 km north (this study) and at least 80 km south (e.g., Legg Sorlien et al., 2015) and from shallow sediment cores and grab samples (Nar- et al., 2007) of the basin (Figs. 2B, 3). The SCF strikes northwest, roughly in din et al., 1979; Barron, 1986; Vedder et al., 1986; Vedder, 1990; Normark et al., line with the Pacific–North America plate motion vector of ~321° through the 2004b). Acoustic basement throughout the ICB (which we refer to herein as region (DeMets et al., 2010); however, several local bends along the fault have simply “basement”) consists of the Catalina Schist, as well as intrusive granitic been associated with pull-apart basins and popup structures (e.g., Legg et al., and volcanic rocks in some areas (e.g., Crouch and Suppe, 1993; Bohannon and 1999, 2007). The SCF has long been considered the dominant fault in the Geist, 1998). Atop the acoustic basement, ICB basins contain Miocene sediments region (e.g., Ridlon, 1968) and has been previously interpreted as a right-lat- characterized by subparallel, highly deformed acoustic reflections that drape eral strike-slip fault accommodating ~4–6 mm/yr of dextral motion, with rates the underlying bedrock. A regional unconformity separates deformed Miocene estimated from several potential (though unverified) geologic piercing points strata from a section of Pliocene-Quaternary sediment as much as 1 km thick, and GPS models (Legg, 1985, 1991a, 2005; Larson, 1993; Bennett et al., 1996; which exhibits more flat-lying, subparallel, coherent reflections and contains a Humphreys and Weldon, 1994; Goldfinger et al., 2000). greater proportion of terrigenous material deposited in marine basins follow- The CF, bounding Catalina Basin to the northeast, has had a number of inter- ing Miocene subsidence and associated sea-level rise (Bohannon et al., 2004). pretations, ranging from right lateral to convergent (Kier and Mueller, 1999; Legg Active faults that traverse the ICB include: the Newport-Inglewood fault, et al., 2004a; Chaytor et al., 2008). Previous mapping of the CF within Catalina which carries at least 0.3–0.6 mm/yr of dextral slip at its northernmost extent Basin was done without comprehensive modern high-resolution bathymetry or near Huntington Beach (Grant et al., 1997), with slip rates south of Dana Point seismic-reflection data, leading to debate about the fault geometry and relation- increasing to 1–2 mm/yr (Fischer and Mills, 1991); the Rose Canyon fault, which ship with neighboring faults (e.g., Legg et al., 2004a, 2007, 2015; Ryan et al., 2012). also accommodates 1–2 mm/yr of right-lateral slip (Lindvall and Rockwell, The CF has been previously mapped as connecting or interacting with either the 1995); the Palos Verdes fault, which has a dextral slip rate of 1.6–1.9 mm/yr Catalina Ridge (Fig. 2A; e.g., Legg et al., 2015) or the Santa Cruz fault zone to the offshore (Brothers et al., 2015) and 2–4 mm/yr onshore (McNeilan et al., 1996; north (Fig. 2B; e.g., Legg et al., 2004a), and/or the San Diego Trough fault to the Ward and Valensise, 1994); and the San Diego Trough fault, which carries 1.2– south (Fig. 2B; e.g., Legg et al., 2007). Each of these fault configurations would 1.8 mm/yr of right-lateral slip (Ryan et al., 2012). There is no clear consensus on serve to extend the length of the CF and therefore its seismogenic potential (e.g., how slip is partitioned amongst individual faults throughout the ICB, owing in Wells and Coppersmith, 1994; Legg et al., 2004a, 2015). The variable strike of the large part to uncertainty in mapping of active fault traces and to sparse robust CF is generally northwest, similar to the SCF, but is obliquely convergent with the slip-rate estimates along offshore fault segments. Only four studies have pro- Pacific–North America plate motion vector in some areas (DeMets et al., 2010). vided direct evidence for Late Pleistocene–Holocene lateral offset along faults Because of this obliquity, the Catalina fault has been thought to be transpres- located offshore (McNeilan et al., 1996; Ryan et al., 2012; Brothers et al., 2015; sional and potentially tsunamigenic (Legg and Borrero, 2001; Legg et al., 2004a, Conrad et al., 2017). Brothers et al. (2015) noted a 3–5 mm/yr deficit between 2007, 2015; Chaytor et al., 2008). Slip rates on the CF are poorly defined, but have summed slip rates of the other significant faults in the ICB (i.e., Newport–Ingle- been estimated at ~2.5 mm/yr based on an inferred restoration of Catalina Ridge wood–Rose Canyon, Palos Verdes, San Diego Trough) and the total available back to an embayment on western Santa Catalina Island (Chaytor et al., 2008). slip across the region based on GPS data (6–8 mm/yr); they proposed that the In order to better understand the recency of deformation along Catalina majority of the 3–5 mm/yr deficit is accommodated by the SCF. Basin fault zones, we consider the age, source, and type of sediment in the basin.

A KF CF B KF CF

SCF SCF

118.8° W gullies gullies 118.8° W ridge ridge Figure 3. (A) Map of the Kimki Ridge area showing high-resolution bathymetry collected in 2016, gridded at 25 m resolution (Dartnell et al., 2017). Image KRN CB seeps KRN CB shows grayscale relief beneath 75% transparent KB color bathymetry. Mapped Quaternary fault traces KB from this study are shown as thin solid lines. (B) Map of backscatter intensity data gridded at 15 m resolution over the same region as A (Dartnell et al., 2017). Image highlights seeps along Kimki Ridge as SCR KRS SCR KRS bright spots (Conrad et al., 2018). Location of figure secondary faults secondary faults is shown in Figure 2A. KB—Kimki Basin; SCR—San N headwall headwall Clemente Ridge; KF—Kimki fault; KRN—Kimki Ridge

scarps? SCF scarps? SCF (north); KRS—Kimki Ridge (south); SCF—San Clem- ente fault; CB—Catalina Basin; CF—Catalina fault. 33.2° N 33.2° N 0 N 0 2 4 8 0 2 4 8

bathy −1500

elev. (m) km km

The San Gabriel Channel system dominated Late Pleistocene Catalina Basin fill 2020], for data releases and detailed survey information). High-resolution MCS with terrestrial-sourced sediments (Maier et al., 2018), and in the latest Pleisto- reflection and sub-bottom CHIRP data were collected during both the 2014 and cene and Holocene, hemipelagic sedimentation draped San Gabriel deep-sea 2016 geophysical surveys (Fig. 2B). The 2014 R/V Robert Gordon Sproul data set fan sediments (e.g., Maier et al., 2018). The San Gabriel depositional system has includes ~1238 km of coincident 48-channel sparker MCS and 3.5 kHz Knudsen been interpreted as primarily receiving terrestrially derived sediment during CHIRP data, ~906 km of which is located within Catalina Basin, the remainder sea-level lowstands due to the ca. 15 ka extrapolated age of the base of the located to the north in Santa Cruz Basin (e.g., Brothers et al., 2018). The 2016 hemipelagic drape layer (Normark et al., 2009; Ryan et al., 2012; Maier et al., survey aboard the R/V Thomas Thompson acquired 24-channel minisparker MCS 2018) and the 8–18 km distance between the shoreline and San Gabriel canyon reflection data and coincident 3.5 kHz Knudsen CHIRP data totaling ~726 line heads (Fig. 2A) (Alexander and Lee, 2009; Sommerfield et al., 2009; Maier et al., kilometers (spacing generally 3–4 km), all within Catalina Basin. High-resolution 2018). Catalina Canyon (Fig. 4), on the northern basin margin, has provided multibeam bathymetry data were also acquired using the R/V Thomas Thomp- lesser sediment input to Catalina Basin from Santa Catalina Island (Maier et al., son’s Kongsberg EM302 hull-mounted multibeam echosounder (MBES) system. 2018). Currently, the best sedimentary age control in the Catalina Basin region All MCS data were processed to poststack time migration using commercial consists of paleontological dating of seafloor dart cores (Barron, 1986; Vedder software (see Balster-Gee et al., 2017, 2020, for details). The MBES data were et al., 1986; Vedder, 1990). Although the subsurface fill of Catalina Basin lacks edited and processed using Caris HIPS and SIPS (www.caris.com/products age control, stratigraphic relationships and seismic facies analysis can be used /hips-sips); associated backscatter data were processed using the SonarWiz to infer correlative allostratigraphic units across ICB basins due to their similar software package. Seafloor data were then gridded at 10 and 15 m resolution genetic histories (e.g., Emery, 1960; Moore, 1969; Gorsline et al., 1984; Gorsline throughout Catalina Basin depending on water depth. The 2016 R/V Thomas and Douglas, 1987; Teng and Gorsline, 1989; Bohannon et al., 2004). Thompson MBES data were merged with other high-resolution MBES data sets collected in the ICB since 2010 (Dartnell et al., 2015, 2017). A regional mosaic was generated and gridded at 25 m resolution. Lastly, the Southern Califor- ■■ DATA AND METHODS nia Coastal Relief Model (CRM, version 2; Calsbeek et al., 2013) was used for regions that do not presently contain MBES coverage. Marine Geophysical Data

The primary data sets used to examine the seafloor and subsurface were Additional Geophysical Data Sets acquired in 2014 and 2016 aboard the R/V Robert Gordon Sproul (U.S. Geological Survey [USGS] cruise ID 2014-645-FA) and R/V Thomas Thompson (cruise ID Legacy academic and industry MCS data are also available in the Cata- 2016-616-FA), respectively (see Dartnell et al., 2017, and Balster-Gee et al., 2017, lina Basin (e.g., Fig. 2B) and were used to examine the entire sedimentary

A offset CR 0 0 2 4 8 N B offset CR 0 2 4 8 N Figure 4. (A) Map of the Catalina fault area km km showing high-resolution bathymetry col- bathy −1500 lected in 2016, gridded at 25 m resolution elev. (m) (Dartnell et al., 2017). Image shows gray- CRF CRF scale relief beneath 75% transparent color CR Santa CR Santa bathymetry. Annotations include gullies, Catalina Catalina landslides, and areas with exposed Plio- Island Island cene sediment (from Vedder et al., 1986); A′ gullies gullies dash-dot line represents an onlap uncon- A formity in Catalina Basin (Fig. 8). Mapped 33.4° N 33.4° N Quaternary fault traces from this study S landslides S landslides C CC C CC K secondary faults K secondary faults are shown as thin solid lines, and pre- F ridge F ridge F F Quaternary faults as thin dashed lines. Inset C C F F shows a bathymetric profile (A-A′ line) of 8.8° W an ~10 m scarp across the Catalina fault. 1

1 118.8° W 118.8° (B) Map of backscatter intensity data grid- CF profile A A′ exposed Pliocene exposed Pliocene ded at 15 m resolution over the same region −1190 exposed Quaternary exposed Quaternary as part A (Dartnell et al., 2017). Location of −1194 ?? ?? figure is shown in Figure 2A. CC—Catalina −1198 Canyon; CF—Catalina fault; CR—Catalina Elevation (m) −1202 Ridge; CRF—Catalina Ridge fault; SCF—San 0 200 400 600 800 (SW) Distance along profile (m) (NE) Clemente fault; KF—Kimki fault.

118.6° W 118.6° W 118.6°

basin fill and character of the acoustic basement, and also to extend fault ESRI ArcGIS software. Subsurface (MCS and CHIRP) data interpretation was mapping beyond the edges of the high-resolution data coverage. All legacy completed using IHS Kingdom Suite seismic reflection interpretation software. data sets crossing our survey area are deeper-penetrating, lower-resolution, Seismostratigraphic horizons presented in this study represent unconformity crustal-scale MCS data. Two notable legacy MCS surveys crossing Catalina surfaces. Isochron sediment thickness maps were generated for key marker Basin include EW9415 (the Los Angeles Region Seismic Experiment [LARSE] horizons, with thickness reported in two-way travel time. study; e.g., ten Brink et al., 2000) and L490SC (Triezenberg et al., 2016), col- New fault maps were generated using a combination of MBES, high-resolu- lected aboard the USGS R/V Lee in 1990. Other relevant exploration industry tion MCS, and CHIRP data through iterative verification of features mapped at surveys are publicly available through the USGS National Archive of Marine the seafloor and in the subsurface in ArcGIS and Kingdom Suite, respectively. Seismic Surveys (NAMSS; Triezenberg et al., 2016), including B685SC, W385SC, The 2014 R/V Robert Gordon Sproul and 2016 R/V Thomas Thompson data sets and W582SC. provide dense high-resolution multibeam and seismic-reflection data within We integrate several other regional geophysical data sets with the high-res- and on the margins of Catalina Basin, where we map the Quaternary-active olution marine geophysical observations, including an earthquake relocation SCF, CF, and KF, as well as secondary faults interpreted to be Quaternary active. catalog from Hauksson et al. (2012), focal mechanism calculations for a subset Holocene (active-fault) offsets were interpreted from offset of the Holocene of these events (Yang et al., 2012), and significant earthquakes (>M4) occurring drape layer (Maier et al., 2018) using CHIRP sub-bottom profile data. in the ICB prior to 1981 from the USGS earthquake catalog (earthquake.usgs North of Catalina Basin, we slightly modify the geometry of the primary .gov/earthquakes). We also include airborne magnetic anomaly (Langenheim strand of the SCF and the Santa Cruz fault using MBES data and legacy seis- et al., 1993) and Bouguer gravity anomaly (Beyer, 1980; Bankey et al., 2002) mic-reflection data sets. We also extend the geometry of the Catalina fault data, which have been gridded for the ICB at ~1 km. Finally, we use GPS veloc- north into Santa Monica Basin using MBES data, high-resolution MCS data ity data from the Southern California Crustal Motion Map, version 4.0 (Shen across Catalina Ridge, and legacy MCS data within Santa Monica Basin. South et al., 2011), to examine the regional strain field. of Catalina Basin, we slightly update the primary strand of the SCF from maps provided by Ryan et al. (2009) using MBES data. We do not map or remap any Quaternary fault strands (solid lines in our figures) where we do not explicitly Data Analysis image clear seafloor offset in one or more high-resolution data sets (MBES, MCS, and/or CHIRP). Dashed faults in our maps indicate unverified (likely Multibeam bathymetric grid processing (including slope and shaded-re- pre-Quaternary or inactive) fault strands. All fault interpretations from this lief map generation), analysis of morphological features (e.g., slope failure, study have been incorporated into the new USGS Quaternary Faults Offshore ridges), and vector data interpretations (i.e., faults) were done largely using of California (QFO) database (Walton et al., 2020) and are publicly accessible.

■■ RESULTS Catalina Basin, Kimki Ridge is an elongate, northwest-trending, ~20-km-long and ~2-km-wide seafloor ridge system. Kimki Ridge has a maximum relief Here, we present observations of new, high-resolution seafloor and sub- of ~400 m, with the ridge peak lying at water depths of ~900–1000 m. Kimki surface geophysical data in the Catalina Basin region. We focus on the SCF Ridge is bounded by steep, scarp-like slopes and is also obliquely cut by a and CF zones, other related structures, and the stratigraphy of Catalina Basin, sharp lineament that divides the ridge into discrete north and south segments and compare our results to plate-motion models, seismicity, and potential-field (Fig. 3), and it forms the eastern margin of the ~10-km-long and ~5-km-wide data sets. Kimki sub-basin (Figs. 2A, 3). Another intrabasin ridge is located downslope of northernmost Santa Catalina Island and ~3 km southwest of the mapped CF (Fig. 4). This ~10-km-long ridge has a maximum relief of ~130 m. Seafloor Morphology and Surficial Geology The San Gabriel Channel (Fig. 5), the primary channel system delivering terrestrial sediment to Catalina Basin (Maier et al., 2018), enters Catalina Basin Catalina Basin narrows from ~40 km wide in the southeast to ~10 km wide between two bedrock highs at the basin’s southeastern corner (Fig. 5). Maier in the northwest (Fig. 2A) and is partially flanked by San Clemente and Santa et al. (2018) describe channels and scours likely associated with the San Gabriel Catalina Islands. There is significant relief between both islands and the basin channel-lobe transition zone on the southeastern basin floor (Fig. 5). Sediment floor (as much as ~1800 m and ~1700 m, respectively), as well as exceptionally supply to Catalina Basin also comes through several canyons and gullies on steep submarine slopes (reaching ~35°) on the basin margins. Conversely, the steep basin margins, including the ~1-km-wide Catalina Canyon (Fig. 4) much of the Catalina Basin floor is relatively flat, with slopes generally less along the northern basin margin, which incises the shelf edge <2 km from than 2°–3° except along channels and scours (e.g., Maier et al., 2018). The the coastline. We also image a number of smaller, ~100–400-m-wide gullies depth of the basin increases from ~1000 m in the southeast where the San and smaller canyons along northern Kimki Ridge (Fig. 3) and downslope of Gabriel Channel enters the basin (Fig. 2A) to ~1300 m in the northwest extent each of Santa Barbara, San Clemente, and Santa Catalina Islands (Figs. 2A, 4). of the basin at the southeastern flank of Santa Barbara Island. Seafloor backscatter data (Figs. 3B, 4B, and 5B) highlight a number of Catalina Basin also contains and is bounded by numerous submerged, sedimentary features throughout Catalina Basin, including relatively low elongate ridges. San Clemente Island and Santa Catalina Island both continue backscatter of the San Gabriel Channel as it enters Catalina Basin (Fig. 5B) as submerged ridges to the north (known as San Clemente Ridge and Catalina and relatively high reflectivity at narrower canyon systems (e.g., Catalina Ridge, respectively), both trending NW-SE (Fig. 2A). These linear ridges, par- Canyon; Fig. 4B). Higher backscatter values are located along ridge systems ticularly Catalina Ridge, have been previously interpreted as being structurally (e.g., southern Kimki Ridge; Fig. 3B), at basin edges where there are older controlled by Quaternary faults (e.g., Legg et al., 2015). In the northwestern sediments (e.g., Fig. 4B), and at bedrock highs (e.g., Fig. 5B). Northern Kimki

A San Gabriel B San Gabriel Channel Channel

118° W 118° W 118°

118.2° W118.2° Figure 5. (A) Map of the southeastern Catalina Ba- sin showing high-resolution merged bathymetry, gridded at 25 m resolution (Dartnell et al., 2017). SDTF SDTF Image shows grayscale relief beneath 75% transparent color bathymetry. Annotation shows the San Gabriel Channel as it enters Catalina Basin, the Qua- 33.2° N 33.2° N ternary San Diego Trough fault (SDTF) highlighted with a thin black solid line, and the escarpment at re-channelized re-channelized the base of Santa Catalina Island highlighted with in basin in basin a thin black dashed line. Sediment pathways are highlighted with thick semitransparent gray lines. channel-lobe channel-lobe (B) Gridded backscatter intensity data over the same transition transition region as part A. Note that both images include data from the 2016 R/V Thomas Thompson survey (15 m resolution) as well as data from a 2013 R/V Melville bedrock bedrock survey (20 m resolution; see Dartnell et al., 2015). highs highs Location of figure is shown in Figure 2A. 0 m N 0 2.5 5 10 N 0 2.5 5 10 bathy

elev. (m) −1500 m km km

118.2° W118.2°

Ridge has three notable bright spots located along the ridge crest (Fig. 3B), 0 - W E which have been sampled and determined to be active methane seeps (Con- A Fig. 7 rad et al., 2018), and a fourth seep was recently discovered on the crest of southern Kimki Ridge (J. Conrad, 2019, personal commun.; Fig. 3B). Another Catalina Basin 1 - similar bright spot is apparent on the crest of the smaller ridge in the north- CF sub-basin eastern basin (Figs. 3B, 4B). SCF The Catalina Basin seafloor is composed of exposed Quaternary sediment with a Holocene drape layer. Uplifted pre-Quaternary sediment and structure 2 - crop out at the basin margins; for example, the Kimki Ridge system (Fig. 3) is Ps a fault-bisected anticline composed of Pliocene and older sedimentary rocks Mb TWTT (s) TWTT (Vedder et al., 1986; Vedder, 1990). Similarly, the ~10-km-long ridge downslope 3 - of Santa Catalina Island (Fig. 4) is composed of Pliocene and older sedimentary

deposits (Vedder et al., 1986; Vedder, 1990). Pliocene stratigraphy is commonly ~1 km also overlain by hemipelagic drape. 4 - 3 km ~3.5x VE 0 - W E B Santa Monica Basin Catalina Basin Stratigraphy SCrF ODP 1015 1 - SPBFZ The new 2014 and 2016 seismic-reflection surveys (Fig. 2B) reveal basin CF (projected) architecture that we interpret within the general allostratigraphic and seismic facies framework for ICB basins summarized by Teng and Gorsline (1989; 2 - Ps Fig. 6). We identify the same units and unconformities in the Catalina Basin, TWTT (s) TWTT Mb including a notable regional unconformity (Miocene surface, MS; see Fig. 7), which we define as the upper boundary of deformed, tilted, and acoustically 3 -

chaotic seismic reflections and/or reflection-free zones. The chaotic and semi- ~1 km transparent facies are the acoustic basement here, and the MS unconformity 4 - 6 km ~7x VE is atop layered, deformed seismic reflections that conform to the underlying basement topography. The MS unconformity commonly truncates the under- Figure 6. (A) Crustal-scale seismic reflection line 116 from legacy survey L490SC crossing the Catalina Basin (Triezenberg et al., 2016) (line location shown in Fig. 1). A regional unconformity, lying reflections and is likely late Miocene in age based on comparison to likely late Miocene in age, is mapped as a solid red line. Quaternary faults are mapped as ages and seismic facies in adjacent ICB basins (e.g., Teng and Gorsline, 1989; subvertical black solid lines. Intersection with Figure 7 is annotated at the top of the image. Schindler, 2010). We map another surface similar to the MS unconformity (B) Crustal-scale seismic reflection line 03e from legacy survey EW9415 crossing the Santa locally through northwestern Catalina Basin (basement or bedrock surface, BB; Monica Basin (ten Brink et al., 2000) (line location shown in Fig. 1). Fault and unconformity annotation matches that of A. Nearby Ocean Drilling Program (ODP) Leg 167 Site 1015 (blue) e.g., Fig. 8). Similar to the MS unconformity, the BB surface caps chaotic and has been projected onto this line (location from Normark et al., 2004b). Ps—Pliocene and semitransparent seismic facies of the acoustic basement (e.g., Fig. 8). The BB younger marine sediments; Mb—Miocene and older sediments and basement rock; CF—Cata- surface may be contemporaneous with the MS unconformity, but we cannot lina fault; SCF—San Clemente fault; SCrF—Santa Cruz fault; SPBFZ—San Pedro Basin fault zone. Vertical scale depth conversions assume 2 km/s two-way travel time (TWTT) velocity. directly link the two surfaces using the available data. VE—vertical exaggeration. Post–late Miocene strata both downlap onto and onlap the MS unconformity (Fig. 7) and are generally less deformed and more flat lying than sediments below the MS unconformity. These overlying strata fill several local onto it. This uppermost stratigraphic unit has variable thickness, generally paleo-lows (previous sedimentary depocenters), creating localized sub-basins thickest in the region of the San Gabriel Channel (e.g., Fig. 7) and in north- (Fig. 7). Sub-basins contain localized growth strata in their centers; in other western Catalina Basin, and thinnest in the center of Catalina Basin (Fig. 9). words, with increasing depth, reflection dip increases toward sub-basin edges, The age of the PS unconformity is likely Pliocene based on ages from seafloor layer thicknesses increase toward the sub-basin center, and sediment wedges samples of the surface where it crops out on the basin margins (Vedder et al., pinch out at sub-basin edges (Fig. 7). 1986; Vedder, 1990; also see, e.g., Pliocene structure map, Fig. 9). Sub-basins A relatively transparent package above the MS unconformity is capped by exhibit evidence for growth strata above and below the mapped PS unconfor- a second notable regional unconformity (Pliocene surface, PS; see Fig. 7). The mity, and the most recently deposited sediments are parallel, flat lying, and PS unconformity is overlain by stratified, wedge-shaped strata that downlap undeformed, except near the CF and SCF (e.g., Fig. 7).

A NW SE Fig. 6A SCR 1.2 - faults San Gabriel Fan deposits 1.4 - downlap surface B 1.6 - sub-basin PS (s) Figure 7. (A) High-resolution multichannel seis- SCF MS mic (MCS) line TT04 from the 2016 R/V Thomas Thompson survey (line location shown in Fig. 2B). TWT T 1.8 - Line shows basin sedimentary architecture through the Catalina Basin, with faults marked by thin black lines and regional unconformities marked by solid 2.0 - onlap surface colored lines. Local growth strata in the labeled C sub-basin (characterized by thickening toward the basin center and layers pinching out at basin edges) 2.2 - are highlighted with a blue bracket and label. In- ~200 m growth tersection with Figure 6A is annotated at the top strata 5 km older faulting and folding ~20x VE of the image. MS—Miocene surface; PS—Pliocene surface. (B) Enlarged MCS data showing faulting ~13.5x VE 500 m B ~13.5x VE 500 m C on the San Clemente Ridge (SCR) adjacent to the San Clemente fault (SCF). Mixed or unclear fault offsets are labeled with “?”. Inset location is shown 50 m SCF 50 m in A. (C) Enlarged MCS data showing the SCF. Inset location is shown in A. An automatic gain control with a window of 250 ms has been applied to all images in this figure. Vertical scale depth conversion for all figures assumes 1.5 km/s two-way travel time (TWTT) velocity. VE—vertical exaggeration.

? ?

Two-way travel time structure contour and isochron sediment thickness the basin. Maier et al. (2018) noted that the shallowest depositional lobes of maps were generated for the stratigraphic interval between the MS and PS the San Gabriel depositional system appear in this same southwestern region regional unconformities, as well as between each of the MS and PS unconfor- due to a broad topographic low that persists in the basin today. mities and the seafloor (Fig. 9). The structure contour maps of the MS and PS In Figure 6, we directly compare the sedimentary deposits of Catalina unconformities reveal the changing geometry of the localized paleo-lows (or Basin to those in another ICB basin, the Santa Monica Basin. The unconformity depocenters; Fig. 9) through time, which do not mimic the modern basin mor- between Miocene and Pliocene deposits is similar in both basins, but apart phology. Isochron thickness maps indicate areas of thick sediment within these from the deep localized sub-basins in Catalina Basin, Pliocene–Quaternary localized sub-basins. The seafloor-Pliocene isochron thickness map shows the sediment thickness is generally greater in Santa Monica Basin than in Catalina most uniform distribution of sediment throughout the basin (Fig. 9), with the Basin. This difference is due to the proximity of Santa Monica Basin to active thickest sediment in the package located in a persistent sub-basin along south- terrestrial sediment sources, leading to relatively thick Quaternary sediment western Catalina Basin and near the entrance of the San Gabriel Channel into cover there (Romans et al., 2009).

Catalina Basin Faults 1.0 - A S Pliocene sediment N Fig. 14 Fig. 10 exposed growth San Clemente Fault strata Kimki Ridge Catalina 1.5 - Kimki KF Basin Basin CF Faults in the Catalina Basin generally exhibit linear expressions in the new SCF (s) ICB bathymetric compilation of Dartnell et al. (2017) due to seafloor scarps (Fig. 3A). The SCF extends along the entire ~90 km length of Catalina Basin with 2.0 - PS an average strike of ~327°. For most of this stretch, the SCF is located along TWT T the westernmost Catalina Basin floor and characterized by an ~2–4-m-high, southwest-up scarp on the seafloor. A few kilometers south of the Kimki Ridge 2.5 - BB system, the SCF makes a broad ~20° releasing bend and traverses across Kimki ~250 m 3 km ~10x VE Ridge and the flat floor of the Catalina Basin. In the releasing bend area, the CHIRP data over CF CHIRP (flattened) sense of offset across the SCF changes to be northeast-side-up, as defined by B C an ~2-m-high seafloor scarp. Bathymetric data show the main trace of the SCF making a broad ~30° left bend north of the ~20° releasing right bend in northernmost Catalina Basin. The SCF thus trends toward and connects farther north with the Santa Cruz fault, an ~80-km-long structure with an average strike of ~311° that runs along a ridge between Santa Cruz and Santa Monica Basins (Figs. 1, 2B). South of offset drape Catalina Basin, the SCF continues for at least another 80 km with an ~318° strike,

nearly parallel with the average Pacific plate motion (321°) through the region 25 m (DeMets et al., 2010). The total mapped length of the main strand of the SCF thus exceeds 250 km. Although small (300–400-m-wide) gullies along north- ~40x VE 1 km ern Kimki Ridge appear to have been offset by the SCF, correlative features Figure 8. (A) High-resolution multichannel seismic (MCS) line 12 from the 2014 R/V Robert on either side of the fault cannot be identified with certainty (Fig. 3), and we Gordon Sproul survey highlighting the Catalina fault (CF; line location shown in Fig. 2B). could not identify any other horizontal SCF piercing points on the seafloor in Line also crosses the Kimki fault (KF) and the San Clemente fault (SCF) in the north-central Catalina Basin. Regional unconformities are highlighted in color (PS—Pliocene surface; BB— the new high-resolution seafloor data. basement or bedrock surface). Local, nonregional unconformities are highlighted in blue. In the subsurface, the SCF exhibits a near-vertical dip with offset seismic A depth range containing growth strata along the CF is bracketed and labeled. Intersection reflections to the seafloor. Vertical offset is southwest-up through the southern with Figures 10 and 14 is annotated at the top of the image. An automatic gain control with a window of 500 ms has been applied. (B) Coincident CHIRP data over the CF. Inset location basin adjacent to San Clemente Island (Figs. 7A, 7C) and northeast-up through is shown with black dashed box in A. (C) Seafloor-flattened CHIRP data over approximately northern Catalina Basin (Figs. 10A, 10B). CHIRP data image offsets of the base the same area and scale as B. Vertical scale depth for all figures assumes 1.5 km/s two-way of the drape layer along the SCF (Figs. 10B, 10C), which was likely deposited travel time (TWTT) velocity. VE—vertical exaggeration. See Figure 10 for CHIRP images of the SCF and Figure 11 for CHIRP images of the KF. starting ca. 15 ka (Normark et al., 2009; Ryan et al., 2012; Maier et al., 2018). The shallowest, 0–200-m-deep seismic reflections are traceable across the SCF, but deeper layers generally cannot be correlated across the fault (e.g., Fig. 10A). map three additional, subparallel secondary faults on San Clemente Ridge (Fig. 3). These secondary faults feature <2-m-high scarps that are generally Kimki Fault and Other Splays of the San Clemente Fault smoother than the SCF scarp, but still resolvable in the 25 m high-resolution bathymetry, and do not explicitly connect with the SCF. In northern Catalina Basin, the SCF bisects and divides the Kimki Ridge sys- At 33 km long, the KF is the third-longest fault in the Catalina Basin (after tem into north and south ridge segments (Fig. 3). Here, the fault zone becomes the SCF and the CF). It strikes subparallel to the SCF through northernmost more complicated, and we map a number of secondary faults and subparallel Catalina Basin with the same average strike of 327°. We consider the KF a SCF splays, including the Kimki fault (KF), a fault originally discovered and splay of the SCF because its southern tip connects with the SCF. Where the informally named by Ford and Normark (1980). The KF runs along and north of KF bounds northern Kimki Ridge on its west side, vertical offset of the KF is the western side of northern Kimki Ridge, and an unnamed splay marks the east northeast-side-up, but the fault transitions to show southwest-side-up north side of the southern ridge (Fig. 3). Both the KF and the unnamed splay connect of the Kimki Ridge system through northern Catalina Basin. In contrast to the with the SCF at their southern tips (Fig. 3). West of southern Kimki Ridge, we sharper scarps along the SCF, the KF is characterized by smooth seafloor and

118.8° W 118.6° W 118.4° W 118.2° W 118.8° W 118.6° W 118.4° W 118.2° W

Seafloor structure faults Seafloor-Pliocene isochron faults N N Depth (m) Thickness 0 TWTT (ms) 1100 500 750 100

33.4° N 250 20 1500 0 0 CI = 100 m 100 CI = 100 ms 1100 100 1300 20 0 20 0 1200 33.2° N 1100 0 200 10

0 1000 20 10 900 0 depocenters 0 10

0 4 8 16 800 0 4 8 16 33° N km km Pliocene structure faults Pliocene-Miocene isochron faults N Depth N Thickness TWTT (ms) TWTT (ms) 500 500 Figure 9. Gridded structure (left) and isochron sed- 0 iment thickness (right) maps within the Catalina 190 190

33.4° N 0 Basin based on mapped multichannel seismic sur- 170 1500 250 0 160 10 160 0 500 faces. Seafloor-Pliocene, Pliocene-Miocene, and 0 0 180 2500 0 Seafloor-Miocene labels indicate the pairs of sur- 0 faces that have been differenced to generate the CI = 100 ms CI = 100 ms 200 isochron thickness maps. Except for the seafloor, 0 20 0 depths and thicknesses are reported in two-way 0 20 travel time (TWTT). Quaternary faults (this study) 33.2° N 0 10 are shown as solid lines, probable pre-Quaternary 180 0 100 30 150 10 0 faults as dashed lines. Location of figure shown in 0 0 0 20 0 0 170 0 Figure 2A. CI—contour interval. seafloor 0 0 0 120 1600 150 exposure depocenters 0

100 0 4 8 16 0 4 8 16 33° N km km Miocene structure faults Seafloor-Miocene isochron faults N Depth N Thickness TWTT (ms) TWTT (ms) 500 500

33.4° N 210 1500 250 0 40 0 20 0 30 200 0 0 0 2500 0 local 30 210 0 CI = 100 ms 0 20 CI = 100 ms 200 lows 0 220 0 50 0 0 180 0 40 0 30

33.2° N 200 400 0 0 170 180 190 0 0 190 30 local lows 0 0 40 190 0 100 0 0 150 20 0 1700 0 130 depocenters 100 0 160 0 4 8 16 basement 150 0 0 4 8 16 33° N km highs km

S N S N 0.5 - CR CF A Fig. 8 Fig. 14 Miocene sediment CR exposed Miocene sediment 1.0 - exposed local 1.0 - Pliocene sediment exposed unconformities SCF on northern Kimki Ridge northernmost Catalina Basin (s) (s) 1.5 - KF KF 1.5 - northern BB Catalina Basin TWT T SCF TWT T 2.0 - 2.0 - local unconformities 2.5 - ~300 m ~300 m 2.5 - A ~9.5x VE 3 km ~9.5x VE 3 km B CHIRP data over SCF C CHIRP (flattened) B CHIRP data over KF C CHIRP (flattened) no offset? inverted fold

growth

offset offset drape 20 m 20 m ~20x VE 250 m ~41.5x VE 500 m Figure 10. (A) High-resolution multichannel seismic line TT10 from the 2016 R/V Thomas Thompson survey highlighting the San Clemente fault (SCF; line location shown in Fig. 2B). Figure 11. (A) High-resolution multichannel seismic line TT08 from the 2016 R/V Thomas Line crosses the three major fault systems (thin black lines) in the northern Catalina Basin: Thompson survey highlighting the Kimki fault (KF; line location shown in Fig. 2B). Line also the SCF, the Catalina fault (CF), and the Kimki fault (KF). The CF shown here is located north crosses the San Clemente fault (SCF) at the base of the Catalina Ridge (CR). Basement or of Catalina Ridge (CR). Local, nonregional unconformities are highlighted in blue. Intersection bedrock regional surface (BB) is highlighted in green. Local, nonregional unconformities are with Figures 8 and 14 is annotated at the top of the image. An automatic gain control with a highlighted in blue. An automatic gain control with a window of 250 ms has been applied. window of 250 ms has been applied. (B) Coincident CHIRP data over the SCF. Inset location (B) Coincident CHIRP data over the KF. Inset location is shown with a black dashed box in A. is shown with a black dashed box in A. (C) Seafloor-flattened CHIRP data over approximately (C) Seafloor-flattened CHIRP data over approximately the same area and scale as B. Vertical the same area and scale as B. Vertical scale depth conversion for all figures assumes 1.5 km/s scale depth conversion for all figures assumes 1.5 km/s two-way travel time (TWTT) velocity. two-way travel time (TWTT) velocity. VE—vertical exaggeration. See Figure 11 for CHIRP im- See Figure 10 for CHIRP images of the SCF. ages of the KF and Figure 8 for CHIRP images of the CF.

an inflection in seafloor slope. The northernmost reach of the KF traverses up splay bounding the eastern edge of the southern ridge in the Kimki Ridge a canyon system on the steep slope flanking southern Santa Barbara Island. system (Fig. 3) exhibits small offsets in the subsurface that diminish moving Here the KF becomes unresolved in existing geophysical data, and may die north. The secondary faults on San Clemente Ridge (Fig. 3) generally exhibit out as shown in Figure 2B. We were not able to identify piercing points across small normal offsets, and three of these faults offset the seafloor (Figs. 3, 7B). the KF; several small-scale gullies located west of the KF may cross the KF, but we cannot resolve or correlate the gullies across the fault with the available high-resolution bathymetric data (e.g., Fig. 3). Catalina Fault Like the SCF, MCS data reveal that the KF is a subvertical fault in the shallow subsurface (Fig. 11A), but the offset at the base of the seafloor drape does The CF is confined to northeastern Catalina Basin and has a mapped length not appear to be offset in CHIRP data crossing the KF (Figs. 11B, 11C). The SCF of ~63 km. From south to north, the CF bends from a strike of ~295° to ~335°

and back to ~310°. It has a 315° average strike as mapped, slightly convergent Fault Geometry, Seismicity, and Potential-Field Data with Pacific plate motion of 321° (Fig. 4). Southeast of Catalina Canyon, the CF is expressed at the seafloor as an inflection in seabed slope that decreases Geometric Analysis toward Santa Catalina Island (Fig. 4). Along the southern margin of Santa Cata- lina Island, we lack high-resolution bathymetry data along much of the steep, New, high-resolution geometric constraints on faults allow us to evaluate nearshore slope; however, the CF may continue along and be represented by the relationship between plate motion and fault geometry. We plot the differ- the escarpment itself at the base of Santa Catalina Island (Fig. 4), making the ence between the fault strike of the SCF, KF, and CF and Pacific–North America total fault length longer than the explicitly mapped 63 km. Ryan et al. (2012) plate motion from MORVEL (DeMets et al., 2010) at the midpoints of discrete, interpreted that the CF does not connect with the San Diego Trough fault south- 5 km segments along the faults (Fig. 12). The Santa Cruz fault and secondary east of Santa Catalina Island where the San Gabriel Channel enters Catalina faults local to Catalina Basin (Figs. 2B, 3, 4) are included in the plot and con- Basin, as has been suggested by other previous studies (e.g., Hauksson and sidered to be a part of their respective primary fault zones (the SCF, KF, or CF). Jones, 1988; Legg et al., 2004a, 2004b, 2007), though we also note that Maier South of ~33.2°N, the SCF is approximately parallel with average Pacific–North et al. (2018) mapped several discontinuous fault structures in this area that America plate motion of ~321° (DeMets et al., 2010; Fig. 12). Between ~33.2°N may be related to either the Catalina or San Diego Trough fault zone. and ~33.6°N, the SCF takes a broad ~20° right bend through Catalina Basin, In northeastern Catalina Basin, the CF crosses Catalina Canyon, which diverging as much as 25° from Pacific plate motion (Fig. 12). Through this makes a northwestern bend just south of the CF, but we cannot resolve any same region, the CF is as much as 25° convergent with Pacific plate motion clear offsets at the canyon walls (Fig. 4). Just north of Catalina Canyon, the CF and the KF strike varies from 16° convergent to 12° divergent (Fig. 12). We also exhibits a significant, northeast-side-up scarp with relief of ~10 m. The height note that the CF and the KF exist at the same latitudes as a broad right step of this scarp lessens moving northward as the CF traverses through Pliocene in the SCF, though the average convergence angle of these three fault zones sediment east of a small ridge and west of the downslope limit of two discrete is still in line with Pacific plate motion (Fig. 12). North of 33.6°N, the CF and submarine landslide scarps and debris aprons (Fig. 4). Near these landslides, KF reach their terminations and the SCF becomes convergent (~10°–15°) rela- the CF scarp transitions from northeast-up to southwest-up, and scarp height tive to Pacific plate motion, with a strike closer to ~307° as it approaches the increases from ~5 m near the landslides to ~10 m where the fault exits Catalina Channel Islands in the southern Western Transverse Ranges province (Fig. 12). Basin to the north (Fig. 4A). North of Catalina Basin, the CF has no obvious geomorphic expression. In MCS data, the CF exhibits a subvertical dip with some folding and Seismicity Analysis offsets of adjacent seismic reflections from the seafloor to the base of seismic imaging (Fig. 8A). The CF cuts through a sub-basin between basement Using the Hauksson et al. (2012) earthquake catalog, we observe quantifi- highs exhibiting wedge-shaped growth strata in Pliocene and older sedi- ably heightened seismicity on the SCF along the northern and southern thirds Cumulative moment release 1 along the SCF since 1981 119.5° W 119° W ments that thicken toward the CF (Figs. 8A, 8B). This sub-basin is similar of the mapped SCF (Fig. 2B; Fig. S1 in the Supplemental Material ). The earth- 34.5 50 SC-SCF N

° to the prominent southwestern sub-basin discussed previously (Fig. 7A). quake catalog includes the 1981 M 6.0 Santa Barbara Island earthquake (e.g., N w 45 The base of the seafloor drape layer at the CF is visibly offset in the CHIRP Astiz and Shearer, 2000), which accounts for much of the seismicity cluster 40 higher seismicity from 1981 M5.3 SBI earthquake data (Figs. 8B, 8C), but vertical offset is less than the vertical offset of the along the Santa Cruz fault (Fig. 2B). Although a large earthquake also occurred 35 34 °

N drape across the SCF. along the southern segment of the SCF (the 1951 Ms 5.9 San Clemente Island 30 seismic While there are no resolvable piercing points along the CF north of Catalina earthquake, one of the largest offshore earthquakes recorded in the ICB; e.g., gap? 25 Depth (m) CF Basin or across Catalina Ridge, there is a subtle offset across the ridge in line Astiz and Shearer, 2000), the earthquake catalog shown here does not include 0

33.5 20

° with the trend of the mapped CF in Catalina Basin (Fig. 4A). In conjunction the 1951 mainshock or its aftershocks within 30 yr of the event.

? N Bin number (+ moving north) 15 1500 with bathymetric data, we use relatively densely spaced (3–4 km) legacy and Using the Hauksson et al. (2012) catalog, we also quantify the cumulative

higher seismicity <5 <2 Magnitude (size) 10 <10 2-3 new high-resolution MCS data to map the CF across Catalina Ridge into Santa moment release along the main trace of the SCF between 1981 and 2011 (see <15 3-4 33 <25 4-5 5 ° Monica Basin (Figs. 4A, 6B, 10A), and note that this is a significant departure Fig. S1 in the Supplemental Material [footnote 1] for detailed methods, figures, Depth (km) (color)

25+ 5+ N USGS mag. >4 historical events 0 from previous mapping (e.g., Legg et al., 2015). Subsurface offset across the and analysis). During that 30 yr period, the Hauksson et al. (2012) catalog indicates 18 19 20 21 22 23 24 25 10 10 10 10 10 10 10 10 0 15 30 60 km Pacific plate Seismic moment (dyne cm) motion (321.2°) CF is similar immediately north and south of Catalina Ridge (Figs. 8A, 10A), several orders of magnitude less seismic moment release over ~50 km of the but lessens moving northward into Santa Monica Basin (Fig. 6B), where we central SCF through northern Catalina Basin between San Clemente Island and 1 Supplemental Material. Detailed methods, figures, interpret the northern termination of the CF (Fig. 2B). The southern termination Santa Barbara Island (Fig. 2B; Fig. S1). The low-seismicity area also correlates and analysis. Please visit https://doi.org/10.1130 of the CF lacks new definition from MCS data due to absence of nearshore with the broad 20° right bend in the SCF and is at similar latitudes to the CF, KF, /GEOS.S.12379910 to access the supplemental material, and contact [email protected] with any high-resolution geophysical coverage and difficulty of seismic imaging along splays, and secondary faults of the SCF. Although we do not quantify seismicity questions. the steep escarpment at the base of Santa Catalina Island. along the CF, KF, and other secondary faults, we note there is qualitatively very

Angle of convergence with Pacific plate (+ convergent, - divergent) 119.5° W 119° W 34.5 34.0 Figure 12. Illustration of the difference between the

= SCF ° Pacific plate motion vector (from MORVEL; DeMets

= KF SC-SCF N et al., 2010) and fault strike along the San Clem- = CF SCB ente fault (SCF), Kimki fault (KF), and Catalina fault 33.8 (CF) systems. Right image shows high-resolution bathymetry in the Inner Continental Borderland SMB (Dartnell et al., 2017) rotated to align with the direc- tion of average Pacific plate motion through the data 33.6 2 SNI1 2 frame (location of figure is shown in Fig. 1). Solid

34 black lines represent Quaternary faults mapped in BAR1 this study, black dashed lines represent probable ° CF

N pre-Quaternary faults. Crustal Motion Map version 33.4 broad KF right step 4.0 (CMM4) GPS stations highlighted in Tables S1 SPB in SCF and S2 (Supplemental Material [text footnote 1]) CAT1 (Shen et al., 2011) are shown in yellow. Left plot N) ° 33.2 shows difference between Pacific plate motion 1 SNB 1 and fault strike along discrete 5 km segments of CAT2 mapped fault traces (plotted as degrees of differ- CB

SCF ence). Convergent segments are plotted as positive, 33.5 and divergent segments are plotted as negative.

Latitude ( 33.0 BLUF SCIP SBCC

° Dashed colored lines were fit to the data for the BOUL Depth (m) ? N SCF, KF, and CF using a locally weighted linear re- 0 gression over 10 data points. Plot has been scaled SDT 32.8 vertically to approximately align with the bathymetry image for visualization purposes, but note that due to the bathymetry image’s rotation, the vertical axes do not match. Arrows 1 and 2 in both the 32.6 plot and the bathymetry image highlight the end 1500

33 points of a broad right step in the SCF. SC-SCF— faults Santa Cruz–San Clemente fault; SCB—Santa Cruz °

SCF

N N Basin; SMB—Santa Monica Basin; SPB—San Pedro 32.4 Basin; CB—Catalina Basin; SNB—San Nicolas Basin; SDT—San Diego Trough. convergent divergent 0 15 30 60 km Pacific plate 30° 20° 10° 0° −10° −20° −30° motion (321.2°)

little to no seismicity along these fault zones, with the exception of a cluster of Potential-Field Data events along the central CF adjacent to north-central Santa Catalina Island (Fig. 2B; Fig. S1). Seismicity decreases outward from this central cluster along the CF to Gridded magnetics (Bankey et al., 2002; Fig. 13A) and gravity (Beyer, 1980; the north and south, with only around three events located along the escarp- Fig. 13B) data sets in the offshore California borderland have coarse ~1 km ment offshore of the southern margin of Santa Catalina Island (Fig. 2B; Fig. S1). resolution, but illuminate regional patterns in crustal structure through our Focal mechanisms determined for some earthquakes in this region (Yang study area (Fig. 13). Notably, both the gravity and magnetics data show broad, et al., 2012) are generally the most uncertain offshore—most events are of regional lows along and east of the SCF; this area has been interpreted as lowest “D” quality, meaning they have a large azimuthal gap between observa- thinned crust formed during extension and exhumation in the late Miocene

tions or very few VP/VS amplitude ratios—but can still provide some important (Bohannon and Geist, 1998; ten Brink et al., 2000; Miller, 2002). The main insights for kinematic interpretations. SCF focal mechanisms are generally trace of the SCF, and even its splay, the KF, traverse across lows in the gravity strike-slip, particularly in the areas with higher seismicity rates along the south- data, which correspond with the weaker, thinned crust (Fig. 13B). West of San ern and northern fault (Fig. S1 [footnote 1]). Reported focal mechanisms along Clemente Island in the Outer Continental Borderland, gravity values are gen- the CF all exhibit thrust mechanisms (Fig. S1). erally higher but regionally smooth, whereas magnetics data show localized,

119º W 118.5º W 118º W 119º W 118.5º W 118º W 2000 -30 A low N 0 10 20 40 B N 0 10 20 40 SCrF km 30 SCrF km -20 1700 1900 -10 -10 0 Figure 13. (A) Magnetic anomaly data across the CF CF releasing 2000 releasing 0 Catalina Basin region gridded at 1 km, from Ban- 2000 30 key et al. (2002). Values are reported in nT and the 1900 bend in SCF bend in SCF -10 contour interval is 100 nT. (B) Bouguer gravity anom- 33.5º N 33.5º N 2200 50 10 KF KF aly data across the Catalina Basin region gridded 2400 1800 60 40 at 1 km, from Beyer (1980). Values are reported in 80 40 30 mgal and the contour interval is 10 mgal. The thick, 2100 1900 10 semitransparent dashed white line shows the ap- 1800 10 2200 2000 1700 extended ? proximate Inner Continental Borderland (ICB)–Outer ? 20 SCF 60 SCF Continental Borderland (OCB) boundary from Legg 1600 50 low crust (ICB) et al. (2015). Quaternary faults (this study) are shown 40 30 80 70 as solid white lines, probable pre-Quaternary faults asperities 40

40 as dashed white lines. Extent of figures is the same 33º N 33º N (highs) 20 20 2000 as that shown in Figure 2. CF—Catalina fault; KF— 2500 non-extended 30 1900 20 Kimki fault; SCF—San Clemente fault; SCrF—Santa 2300 crust (OCB) 40 Cruz fault. 50 50 1900 2200 20 SCF 60 2000 1900 SCF 1700 1800 30 1900 40

concentrated highs (asperities) indicating relatively hard crustal blocks with and other strain structures. All crustal fabrics can be either “pre-existing” (in more abundant magnetic minerals. Fault pathways also correlate with mag- our study, pre-Quaternary or “inherited”) or “active” (Quaternary, unless oth- netic lows, and the SCF appears to divert around a prominent magnetic asperity erwise indicated). For the time frame described by our study, crustal blocks west of the northernmost Catalina Basin (Fig. 13A). are always pre-existing, whereas crustal structures can be pre-existing, active, or both, if the structure has persisted since the pre-Quaternary or has been reactivated in the Quaternary. On the surface of the crust, it is important to be ■■ DISCUSSION able to differentiate between inherited and active structures, both of which are manifested in the physiography and tectonic geomorphology of the seafloor. Here, we utilize the above observations to address the following hypotheses: (1) the SCF accommodates significant (as much as 4–6 mm/yr) right-lateral slip, and the CF accommodates primarily convergent stress; (2) modern Active Deformation and Hazards physiography does not necessarily indicate where active faults exist, and Quaternary faults can overprint pre-existing crustal structures; and (3) pre-ex- San Clemente Fault isting crustal fabric is an important control on Quaternary fault geometry, with pre-existing crustal blocks being more important than pre-existing faults. We Using direct seafloor and subsurface observations of the SCF, we consis- begin by examining the geometry and structure of shallow active faults and tently observe offset of the seafloor and Holocene drape layer (Maier et al., then discuss their relationship to crustal-scale structures and development 2018) along the SCF (e.g., Fig. 10), and thus, we interpret the SCF as Holocene since the late Miocene. active. Moreover, the SCF has both the sharpest and largest seafloor offset Importantly, throughout our interpretations, we differentiate between dif- (Figs. 3, 7, 10) and the most advanced subsurface deformation (Figs. 7C, 8A, ferent types of crustal fabric, with “crustal fabric” being the umbrella term 10) of the faults we map in the Catalina Basin region. Subsurface deformation, for all crustal features. We define crustal fabric as including two broad cat- including difficulty correlating seismic reflections across the SCF, especially egories of crustal features: “crustal blocks” and “structures.” Crustal blocks deeper reflections (Figs. 8A, 10A), indicates significant lateral slip and/or a long- can be differentiated from each other using physical characteristics such as lived fault. These subsurface observations, along with the continuity, length thickness, rock type, and density of the crystalline crustal rock underlying (>250 km), and linear seafloor expression of the SCF, are all consistent with sediment; potential-field data (gravity, magnetics) are commonly diagnostic significant offsets and Holocene slip on the SCF. The SCF bisects and offsets of these properties, and can thus differentiate between crustal blocks. Crustal the Kimki Ridge system, which is composed of Pliocene and older deposits structures, the other broad category under crustal fabric, include faults, folds, (Vedder et al., 1986; Vedder, 1990; Figs. 3, 11). This relationship suggests that

Kimki Ridge is a pre-Quaternary structure, that folding along the ridge is not of restraining and releasing bends along the ~250 km fault (Fig. 12; e.g., Legg active (which is also supported by MCS data; Fig. 8A), and that significant et al., 1999, 2007; Legg and Borrero, 2001) could also potentially inhibit rupture lateral SCF slip is younger than Kimki Ridge. propagation (King and Nábělek, 1985; Wesnousky, 2006). However, we also Mapped splays and secondary faults of the SCF generally appear to be note that strike-slip earthquakes can rupture through large bends (e.g., the Quaternary faults with smaller offsets than the SCF. Like the SCF, the clear 1857 M7.9 Fort Tejon earthquake in south-central California; Sieh, 1978), and seabed expression of the KF (Fig. 3A) suggests Quaternary activity, though that large, complex ruptures have been known to occur elsewhere along the ambiguous subsurface offset of the hemipelagic drape layer (Figs. 11B, 11C) California plate boundary (e.g., the 1992 M7.3 Landers earthquake in southern suggests that the KF is accommodating much smaller amounts or slower rates California; Hauksson et al., 2012) and along other strike-slip plate boundaries of deformation, and perhaps has not been deforming in the Holocene. Other (e.g., the recent Kaikoura event in New Zealand, Hollingsworth et al., 2017; the faults within the SCF zone, including the splay bounding the east flank of Haida Gwaii event along the Queen Charlotte fault offshore of British Columbia, southern Kimki Ridge and subparallel, subvertical faults on the San Clemente Canada, Lay et al., 2013). Ridge (Figs. 3, 7B) are short and discontinuous, hence considered inactive or Fault geometry also illuminates the kinematics of Catalina Basin fault zones accommodating small, localized offsets. (Fig. 12). The strike of the SCF south of ~33.2°N is consistent with an interpre- Our seafloor mapping shows clearly that the SCF connects with the Santa tation of dominantly right-lateral strike-slip fault (Fig. 12), and deformation and Cruz fault to the north. This is a substantial departure from some published seismicity along the SCF are consistent with strike-slip behavior (e.g., Fig. 7C; mapping of the SCF, particularly through the northern Catalina Basin (e.g., Fig. S1 [footnote 1]). Between ~33.2°N and ~33.5°N, a broad right step in the Fig. 2B), where previous mapping has commonly placed the SCF ~10 km SCF results in an ~50-km-long releasing bend along the fault (Fig. 12), though west of our interpretation and/or as ending in the northern Catalina Basin limited evidence for Quaternary subsidence or normal faulting exists in the (e.g., Ford and Normark, 1980; Vedder et al., 1986; U.S. Geological Survey Catalina Basin region. Late Quaternary subsidence of Pilgrim Banks just north and California Geological Survey, Quaternary Fault and Fold Database for of Catalina Basin on the west side of the Santa Cruz fault (Castillo et al., 2018) the United States, https://www.usgs.gov/natural-hazards/earthquake-hazards/ is potentially consistent with subsidence along the SCF releasing bend, and faults); additionally, the Santa Cruz fault has previously been interpreted as the sub-basin we map just south of the Kimki Ridge system (Fig. 7A) has local- connecting with the Catalina Ridge fault, with the combined structure previ- ized growth strata that might suggest local sub-basin subsidence and/or uplift ously referred to as the Santa Cruz–Catalina Ridge fault zone (e.g., Legg et al., along the San Clemente Ridge through the Pliocene–Quaternary. However, 2004a). Recent seismicity along the Santa Cruz fault, largely a result of the 1981 tectonic geomorphology near the SCF releasing bend also seems to indicate

Mw 6.0 Santa Barbara Island earthquake (Astiz and Shearer, 2000; Hauksson post-Pliocene transpression (Kimki Ridge), and the only normal faults we et al., 2012; Fig. 2B; Fig. S1 [footnote 1]), indicates that the fault is probably image in or on the margins of Catalina Basin are the short Quaternary faults active, and focal mechanisms (Yang et al., 2012) are consistent with a strike- on San Clemente Ridge (Fig. 7B), which could be nontectonic gravitational slip interpretation, making the kinematics likely similar between the SCF and faults. Because pervasive transtensional deformation associated with the Santa Cruz fault zones. The SCF and Santa Cruz fault have not previously been SCF releasing bend is limited, our observations suggest that the right bend connected in fault nomenclature, but we suggest that the combined fault zone along the SCF may be a relatively recent (Quaternary) geometric adjustment. might be more accurately described as the Santa Cruz–San Clemente fault zone (SC-SCF; shown in Fig. 12), given the clear connection between the fault zones evident in our data. Catalina Fault The increased length (>250 km) of a combined SC-SCF has implications for earthquake hazards. The SCF has been considered capable of rupturing There is clear offset of the Holocene drape layer along the CF (e.g., Fig. 8), in infrequent >M7 events (Legg and Borrero, 2001), and the Santa Cruz fault and we thus consider it a Holocene-active fault (Maier et al., 2018). Our kine-

likely ruptured in the 1981 Mw 6.0 Santa Barbara Island earthquake (Astiz and matic analysis based on fault geometry supports convergent deformation Shearer, 2000; Fig. 2B; Fig. S1 [footnote 1]). A 250 km fault length is capable of across the CF, particularly between ~33.2°N and ~33.5°N along the northeast- generating earthquakes up to magnitude ~8 (assuming a 10-km-wide rupture ern margin of Catalina Basin (Fig. 12) where the CF becomes as much as patch and 15–20 m of slip), the theoretical maximum for terrestrial strike-slip 25° convergent with Pacific plate motion. Strain along the CF indicates the earthquakes (Wells and Coppersmith, 1994). We speculate, however, that a influence of both convergence and transform motion. Gentle folding in the magnitude 8 event on the SCF is unlikely given the extended crust in the ICB, shallow subsurface is consistent with a component of convergence (Fig. 8), which has resulted in relatively high heat flow, and thus, potentially a rela- and strata diverging into the CF (Fig. 8A) could indicate growth along the fault tively shallow brittle-ductile transition (Lee and Henyey, 1975; ten Brink et al., or basin growth offset by the fault. Seafloor scarps as high as 10 m (Fig. 4A) 2000); relocated earthquake depths of generally <10 km along the Santa Cruz with changing sense of offset along strike are consistent with lateral motion; fault and SCF support this idea (Hauksson et al., 2012; Figs. 2B, 10). A number in one spot, the fault scarp relief appears to have limited the downslope extent

of a submarine landslide debris apron (Fig. 4). We do not observe any piercing Santa Catalina Island and across the San Pedro Basin fault (Fig. 6) on the points along the CF, but its proximity to insular sediment routed through Cata- island’s northern margin (Castillo et al., 2018). Our imaging instead suggests lina Canyon may have resulted in burial or erosion of evidence of transform convergence or transpression across the CF, requiring a different mechanism motion. Relatively small subsurface vertical offsets and correlatable unconfor- for Santa Catalina Island subsidence. We suggest that the relatively thick mities across the CF in MCS data (Fig. 8A) indicate (1) relatively small amounts Pliocene–Quaternary sediment in Santa Monica Basin (Fig. 6), along with of cumulative compressional strain along the CF; (2) a relatively young or continuous deposition in Santa Monica Basin through the Quaternary (Romans short-lived fault, which would not exhibit large amounts of cumulative strain; et al., 2009), might have led to flexural subsidence in the Santa Catalina Island (3) both (1) and (2); or (4) predominantly strike-slip deformation, which seems region, accounting for both the observed 0.08–0.27 mm/yr subsidence since at unlikely given the convergent CF geometry (Fig. 12). Secondary faults in the least 1.15 Ma and the 1.5° northward asymmetric tilt of Santa Catalina Island. CF zone (Fig. 4), like the secondary faults in the SCF zone (Fig. 3), are short and discontinuous and exhibit little to no offset of the hemipelagic drape layer; therefore, the most recent slip on these minor faults is likely pre-Holocene. Slip Rates The CF exhibits notable changes from previously inferred geometry. We use the trend of the CF as observed in merged bathymetry and in the 2014–2016 Despite the generally clear expression of the SCF, KF, and CF in high-res- high-resolution MCS data to map the active CF across Catalina Ridge into olution bathymetric data and numerous fault crossings of seabed landforms southern Santa Monica Basin, where we image the fault in legacy industry (e.g., ridges, Fig. 3; gullies and canyons, Fig. 4), we do not observe any clearly seismic-reflection data (Figs. 2B, 6B, 10A, 12). The surficial bedrock geology of offset piercing points that can be used to verify lateral slip rates. It is tempting the submerged Catalina Ridge is Miocene in age (Vedder et al., 1986; Vedder, to reconstruct the offset Kimki Ridge along the SCF (e.g., Fig. 3), but the pre- 1990), suggesting a Miocene (or later) generation of Catalina Ridge followed cise age of Kimki Ridge is uncertain, and the Pliocene deposits of the northern by initiation of the CF. Due to the cross-cutting relationship of the Holocene CF Kimki Ridge and the Miocene rocks of the southern ridge (Vedder et al., 1986; traversing across Catalina Ridge, we suggest that Catalina Ridge is an inherited Vedder, 1990; Fig. 3) also suggest that this is not a valid reconstruction. Sim- structure formed during an earlier episode of deformation that is no longer ilarly, gullies along the eastern flank of northern Kimki Ridge (Fig. 3) do not active, although it is still possible that it is an early Quaternary structure or correlate across the SCF and thus are not viable piercing points. at least may become reactivated. Our mapping is also inconsistent with the Previous studies have estimated SCF slip rates as high as ~4–6 mm/yr (Legg, interpretation of the Catalina–Catalina Ridge fault zone as a right stepover, 1985, 1991a, 2005; Larson, 1993; Bennett et al., 1996; Humphreys and Weldon, as has been suggested previously (Legg et al., 2007). Additionally, the CF as 1994; Goldfinger et al., 2000). Our work supports a maximum of ~3.6 mm/yr of mapped in our study does not appear to connect with the Santa Cruz fault as right-lateral strain accommodated within the Catalina Basin fault zones, with the has commonly been previously interpreted (e.g., Legg et al., 2004a). Because majority of this slip likely taken up by the SCF, because it exhibits more cumu- of the likely kinematic and age differences between the Santa Cruz and Catalina lative deformation than the CF on the seabed and in the subsurface. GPS data Ridge faults, we argue against connecting these two faults in nomenclature help to verify hypothesized slip rates in the absence of viable piercing points. (i.e., Santa Cruz–Catalina Ridge fault) going forward. Offshore GPS data from the Southern California Crustal Motion Map, version The kinematic activity along the southern end of the CF cannot be better 4.0 (CMM4; Shen et al., 2011; Tables S1 and S2 [footnote 1]; Fig. 12), show that characterized with our new data due to lack of coverage along the escarpment between 2.2 mm/yr and 3.6 mm/yr of differential lateral slip is accommodated (Fig. 2B). Thus, our observations could be consistent with either (1) an active between San Clemente Island and Santa Catalina Island (Tables S1 and S2; southern CF along the escarpment, consistent with demonstrable Holocene Fig. 12). The aggregate of geologic ICB slip rates suggests a similar slip deficit activity observed along its western end; or (2) an inactive fault, consistent with of 2.7–3.7 mm/yr accommodated along Catalina Basin faults (see text accom- waning seismicity along the escarpment (Fig. 2B). An inactive (early Quater- panying Tables S1 and S2 for analysis). Thus, results from previous studies and nary or pre-Quaternary) fault would also be consistent with the observations the morphology of the SCF leads us to prefer (1) a slip rate on the higher end of of Normark et al. (2004b) and Ryan et al. (2012) that the fault does not deform the range suggested by GPS results (~3.6 mm/yr) and (2) that this slip is largely the youngest sediments or connect with the San Diego Trough fault. Addition- accommodated along the SCF in Catalina Basin. We note that slip rates for the ally, sparker seismic reflection data collected in 2008 (Sliter et al., 2017) show SCF and all other faults can vary along their length (e.g., the Newport-Inglewood onlap onto Catalina Ridge and no deformation of the upper ~60 m of sediment. fault; Fischer and Mills, 1991; Grant et al., 1997), and that the rates we report here A sedimentation rate of ~4 cm/k.y. from radiocarbon dating in Catalina Basin are representative maximum slip rates throughout the Catalina Basin region. (McGann and Conrad, 2018) suggests that the 60 m sequence of undeformed Kinematics (Fig. 12) and subsurface deformation (Fig. 8) suggest that the sediment could represent as much as 1.5 m.y. of fault inactivity. CF is accommodating convergent or transpressive stress across the northern Late Quaternary asymmetric subsidence of Santa Catalina Island has been Catalina Basin. The CF may be accommodating a small part of the ~3.6 mm/yr attributed to paired normal motion across the CF on the southern flank of of right-lateral motion taken up across Catalina Basin, but the geometry and

subsurface deformation along the CF imply that the CF is a small part of the our interpretations of subsurface deformation (e.g., Figs. 8, 10) and kinematics lateral slip budget and that it is taking up primarily convergence, so we prefer (Fig. 12) along these fault zones, supporting dominantly strike-slip motion along lateral slip rates smaller than the 2.5 mm/yr found by Chaytor et al. (2008) for the combined SC-SCF and active convergent thrusting deformation along the the CF. The convergent component of slip along the CF could potentially be CF. Several focal mechanisms potentially located along the KF indicate normal as high as 1.5 mm/yr where the restraining angle reaches a peak of 25°, if we offsets (Fig. S1), which may reflect transtensional strain resulting from the assume a high-end right-lateral slip rate of 3.6 mm/yr along Catalina Basin right bend in the SCF being accommodated partly by the KF. faults (namely the SCF).

Fault Generation and Basin Evolution Seismicity Our results indicate that inherited crustal fabric in the ICB is an important While a 30 yr window of seismicity (Hauksson et al., 2012) is not indicative of control on fault geometry, that crustal-scale blocks are the most important influ- long-term fault behavior, it can tell us where faults are actively deforming and ence on evolving fault systems, and that pre-existing crustal structures can be provide insights into crustal rheology. Qualitatively, recent seismicity (Hauks- overprinted by Quaternary faults. The most important supporting observations son et al., 2012; Yang et al., 2012) has occurred primarily along the northern are that several demonstrably active faults cut across pre-Quaternary crustal

and southern SCF (Fig. 2B; Fig. S1 [footnote 1]). The 1981 Mw 6.0 Santa Barbara structures—notably, the SCF bisects Kimki Ridge (Fig. 3) and the CF traverses Island earthquake, which is included in the earthquake catalog we use here, obliquely across Catalina Ridge (Figs. 2B, 10A, 12)—suggesting that evolving likely occurred along the Santa Cruz fault and thus partly accounts for elevated faults can cut pre-existing crustal structures in some cases. Potential-field data seismicity in that region. In general, high seismicity rates along the northern (Fig. 13) provide some important insight into why this might be occurring in and southern SCF are consistent with our subsurface observations that the the Catalina Basin region. The gravity and magnetics data (Fig. 13) illuminate SCF is actively deforming and that deformation is accommodated across a physical properties of the deeper crust (i.e., crustal blocks), showing strong relatively narrow deformation zone in these areas. Fewer events have occurred correlations between lows in gravity and magnetics data (indicating weak, thin, along the CF than along the SCF, but their presence supports our interpretation extended crust) and mapped fault geometry, particularly SCF geometry. This of active deformation along this fault zone as well. correlation suggests that crustal blocks may be a more important control on One of the more striking observations of seismicity distribution is the quan- fault geometry than other inherited crustal structures, perhaps especially for tifiable lack of seismicity through the central segment of the SCF between crustal-scale faults like the SCF and CF. Our study thus supports crustal blocks northern San Clemente Island and just south of Santa Barbara Island, where as “essential” features, and crustal structures like faults and fault-controlled the SCF has a broad releasing bend (Fig. 2B; Fig. S1 [footnote 1]). We spec- ridges (including the Kimki and Catalina Ridges) as “incidental” structures ulate that the lower seismicity rates could imply more diffuse or distributed as described by Christie-Blick and Biddle (1985); in other words, we find that slip distribution here, which could potentially be supported by the existence crustal blocks have had more influence on strike-slip fault deformation in the of multiple fault zones (KF, CF) and thus a wider deformation zone at these Catalina Basin region than inherited faults and ridges. While we prefer the latitudes. At the latitudes of the SCF releasing bend, we also note an apparent interpretation that crustal blocks have influenced the geometry of faults, it is shoreward shift in ICB seismicity indicated by heightened earthquake activ- also possible that the translation of crustal blocks along the fault strands (per- ity along the San Diego Trough, Palos Verdes, and Newport-Inglewood fault haps up to tens of kilometers on the SCF alone) has led to the configuration zones, possibly suggesting more slip accommodation closer to shore here of crustal blocks that we observe today. as well. While the SCF releasing bend may exhibit a more widespread strain If crustal blocks do in fact have some control on fault geometry, it is possible distribution due to increased obliquity with plate motion, it is also possible that the SCF has undergone geometric modifications over time in response to that the reduced seismicity through the northern Catalina Basin is caused by changes in the regional stress field or the positions of essential crustal fabrics a seismic gap along the fault zone (Fig. S1), and that the SCF is locked and as they translate along faults. For example, we interpret a prominent magnetic poised to rupture in a relatively large (>M5) event here; the seismicity gap could asperity located just northwest of Catalina Basin (Fig. 13A) as a relatively hard also indicate fault creep, resulting in similar net slip but lower seismicity rates. crustal block that may have affected SCF fault geometry. We speculate that the Earthquake depth distribution along the SCF and CF is consistent with geometric releasing bend in the SCF through northern Catalina Basin may be a deformation of extended continental crust, with most events occurring at “sidewall ripout” (as described by Swanson, 1989, 2005, and Mann, 2007), forced ~10 km or shallower depths (Fig. 2B; Fig. S1 [footnote 1]). Some slightly by the presence of the hard crustal block (Fig. 13A) as it translated north into deeper earthquakes (5–15 km) along the northern and southern SCF and the this position. If this is the case, we also speculate that it is possible that the KF CF (Fig. 2B; Fig. S1) support crustal-scale deformation of these fault zones. once accommodated more slip in the SCF zone and was perhaps abandoned Focal mechanism observations (Fig. S1; Yang et al., 2012) are consistent with once the asperity had translated far enough north to inhibit or deoptimize slip

along the KF. Castillo et al. (2018) suggested late Quaternary abandonment of 0 - SW Uninterpreted NE the KF strand, and Conrad et al. (2018) similarly suggested that the KF was active as a restraining bend in the early Pleistocene and involved in the forma- 1 - tion of the Kimki Ridge anticline, possibly also accommodating some amount of strike-slip motion that later transferred over to the SCF. The interpretation 2 -

that the KF was a paleo-SCF and abandoned in the Quaternary is consistent (s) with our observations of waning deformation in the subsurface along the KF 3 - (Figs. 11B, 11C). Consequences of the SCF diverting around this crustal block TWT T may also include the bisection of the Kimki Ridge system by the SCF and/or 4 - generation of secondary fault strands (e.g., CF) in order to best accommodate plate motion through the region containing the forced releasing bend. 5 -

With a better understanding of active faults and their relationships to ~8x VE ~1 km 10 km pre-existing topography, we can now revisit our observations of crustal struc- SW Fig.8 Fig. 10 NE ture and seismic stratigraphy to piece together a loosely defined picture of 0 - OCB ICB CR Catalina Basin evolution and the role of faults and other structures in basin Catalina Redondo SCR formation through time (Figs. 14, 15). On a crustal scale, tilted blocks evi- 1 - Basin CF Knoll dent in legacy seismic-reflection data (Fig. 14) suggest that block rotation, a San Nicolas KF SCF basin-and-range–style extensional basin growth mechanism, occurred in the 2 - Basin Mb ICB. This block rotation (which has been suggested in previous studies; e.g., (s) Mb Legg and Kamerling, 2003) could be responsible for basin opening and the 3 -

horst-and-graben–style morphology we observe in the ICB today, including TWT T Mb steeply sloped, asymmetric islands and the ICB basins themselves. However, 4 - Mb there is no current consensus on exactly how the ICB basins developed; other Miocene hypotheses include nonuniform crustal thinning (e.g., Bohannon et al., 2004) 5 - Pliocene (?) and basin formation along local strike-slip releasing bends (e.g., Legg et al., ~8x VE Quaternary ~1 km 10 km 2007). One possibility is that Catalina Basin itself is a “lazy-Z” basin (Mann, 2007) along the broad SCF releasing bend in northern Catalina Basin (Fig. 12). Figure 14. Crustal-scale seismic reflection line SC85-308 from legacy industry survey B685SC (Triezenberg et al., 2016) showing “basin-and-range” style deformation across the Catalina Ba- Legg et al. (2007) also noticed the SCF releasing bend and determined that sin (line location is shown in Figs. 1, 2B). Interpreted section (bottom) includes a regional late northernmost Catalina Basin was formed as a pull-apart basin along the bend. Miocene unconformity mapped as a red line, and faults that formed in the Miocene–Quaternary New observations of seismic stratigraphy and subsurface deformation help mapped as subvertical black lines. Intersections with Figures 8 and 10 are annotated at the top of the image. Mb—Miocene and older sediments and basement rock; SCR—San Clemente Ridge; illuminate the viability of the block-rotation and lazy-Z models for Catalina KF—Kimki fault; SCF—San Clemente fault; CF—Catalina fault; CR—Catalina Ridge; ICB—Inner Basin opening. Our Pliocene-seafloor isochron map shows fairly uniform thick- Continental Borderland; OCB—Outer Continental Borderland. Vertical scale depth conversion ness in Catalina Basin (Fig. 9), suggesting that Catalina Basin resembled today’s assumes 2 km/s two-way travel time (TWTT) velocity. VE—vertical exaggeration. physiography by the end of the Pliocene. In contrast, the MS unconformity and Miocene-Pliocene and Miocene-seafloor isochron maps (Fig. 9) indicate local sub-basins and depocenters that are much smaller than Catalina Basin Although complex fault configuration and stress environments have likely and do not match its modern physiography. We thus interpret two phases of caused tectonic regimes ranging from convergent to divergent to exist in the ICB Catalina Basin growth and infill, starting with sub-basin development in the simultaneously (Bohannon et al., 2004), we prefer a block-rotation mechanism Miocene, followed by broader basin development in the Miocene–Pliocene, for ICB basin opening to support observations of new (e.g., Figs. 7A, 8A, 10A) finally resulting in the Catalina Basin geometry we observe today. The pattern and legacy (Fig. 14) geophysical data. The ICB basins may have started to open of basin growth (Fig. 9) is not consistent with lazy-Z basin development (Mann, in the late Miocene–early Pliocene based on general consensus that ICB kine- 2007), and we see little to no evidence for Quaternary transtension (e.g., sub- matic history includes a period of Miocene–Pliocene extension or transtension sidence, normal faults) along the SCF releasing bend segment (Figs. 8A, 10A). (Crowell, 1974; Legg, 1991b; Crouch and Suppe, 1993; Bohannon and Geist, 1998; A lack of tensile strain might suggest that the SCF releasing bend is a more ten Brink et al., 2000; Legg and Kamerling, 2003; Sorlien et al., 2015; DeMets recent tectonic adjustment and was not involved in Catalina Basin opening in and Merkouriev, 2016), and that uplift of Santa Catalina Island began by earliest the late Miocene–Pliocene, an idea that is also consistent with the previously Pliocene (Castillo et al., 2018). The steep island topography we observe today discussed sidewall-ripout hypothesis and Quaternary abandonment of the KF. (i.e., San Clemente Island, San Clemente Ridge, Santa Catalina Island, and

Late Miocene: Regional transtension forms Early Pliocene: Further block tilting and local deeps; submergence of Miocene rocks subsidence (?) continues sub-basin formation causes marine sedimentation; syntectonic and begins forming the larger ICB basins; deposition creates growth strata in local deeps flat-lying marine strata deposited SCR CB CR Figure 15. Summary schematic illustrating snapshots Ms Ps of fault behavior and sedimentary deposition during Catalina Basin (CB) evolution through time. Arrows Mb Mb Ms indicate regional stresses. Solid lines indicate active Cs Cs faults, long dashed lines indicate Miocene–Pliocene basin-evolution faults, and dotted lines indicate Pleistocene faults (similar annotation in Fig. 14). CF— Catalina fault; CR—Catalina Ridge; KR—Kimki Ridge; KF—Kimki fault; SCF—San Clemente fault; SCR—San Late Pliocene/Pleistocene: Regional Late Quaternary: Strike-slip and Clemente Ridge; Cs—Catalina Schist; Mb—Miocene transpression leads to uplift of Pliocene sediment transpressional structures overprint pre-existing and older sediments and basement rock; Ms—Mio- cene sedimentary deposits; Ps—Pliocene and early (e.g., Kimki Ridge); accelerated deposition from structures, following pathways of pre-existing Pleistocene marine sedimentary deposits; Qs—late the San Gabriel system weakness within the crustal fabric Quaternary marine sedimentary deposits; ICB—Inner Continental Borderland. Feature labels are meant SCR SCR KR CR KR CB CF CR to be analogous to real structures but not directly CB Qs comparable; the closest real-world analogue is the Ps Ps Catalina Basin region in Figure 14. Mb Mb Ms Ms Cs Cs KF SCF

Catalina Ridge) was thus likely largely generated in the Miocene–Pliocene via 2018), allowing the San Gabriel Channel to enter Catalina Basin. This event normal faulting (Fig. 15) along structures like the escarpment south of Santa likely correlates with our mapped PS unconformity and the downlapping San Catalina Island. Some topography may have been preserved from earlier in Gabriel Fan deposits above it. Late Pliocene–Quaternary regional transpression the Miocene during regional extension and volcanism in the ICB (Crouch and (Luyendyk et al., 1980; Ingersoll and Rumelhart, 1999; ten Brink et al., 2000; Suppe, 1993; ten Brink et al., 2000; Miller, 2002). For example, Emery Knoll prob- DeMets and Merkouriev, 2016) may have led to uplifted Pliocene sediment at ably represents an eroded and/or subsided volcanic remnant (comparable to basin margins (e.g., Fig. 8A), as well as a number of local popup structures Santa Barbara Island to the north) given its volcanic composition (Vedder et al., and folds like Kimki Ridge (Fig. 15), which fits with the proposed Pleistocene 1986) and its high acoustic reflectivity, which has been noted previously (e.g., development of Kimki Ridge suggested by Conrad et al. (2018). Today, Qua- Ridgway and Zumberge, 2002) and is observable in our 2014–2016 MCS data. ternary and Holocene strike-slip and transpressional structures (SCF, CF) cut Growth of Catalina Basin began with the sub-basins now evident in MCS across and overprint Miocene–Pleistocene pre-existing structures (Fig. 15). data (Figs. 7A, 15) in the late Miocene, corresponding with regional sea-level rise and our mapped MS unconformity (e.g., Fig. 7A). Basin development may have continued into the Pliocene, given that subsurface structure and isochron ■■ CONCLUSIONS thickness maps (Fig. 9) suggest that Catalina Basin existed in something close to its current form by the end of the Pliocene. Localized growth strata in the Using observations of new, high-resolution geophysical data in conjunc- centers of the sub-basins persist into Pliocene and possibly Quaternary marine tion with crustal-scale data sets in the Catalina Basin region, we put forth the sediments (Figs. 7A, 15), indicating that sedimentation rates did not quite following conclusions about fault geometry, structure, and evolution: keep up with tectonic growth during this time frame. Beginning in the late 1. We map Holocene-active and Quaternary-active traces of the SCF and CF. Pliocene (ca. 3 Ma), deposition of the San Gabriel Fan accelerated due to open- Based on fault geometry, subsurface deformation, and seismicity pat- ing of the southeastern basin along the San Diego Trough fault (Maier et al., terns, we interpret the SCF to be the primary active fault in the Catalina

Basin region, likely a crustal-scale feature accommodating the majority dissemination. Thorough reviews from Sam Johnson, Craig Nicholson, an anonymous reviewer, and the Geosphere Associate Editor helped greatly clarify and improve the paper—thank you. of dextral strike-slip offset (as much as 3.6 mm/yr) in this part of the Additionally, thanks to John Barron, Scott Bennett, Jayne Bormann, Eileen Evans, Vicki Langen- ICB. The CF is likely accommodating smaller amounts (<1.5 mm/yr) of heim, Mark Legg, Tom Parsons, and David Walton for helpful scientific discussion, support, and oblique transpression or convergence. advice. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 2. The SCF connects to the north with the Santa Cruz fault, and the total mapped fault length is >250 km. We map ~60 km of the active CF cutting across Catalina Ridge to where it terminates in western Santa Monica REFERENCES CITED Basin, a significant departure from previous mapping. Alexander, C.R., and Lee, H.J., 2009, Sediment accumulation on the Southern California Bight 3. A 30 yr catalog of seismicity in southern California indicates minimal continental margin during the twentieth century, in Lee, H.J., and Normark, W.R., eds., Earth Science in the Urban Ocean: The Southern California Continental Borderland: Geological cumulative moment release along an ~50-km-long SCF releasing bend Society of America Special Paper 454, p. 69–87, https://doi.org/10.1130/2009.2454(2.4). between northern San Clemente Island and Santa Barbara Island. The Allen, C.R., Silver, L.T., and Stehli, F.G., 1960, Agua Blanca fault—A major transverse structure of relative lack of seismicity may support diffuse slip distributed across a northern Baja California, Mexico: Geological Society of America Bulletin, v. 71, p. 467–482, https://doi.org/10.1130/0016-7606(1960)71[467:ABFMTS]2.0.CO;2. number of secondary faults through this region, a creeping section along Astiz, L., and Shearer, P.M., 2000, Earthquake locations in the inner Continental Borderland, off- this part of the SCF, or a seismic gap and the site of a future earthquake. shore southern California: Bulletin of the Seismological Society of America, v. 90, p. 425–449, 4. Quaternary high-angle faults (namely the SCF and CF) cut across older, https://doi.org/10.1785/0119990022. Miocene to Pleistocene structures (e.g., Catalina Ridge, Kimki Ridge), Balster-Gee, A.F., Kluesner, J.W., Brothers, D.S., Conrad, J.E., and Sliter, R.W., 2017, Minisparker and chirp seismic-reflection data of field activity 2014-645-FA collected in the outer Santa indicating that modern physiography does not necessarily reflect active Barbara Channel, California, 2014-11-12 to 2014-11-25 (ver. 2.0, March 2020): U.S. Geological faulting, and that Quaternary faulting overprints pre-existing crustal Survey data release, https://doi.org/10.5066/F7CV4FW6. structures in some cases. Balster-Gee, A.F., Brothers, D.S., Roland, E.C., Kluesner, J.W., Hart, P.E., Conrad, J.E., Myers, E.K., Ebuna, D.R., and Edwards, J.H., 2020, Multichannel minisparker and chirp seismic reflection 5. The SCF main trace follows low magnetic and gravity anomalies, and it data of U.S. Geological Survey field activity 2016-616-FA collected in the Catalina Basin off- may preferentially occupy this pathway due to extended, thinner, and/ shore southern California in February 2016: U.S. Geological Survey data release, https://doi or weaker crust. Likewise, a large right bend in the SCF in the northern .org/10.5066/P9BLAJ72. Bankey, V., Cuevas, A., Daniels, D., Finn, C.A., Hernandez, I., Hill, P., Kucks, R., Miles, W., Pilking- Catalina Basin may have formed around a hard crustal block as rep- ton, M., Roberts, C., Roest, W., Rystrom, V., Shearer, S., Snyder, S., Sweeney, R., Velez, J., resented by a substantial magnetic asperity; the releasing bend may Phillips, J.D., and Ravat, D., 2002, Digital data grids for the magnetic anomaly map of North also be responsible for the formation of, and/or distribution of, strain America: U.S. Geological Survey Open-File Report 02-414, https://doi.org/10.3133/ofr02414. Barron, J.A., 1986, Paleoceanographic and tectonic controls on deposition of the Monterey across other faults like the CF and KF. Formation and related siliceous rocks in California: Palaeogeography, Palaeoclimatology, 6. Inherited crustal fabric is an important control on Quaternary fault geom- Palaeoecology, v. 53, p. 27–45, https://doi.org/10.1016/0031-0182(86)90037-4. etry, although pre-existing crustal blocks are a more important influence Bennett, R.A., Rodi, W., and Reilinger, R.E., 1996, Global Positioning System constraints on fault on fault geometry than inherited fault structures in this region. slip rates in southern California and northern Baja, Mexico: Journal of Geophysical Research, v. 101, p. 21,943–21,960, https://doi.org/10.1029/96JB02488. 7. Seismic stratigraphy of Catalina Basin fill, relationships between inher- Bent, A.L., and Helmberger, D.V., 1991, Seismic characteristics of earthquakes along the offshore ited and active structures, and regional age constraints suggest several extension of the western transverse ranges, California: Bulletin of the Seismological Society stages of basin formation and deformation: (1) late Miocene relative sea- of America, v. 81, no. 2, p. 399–422. Beyer, L.A., 1980, Offshore southern California, in Oliver, H.W., ed., Interpretation of the Gravity level rise and the onset of marine sedimentary deposition in local deeps Map of California and Its Continental Margin: California Division of Mines and Geology formed due to tectonic subsidence and regional extension, (2) early Bulletin 205, p. 8–15. to mid-Pliocene continuing extension and block tilting leading to the Bohannon, R.G., and Gardner, J.V., 2004, Submarine landslides of San Pedro Escarpment, southwest of Long Beach, California: Marine Geology, v. 203, p. 261–268, https://doi.org/10.1016 basin morphology we observe today, (3) late Pliocene and Pleistocene /S0025-3227(03)00309-8. transpression leading to localized popup structures and continued ridge Bohannon, R.G., and Geist, E., 1998, Upper crustal structure and Neogene tectonic development of uplift, and (4) Quaternary strike-slip overprinting. the California continental borderland: Geological Society of America Bulletin, v. 110, p. 779–800, https://doi.org/10.1130/0016-7606(1998)110<0779:UCSANT>2.3.CO;2. 8. Our results emphasize the importance of the acquisition and analysis of Bohannon, R.G., Gardner, J.V., and Sliter, R.W., 2004, Holocene to Pliocene tectonic evolution of high-resolution geophysical data in order to identify active deformation the region offshore of the Los Angeles urban corridor, southern California: Tectonics, v. 23, structures, contribute to earthquake and hazard analysis, and accurately TC1016, https://doi.org/10.1029/2003TC001504. Brothers, D.S., Conrad, J.E., Maier, K.L., Paull, C.K., McGann, M., and Caress, D.W., 2015, The Palos interpret tectonic geomorphology. Verdes Fault offshore Southern California: Late Pleistocene to present tectonic geomorphology, seascape evolution, and slip rate estimate based on AUV and ROV surveys: Journal of Geophysical Research: Solid Earth, v. 120, p. 4734–4758, https://doi .org /10 .1002 /2015JB011938 . ACKNOWLEDGMENTS Brothers, D.S., Maier, K.L., Kluesner, J.A., Conrad, J.E., and Chaytor, J.D., 2018, The Santa Cruz We thank all the scientists and crew who assisted with the 2014 R/V Robert Gordon Sproul survey Basin submarine landslide complex, southern California: Repeated failure of inverted basin and the 2016 R/V Thomas Thompson survey for their help with data collection. We specifically sediment, in Marsaglia, K.M., Schwalbach, J.R., and Behl, R.J., eds., From the Mountains to thank Alicia Balster-Gee, Pat Hart, and Ray Sliter for their assistance with data processing and the Abyss: The California Borderland as an Archive of Southern California Geologic Evolution:

SEPM (Society for Sedimentary Geology) Special Publication 110, http://dx.doi.org/10.2110 Science in the Urban Ocean: The Southern California Continental Borderland: Geological /sepmsp.110.05. Society of America Special Paper 454, p. 229–250, https://doi.org/10.1130/2009.2454(4.2). Calsbeek, N., Eakins, B.W., and Love, M., 2013, Revised coastal relief model of southern Califor- Ford, G.A., and Normark, W.R., 1980, Map showing a deep-tow geophysical study of the north nia: Procedures, data sources, and analysis: Boulder, Colorado, National Geophysical Data end of the San Clemente Fault, California Borderland: U.S. Geological Survey Miscellaneous Center, 25 p. Field Studies Map 1230, 1 sheet, https://doi.org/10.3133/mf1230. Castillo, C.M., Klemperer, S.L., Ingle, J.C., Jr., Powell, C.L., Legg, M.R., and Francis, R.D., 2018, Late Goldfinger, C., Legg, M.R., and Torres, M., 2000, New mapping and submersible observations of recent Quaternary subsidence of Santa Catalina Island, California Continental Borderland, demon- activity on the San Clemente Fault: Eos (Transactions, American Geophysical Union), v. 81, p. 1068. strated by seismic-reflection data and fossil assemblages from submerged marine terraces: Gorsline, D., and Douglas, R., 1987, Analysis of sedimentary systems in active-margin basins: Geological Society of America Bulletin, v. 131, p. 21–42, https://doi.org/10.1130/B31738.1. California Continental Borderland, in Ingersoll, R.V., and Ernst, W.G., eds., Cenozoic Basin Chaytor, J.D., Goldfinger, C., Meiner, M.A., Huftile, G.J., Romsos, C.G., and Legg, M.R., 2008, Development of Coastal California (Rubey Volume 6): Englewood Cliffs, New Jersey, Pren- Measuring vertical tectonic motion at the intersection of the Santa Cruz–Catalina Ridge and tice-Hall, p. 64–80. Northern Channel Islands platform, California Continental Borderland, using submerged Gorsline, D.S., Kolpack, R.L., Karl, H.A., Drake, D.E., Thornton, S.E., Schwalbach, J.R., Savrda, C.E., paleoshorelines: Geological Society of America Bulletin, v. 120, p. 1053–1071, https://doi.org and Fleischer, P., 1984, Studies of fine-grained sediment transport processes and products in /10.1130/B26316.1. the California Continental Borderland, in Stow, D.A.V., and Piper, D.J.W., eds., Fine-Grained Christie-Blick, N., and Biddle, K.T., 1985, Deformation and basin formation along strike-slip faults, Sediments: Deep-Water Processes and Facies: Geological Society of London Special Publi- in Biddle, K.T., and Christie-Blick, N., eds., Strike-Slip Deformation, Basin Formation, and cation 15, p. 395–415, https://doi.org/10.1144/GSL.SP.1984.015.01.26. Sedimentation: Society of Economic Paleontologists and Mineralogists Special Publication 37, Grant, L.B., Waggoner, J.T., Rockwell, T.K., and von Stein, C., 1997, Paleoseismicity of the north p. 1–34, https://doi.org/10.2110/pec.85.37.0001. branch of the Newport-Inglewood fault zone in Huntington Beach, California, from cone Conrad, J.E., Brothers, D.S., Maier, K.L., Ryan, H.F., Dartnell, P., and Sliter, R., 2017, Right-lateral penetrometer test data: Bulletin of the Seismological Society of America, v. 87, p. 277–293.

fault motion along the slope-basin transition, Gulf of Santa Catalina, southern California, in Hauksson, E., and Jones, L.M., 1988, The July 1986 Oceanside (ML= 5.3) earthquake sequence Marsaglia, K.M., Schwalbach, J.R., and Behl, R.J., eds., From the Mountains to the Abyss: The in the Continental Borderland, Southern California: Bulletin of the Seismological Society of California Borderland as an Archive of Southern California Geologic Evolution: SEPM (Soci- America, v. 78, p. 1885–1906. ety for Sedimentary Geology) Special Publication 110, https://doi.org/10.2110/sepmsp.110.07. Hauksson, E., Yang, W., and Shearer, P.M., 2012, Waveform relocated earthquake catalog for Conrad, J.E., Prouty, N.G., Walton, M.A.L., Kluesner, J.W., Maier, K.L., McGann, M., Brothers, D.S., southern California (1981 to June 2011): Bulletin of the Seismological Society of America, Roland, E.C., and Dartnell, P., 2018, Seafloor fluid seeps on Kimki Ridge, offshore southern v. 102, p. 2239–2244, https://doi.org/10.1785/0120120010. California: Links to active strike-slip faulting: Deep-Sea Research, Part II: Topical Studies in Hollingsworth, J., Ye, L., and Avouac, J.-P., 2017, Dynamically triggered slip on a splay fault in

Oceanography, v. 150, p. 82–91, https://doi.org/10.1016/j.dsr2.2017.11.001. the Mw 7.8, 2016 Kaikoura (New Zealand) earthquake: Geophysical Research Letters, v. 44, Crouch, J.K., and Suppe, J., 1993, Late Cenozoic tectonic evolution of the Los Angeles basin and p. 3517–3525, https://doi.org/10.1002/2016GL072228. inner California borderland: A model for core complex-like crustal extension: Geological Howarth, J.D., Fitzsimons, S.J., Norris, R.J., and Jacobsen, G.E., 2012, Lake sediments record Society of America Bulletin, v. 105, p. 1415–1434, https://doi.org/10.1130/0016-7606(1993)105 cycles of sediment flux driven by large earthquakes on the Alpine fault, New Zealand: Geol- <1415:LCTEOT>2.3.CO;2. ogy, v. 40, p. 1091–1094, https://doi.org/10.1130/G33486.1. Crowell, J.C., 1974, Origin of late Cenozoic basins in southern California, in Dickinson, W.R., ed., Humphreys, E.D., and Weldon, R.J., 1994, Deformation across the western United States: A local Tectonics and Sedimentation: Society of Economic Paleontologists and Mineralogists Special estimate of Pacific–North America transform deformation: Journal of Geophysical Research, Publication 22, p. 190–204, https://doi.org/10.2110/pec.74.22.0190. v. 99, p. 19,975–20,010, https://doi.org/10.1029/94JB00899. Cunningham, W.D., and Mann, P., 2007, Tectonics of strike-slip restraining and releasing bends, in Ingersoll, R.V., and Rumelhart, P.E., 1999, Three-stage evolution of the Los Angeles basin, south- Cunningham, W.D., and Mann, P., eds., Tectonics of Strike-Slip Restraining and Releasing Bends: ern California: Geology, v. 27, p. 593–596, https://doi.org/10.1130/0091-7613(1999)027<0593: Geological Society of London Special Publication 290, p. 1–12, https://doi .org /10 .1144 /SP290 .1 . TSEOTL>2.3.CO;2. Dartnell, P., Driscoll, N.W., Brothers, D., Conrad, J.E., Kluesner, J., Kent, G., and Andrews, B., 2015, Janik, A., Lyle, M.W., and Liberty, L.M., 2004, Seismic expression of Pleistocene paleoceanographic Colored shaded-relief bathymetry, acoustic backscatter, and selected perspective views of changes in the California Borderland from digitally acquired 3.5 kHz subbottom profiles and the Inner Continental Borderland, Southern California: U.S. Geological Survey Scientific Ocean Drilling Program Leg 167 drilling: Journal of Geophysical Research, v. 109, B07101, Investigations Map 3324, 3 sheets, https://doi.org/10.3133/sim3324. https://doi.org/10.1029/2003JB002439. Dartnell, P., Roland, E.C., Raineault, N.A., Castillo, C.M., Conrad, J.E., Kane, R.R., Brothers, D.S., Johnson, S.Y., and Watt, J.T., 2012, Influence of fault trend, bends, and convergence on shallow Kluesner, J.W., and Walton, M.A.L., 2017, Multibeam bathymetry and acoustic-backscatter structure and geomorphology of the Hosgri strike-slip fault, offshore central California: Geo- data collected in 2016 in Catalina Basin, southern California and merged multibeam bathym- sphere, v. 8, p. 1632–1656, https://doi.org/10.1130/GES00830.1. etry datasets of the northern portion of the Southern California Continental Borderland: U.S. Johnson, S.Y., Watt, J.T., Hartwell, S.R., and Kluesner, J.W., 2018, Neotectonics of the Big Sur bend, Geological Survey data release, https://doi.org/10.5066/F7DV1H3W. San Gregorio–Hosgri fault system, Central California: Tectonics, v. 37, p. 1930–1954, https:// De Hoogh, G.L., 2012, Structural and stratigraphic evolution of the central and southern outer doi.org/10.1029/2017TC004724. California Continental Borderland [M.S. thesis]: Long Beach, California State University, 66 p. Junger, A., and Sylvester, A.G., 1979, Origin of Emery Seaknoll, southern California Borderland: DeMets, C., and Merkouriev, S., 2016, High-resolution reconstructions of Pacific–North America Eos (Transactions, American Geophysical Union), v. 60, p. 951. plate motion: 20 Ma to present: Geophysical Journal International, v. 207, p. 741–773, https:// Kier, G., and Mueller, K., 1999, Evidence for active shortening in the offshore Borderlands and doi.org/10.1093/gji/ggw305. its implications for blind thrust hazards in the Coastal Orange and San Diego Counties, in DeMets, C., Gordon, R.G., and Argus, D.F., 2010, Geologically current plate motions: Geophysical Proceedings and Abstracts, Southern California Earthquake Center (SCEC) Annual Meeting, Journal International, v. 181, p. 1–80, https://doi.org/10.1111/j.1365-246X.2009.04491.x. Los Angeles, California, p. 71. Emery, K.O., 1960, The Sea off Southern California: A Modern Habitat of Petroleum: New York, King, G., and Nábělek, J., 1985, Role of fault bends in the initiation and termination of earthquake John Wiley & Sons, 366 p. rupture: Science, v. 228, p. 984–987, https://doi.org/10.1126/science.228.4702.984. Fischer, P.J., and Mills, G.I., 1991, The offshore Newport-Inglewood-Rose Canyon fault zone, Califor- Langenheim, V.E., Halvorson, P.F., Castellanos, E.L., and Jachens, R.C., 1993, Aeromagnetic map nia: Structure, segmentation, and tectonics, in Abbott, P.L., and Elliot, W.J., eds., Environmental of the southern California borderland east of the Patton Escarpment: U.S. Geological Survey Perils, San Diego Region: San Diego, California, San Diego Association of Geologists, p. 17–36. Open-File Report 93-520, 1 sheet, scale 1:500,000, https://doi.org/10.3133/ofr93520. Fisher, M.A., Langenheim, V.E., Nicholson, C., Ryan, H.F., and Sliter, R.W., 2009, Recent devel- Larson, K.M., 1993, Application of the global positioning system to crustal deformation measure- opments in understanding the tectonic evolution of the Southern California offshore area: ments: 3. Result from the southern California borderlands: Journal of Geophysical Research, Implications for earthquake-hazard analysis, in Lee, H.J., and Normark, W.R., eds., Earth v. 98, p. 21713–21726, https://doi.org/10.1029/92JB02116.

Lay, T., Ye, L., Kanamori, H., Yamazaki, Y., Cheung, K.F., Kwong, K., and Koper, K.D., 2013, The Mann, P., 2007, Global catalogue, classification and tectonic origins of restraining- and releasing

October 28, 2012 Mw 7.8 Haida Gwaii underthrusting earthquake and tsunami: Slip partitioning bends on active and ancient strike-slip fault systems, Cunningham, W.D., and Mann, P., eds., along the Queen Charlotte Fault transpressional plate boundary: Earth and Planetary Science Tectonics of Strike-Slip Restraining and Releasing Bends: Geological Society of London Special Letters, v. 375, p. 57–70, https://doi.org/10.1016/j.epsl.2013.05.005. Publication 290, p. 13–142, https://doi.org/10.1144/SP290.2. Lee, H.J., Greene, H.G., Edwards, B.D., Fisher, M.A., and Normark, W.R., 2009, Submarine landslides McGann, M., and Conrad, J.E., 2018, Faunal and stable isotopic analyses of benthic foramin- of the Southern California Borderland, in Lee, H.J., and Normark, W.R., eds., Earth Science ifera from the Southeast Seep on Kimki Ridge offshore southern California, USA: Deep-Sea in the Urban Ocean: The Southern California Continental Borderland: Geological Society of Research, Part II: Topical Studies in Oceanography, v. 150, p. 92–1117, https://doi.org/10.1016 America Special Paper 454, p. 251–269, https://doi.org/10.1130/2009.2454(4.3). /j.dsr2.2018.01.011. Lee, T.-C., and Henyey, T.L., 1975, Heat flow through the Southern California Borderland: Jour- McNeilan, T.W., Rockwell, T.K., and Resnick, G.S., 1996, Style and rate of Holocene slip, Palos nal of Geophysical Research, v. 80, p. 3733–3743, https://doi.org/10.1029/JB080i026p03733. Verdes fault, southern California: Journal of Geophysical Research, v. 101, p. 8317–8334, Legg, M.R., 1985, Geological structure and tectonics of the Inner Continental Borderland offshore https://doi.org/10.1029/95JB02251. northern Baja California, Mexico [Ph.D. thesis]: Santa Barbara, University of California, 410 p. Miller, K.C., 2002, Geophysical evidence for Miocene extension and mafic magmatic addition in the Legg, M.R., 1991a, Sea Beam evidence of recent tectonic activity in the California Continental California Continental Borderland: Geological Society of America Bulletin, v. 114, p. 497–512, Borderland, in Dauphin, J.P., and Simoneit, B.R.T., eds., The Gulf and Peninsular Province https://doi.org/10.1130/0016-7606(2002)114<0497:GEFMEA>2.0.CO;2. of the Californias: American Association of Petroleum Geologists Memoir 47, p. 179–196. Moore, D.G., 1969, Reflection Profiling Studies of the California Continental Borderland: Structure Legg, M.R., 1991b, Developments in understanding the tectonic evolution of the California Con- and Quaternary Turbidite Basins: Geological Society of America Special Paper 107, 136 p., tinental Borderland, in Osborne, R.H., ed., From Shoreline to Abyss: Contributions in Marine https://doi.org/10.1130/SPE107. Geology in Honor of Francis Parker Shepard: SEPM (Society for Sedimentary Geology) Special Namson, J., and Davis, T.L., 1990, Late Cenozoic fold and thrust belt of the southern Coast Ranges Publication 46, p. 291–312, https://doi.org/10.2110/pec.91.09.0291. and Santa Maria basin, California: American Association of Petroleum Geologists Bulletin, Legg, M.R., 2005, Geoloogic slip on offshore San Clemente fault, Southern California, understated v. 74, p. 467–492, https://doi.org/10.1306/0C9B2335-1710-11D7-8645000102C1865D. in GPS data: Eos (Transactions, American Geophysical Union), v. 86, Fall Meeting Supple- Nardin, T.R., Edwards, B.D., and Gorsline, D.S., 1979, Santa Cruz Basin, California Borderland: ment, abstract G53A-0876. Dominance of slope processes in basin sedimentation, in Doyle, L.J., and Pilkey, Orrin H., Legg, M.R., and Borrero, J.C., 2001, Tsunami potential of major restraining bends along subma- eds., Geology of Continental Slopes: Society of Economic Paleontologists and Mineralogists rine strike-slip faults, in Proceedings of the 2001 International Tsunami Symposium, Session Special Publication 27, p. 209–221, https://doi.org/10.2110/pec.79.27.0209. 1, Number 1–9, p. 331–342. Nicholson, C., Sorlien, C.C., Atwater, T., Crowell, J.C., and Luyendyk, B.P., 1994, Microplate capture, Legg, M.R., and Kamerling, M.J., 2003, Large-scale basement-involved landslides, California rotation of the western Transverse Ranges, and initiation of the San Andreas transform as a Continental Borderland: Pure and Applied Geophysics, v. 160, p. 2033–2051, https://doi.org low-angle fault system: Geology, v. 22, p. 491–495, https://doi .org /10 .1130 /0091 -7613 (1994)022 /10.1007/s00024-003-2418-9. <0491:MCROTW>2.3.CO;2. Legg, M.R., Einstein, D.E., and Wang, H.D., 1999, Deformation history of a major restraining Normark, W.R., and McGann, M., 2004, Late Quaternary deposition in the inner basins of the Cal- bend along a right-slip fault: The San Clemente fault offshore northern Baja California, in ifornia Continental Borderland: Part A. Santa Monica Basin: U.S. Geological Survey Scientific Proceedings and Abstracts, Southern California Earthquake Center (SCEC) Annual Meeting, Investigations Report 2004-5183, 21 p., https://doi.org/10.3133/sir20045183. Palm Springs, California, p. 74. Normark, W.R., McGann, M., and Sliter, R., 2004a, Age of Palos Verdes submarine debris ava- Legg, M.R., Borrero, J.C., and Synolakis, C.E., 2004a, Tsunami hazards associated with the Catalina lanche, southern California: Marine Geology, v. 203, p. 247–259, https://doi .org /10 .1016 /S0025 fault in southern California: Earthquake Spectra, v. 20, p. 917–950, https://doi .org /10 .1193 /1 .1773592 . -3227(03)00308-6. Legg, M.R., Nicholson, C., Goldfinger, C., Milstein, R., and Kamerling, M.J., 2004b, Large enig- Normark, W.R., Baher, S., and Sliter, R., 2004b, Late Quaternary sedimentation and deformation matic crater structures offshore southern California: Geophysical Journal International, v. 159, in Santa Monica and Catalina Basins, offshore southern California, in Legg, M., Davis, P., and p. 803–815, https://doi.org/10.1111/j.1365-246X.2004.02424.x. Gath, E., eds., Geology and Tectonics of Santa Catalina Island and the California Continental Legg, M.R., Goldfinger, C., Kamerling, M.J., Chaytor, J.D., and Einstein, D.E., 2007, Morphology, Borderland: South Coast Geological Society Field Trip Guidebook 32, p. 291–317. structure and evolution of California Continental Borderland restraining bends, in Cunningham, Normark, W.R., Paull, C.K., Caress, D.W., Ussler, W., III, and Sliter, R., 2009, Fine-scale relief related W.D., and Mann, P., eds., Tectonics of Strike-Slip Restraining and Releasing Bends: Geolog- to Late Holocene channel shifting within the floor of the upper Redondo Fan, offshore Southern ical Society of London Special Publication 290, p. 143–168, https://doi.org/10.1144/SP290.3. California: Sedimentology, v. 56, p. 1690–1704, https://doi .org /10 .1111 /j .1365 -3091 .2009 .01052 .x . Legg, M.R., Kohler, M.D., Shintaku, N., and Weeraratne, D.S., 2015, High-resolution mapping of Pacheco, J., and Nábělek, J., 1988, Source mechanisms of three moderate California earthquakes two large-scale transpressional fault zones in the California Continental Borderland: Santa of July 1986: Bulletin of the Seismological Society of America, v. 78, p. 1907–1929. Cruz–Catalina Ridge and Ferrelo faults: Journal of Geophysical Research: Earth Surface, v. 120, Platt, J.P., and Becker, T.W., 2010, Where is the real transform boundary in California?: Geochemistry p. 915–942, https://doi.org/10.1002/2014JF003322. Geophysics Geosystems, v. 11, Q06012, https://doi.org/10.1029/2010GC003060. Lindvall, S.C., and Rockwell, T.K., 1995, Holocene activity of the Rose Canyon fault zone in San Richter, C.F., 1958, Elementary Seismology: San Francisco, W.H. Freeman and Company, 768 p. Diego, California: Journal of Geophysical Research, v. 100, p. 24,121–24,132, https://doi.org Ridgway, J.R., and Zumberge, M.A., 2002, Deep-towed gravity surveys in the southern Califor- /10.1029/95JB02627. nia Continental Borderland: Geophysics, v. 67, p. 777–787, https://doi.org/10.1190/1.1484521. Locat, J., Lee, H.J., Locat, P., and Imran, J., 2004, Numerical analysis of the mobility of the Palos Ridlon, J.B., 1968, Bathymetry and structure of San Clemente Island, California, and tectonic Verdes debris avalanche, California, and its implication for the generation of tsunamis: Marine implications for the southern California continental borderland [Ph.D. thesis]: Corvallis, Ore- Geology, v. 203, p. 269–280, https://doi.org/10.1016/S0025-3227(03)00310-4. gon State University, 261 p. Lonsdale, P., 1991, Structural patterns of the Pacific floor offshore of peninsular California, in Romans, B.W., Normark, W.R., McGann, M.M., Covault, J.A., and Graham, S.A., 2009, Coarse- Dauphin, J.P., and Simoneit, B.R.T., eds., The Gulf and Peninsular Province of the Californias: grained sediment delivery and distribution in the Holocene Santa Monica Basin, California: American Association of Petroleum Geologists Memoir 47, p. 87–125. Implications for evaluating source-to-sink flux at millennial time scales: Geological Society Luyendyk, B.P., Kamerling, M.J., and Terres, R., 1980, Geometric model for Neogene crustal rota- of America Bulletin, v. 121, p. 1394–1408, https://doi.org/10.1130/B26393.1. tions in southern California: Geological Society of America Bulletin, v. 91, p. 211–217, https:// Ryan, H.F., Legg, M.R., Conrad, J.E., and Sliter, R.W., 2009, Recent faulting in the Gulf of Santa doi.org/10.1130/0016-7606(1980)91<211:GMFNCR>2.0.CO;2. Catalina: San Diego to Dana Point, in Lee, H.J., and Normark, W.R., eds., Earth Science in the Maier, K.L., Roland, E.C., Walton, M.A., Conrad, J.E., Brothers, D.S., Dartnell, P., and Kluesner, Urban Ocean: The Southern California Continental Borderland: Geological Society of America J.W., 2018, The tectonically controlled San Gabriel channel–lobe transition zone, Catalina Special Paper 454, p. 291–315, https://doi.org/10.1130/2009.2454(4.5). Basin, Southern California Borderland: Journal of Sedimentary Research, v. 88, p. 942–959, Ryan, H.F., Conrad, J.E., Paull, C.K., and McGann, M., 2012, Slip rate on the San Diego Trough https://doi.org/10.2110/jsr.2018.50. fault zone, Inner California Borderland, and the 1986 Oceanside earthquake swarm revisited:

Bulletin of the Seismological Society of America, v. 102, p. 2300–2312, https://doi.org/10 ten Brink, U.S., Zhang, J., Brocher, T.M., Okaya, D.A., Klitgord, K.D., and Fuis, G.S., 2000, Geophysi- .1785/0120110317. cal evidence for the evolution of the California Inner Continental Borderland as a metamorphic Schindler, C.S., 2010, 3D fault geometry and basin evolution in the northern continental border- core complex: Journal of Geophysical Research, v. 105, p. 5835–5857, https://doi.org/10.1029 land offshore southern California [M.S. thesis]: Bakersfield, California State University, 42 p. /1999JB900318. Scholz, C.H., 2002, The mechanics of earthquakes and faulting: Cambridge, UK, Cambridge Uni- Teng, L.S., and Gorsline, D.S., 1989, Late Cenozoic sedimentation in California Continental Bor- versity Press, https://doi.org/10.1017/CBO9780511818516. derland basins as revealed by seismic facies analysis: Geological Society of America Bulletin, Shen, Z.-K., King, R.W., Agnew, D.C., Wang, M., Herring, T.A., Dong, D., and Fang, P., 2011, A uni- v. 101, p. 27–41, https://doi.org/10.1130/0016-7606(1989)101<0027:LCSICC>2.3.CO;2. fied analysis of crustal motion in Southern California, 1970–2004: The SCEC crustal motion Triezenberg, P.J., Hart, P.E., and Childs, J.R., 2016, National Archive of Marine Seismic Surveys (NAMSS): map: Journal of Geophysical Research, v. 116, B11402, https://doi.org/10.1029/2011JB008549. A U.S. Geological Survey data website of marine seismic reflection data within the U.S. Exclusive Shipboard Scientific Party, 1997, Site 1013, Proceedings of the Ocean Drilling Program, Initial Economic Zone (EEZ): U.S. Geological Survey data release, https://doi.org/10.5066/F7930R7P. Reports, Volume 167: College Station, Texas, Ocean Drilling Program, p. 157–174, https://doi Vedder, J.G., compiler, 1990, Maps of California continental borderland showing compositions .org/10.2973/odp.proc.ir.167.107.1997. and ages of bottom samples acquired between 1968 and 1979: U.S. Geological Survey Mis- Sieh, K.E., 1978, Central California foreshocks of the great 1857 earthquake: Bulletin of the Seis- cellaneous Field Studies Map 2122, 3 sheets, scale 1:250,000, https://doi.org/10.3133/mf2122. mological Society of America, v. 68, p. 1731–1749. Vedder, J.G., Greene, H.G., Clarke, S.H., and Kennedy, M.P., 1986, Geologic map of the mid-south- Sliter, R.W., Conrad, J.E., Ryan, H.F., and Triezenberg, P.J., 2017, Minisparker seismic-reflection ern California continental margin, in Greene, H.G., and Kennedy, M.P., eds., Geology of the data of field activity B-1-08-SC: Between Huntington Beach and San Diego, offshore south- Mid-Southern California Continental Margin: California Division of Mines and Geology Con- ern California from 2008-04-28 to 2008-05-05: U.S. Geological Survey data release, https:// tinental Margin Map 2A, scale 1:250,000. doi.org/10.5066/F7NS0S1D. Walton, M.A.L., Papesh, A.G., Johnson, S.Y., Conrad, J.E., and Brothers, D.S., 2020, Quaternary faults Sommerfield, C.K., Lee, H.J., and Normark, W.R., 2009, Postglacial sedimentary record of the offshore of California: U.S. Geological Survey data release, https://doi .org /10 .5066 /P91RYEZ4 . Southern California continental shelf and slope, Point Conception to Dana Point, in Lee, H.J., Ward, S.N., and Valensise, G., 1994, The Palos Verdes terraces, California: Bathtub rings from a and Normark, W.R., eds., Earth Science in the Urban Ocean: The Southern California Con- buried reverse fault: Journal of Geophysical Research, v. 99, p. 4485–4494, https://doi.org tinental Borderland: Geological Society of America Special Paper 454, p. 89–115, https://doi /10.1029/93JB03362. .org/10.1130/2009.2454(2.5). Wells, D.L., and Coppersmith, K.J., 1994, New empirical relationships among magnitude, rupture Sorlien, C.C., Bennett, J.T., Cormier, M.-H., Campbell, B.A., Nicholson, C., and Bauer, R.L., 2015, length, rupture width, rupture area, and surface displacement: Bulletin of the Seismological Late Miocene–Quaternary fault evolution and interaction in the southern California Inner Society of America, v. 84, p. 974–1002. Continental Borderland: Geosphere, v. 11, p. 1111–1132, https://doi.org/10.1130/GES01118.1. Wesnousky, S.G., 2006, Predicting the endpoints of earthquake ruptures: Nature, v. 444, p. 358–360, Stein, R.S., Barka, A.A., and Dieterich, J.H., 1997, Progressive failure on the North Anatolian https://doi.org/10.1038/nature05275. fault since 1939 by earthquake stress triggering: Geophysical Journal International, v. 128, Yang, W., Hauksson, E., and Shearer, P.M., 2012, Computing a large refined catalog of focal mecha- p. 594–604, https://doi.org/10.1111/j.1365-246X.1997.tb05321.x. nisms for southern California (1981–2010): Temporal stability of the style of faulting: Bulletin of Swanson, M.T., 1989, Sidewall ripouts in strike-slip faults: Journal of Structural Geology, v. 11, the Seismological Society of America, v. 102, p. 1179–1194, https://doi.org/10.1785/0120110311. p. 933–948, https://doi.org/10.1016/0191-8141(89)90045-X. Yue, H., Lay, T., Freymueller, J.T., Ding, K., Rivera, L., Ruppert, N.A., and Koper, K.D., 2013, Super-

Swanson, M.T., 2005, Geometry and kinematics of adhesive wear in brittle strike-slip fault zones: shear rupture of the 5 January 2013 Craig, Alaska (Mw 7.5) earthquake: Journal of Geophysical Journal of Structural Geology, v. 27, p. 871–887, https://doi.org/10.1016/j.jsg.2004.11.009. Research: Solid Earth, v. 118, p. 5903–5919, https://doi.org/10.1002/2013JB010594.