Transverse Ranges Research Articles

Kinematics of southern California crustal deformation: Insights from finite-element models

Two sets of finite-element model suites were developed to address discrepancies between geodetic and geologic fault slip rates in southern California, as well as the extent of “off-fault” deformation that does not result from interseismic elastic strain around known, locked faults. The GPS-constrained models represent current deformation, while the strain energy rate (TSE) minimizing models represent a long-term average of deformation that has occurred over thousands to millions of years. Both classes of models suggest low slip rates (<29 mm/yr) for the San Andreas Fault (SAF) in the Mojave and Big Bend regions. A summed slip rate of less than 8 mm/yr is also inferred for thrust faults accommodating shortening across the Transverse Ranges. GPS-constrained, locked models suggest high slip rates for the Imperial Fault and the Coachella SAF (>35 mm/yr and >32 mm/yr) but TSE-constrained, unlocked models suggest much lower slip rates for these two faults. All models are consistent with strain transfer from the SAF Coachella segment to the Eastern California Shear Zone, bypassing the SAF, but the GPS-constrained models suggest this far more strongly. This would seem to support variations in rates of fault slip and regional deformation patterns over millennial timescales, but the comparable Mojave SAF slip rates for both model classes is inconsistent with this hypothesis. Assuming a “propeller” geometry for the SAF (Fuis et al., 2012, 2017) does not alter inferred slip rates. Correcting the GPS velocity field for seismic cycle effects associated with large earthquakes on the 1857 SAF rupture segment increases the inferred slip rate of the Mojave South segment of the San Andreas fault by up to about 5 mm/yr. “Off-fault” deformation accounts for 38% of the total moment accumulation in the preferred GPS-constrained model, broadly consistent with previous estimates.

Spatial variations of rock damage production by earthquakes in southern California

We perform a comparative spatial analysis of inter-seismic earthquake production of rupture area and volume in southern California using observed seismicity and basic scaling relations from earthquake phenomenology and fracture mechanics. The analysis employs background events from a declustered catalog in the magnitude range 2≤M<4 to get temporally stable results representing activity during a typical inter-seismic period on all faults. Regions of high relative inter-seismic damage production include the San Jacinto fault, South Central Transverse Ranges especially near major fault junctions (Cajon Pass and San Gorgonio Pass), Eastern CA Shear Zone (ECSZ) and the Imperial Valley – Brawley seismic zone area. These regions are correlated with low velocity zones in detailed tomography studies. A quasi-linear zone with ongoing damage production extends between the Imperial fault and ECSZ and may indicate a possible future location of the main plate boundary in the area. The regions around the 1992 M6.1 Joshua Tree, M7.3 Landers and M6.3 Big Bear earthquakes have background seismic activity before 1990. This may represent a regional weakening process by damage production in future rupture zones. The depth of background seismicity and damage production decreases steadily from SW of the coastline to NE of the San Andreas fault, and also to the SE near the US–Mexico border. The seismicity and rock damage become more pronounced and continuous along-strike of main faults with increasing depth.

Hourly Analyses of the Large Storms and Atmospheric Rivers that Provide Most of California’s Precipitation in Only 10 to 100 Hours per Year

California is regularly impacted by floods and droughts, primarily as a result of too many or too few atmospheric rivers (ARs). This study analyzes a two-decade-long hourly precipitation dataset from 176 California weather stations and a 3-hourly AR chronology to report variations in rainfall events across California and their association with ARs. On average, 10-40 and 60-120 hours of rainfall in southern and northern California, respectively, are responsible for more than half of annual rainfall accumulations. Approximately 10-30% of annual precipitation at locations across the state is from only one large storm. On average, northern California receives 25-45 rainfall events annually (40-50% of which are AR-related). These events typically have longer durations and higher event-precipitation totals than those in southern California. Northern California also receives more AR landfalls with longer durations and stronger Integrated Vapor Transport (IVT). On average, ARs contribute 79%, 76%, and 68% of extreme-rainfall accumulations (i.e., top 5% events annually) in the north coast, northern Sierra, and Transverse Ranges of southern California, respectively. The San Francisco Bay Area terrain gap in the California Coast Range allows more AR water vapor to reach inland over the Delta and Sacramento Valley, and thus, influences precipitation in the Delta’s catchment. This is particularly important for extreme precipitation in the northern Sierra Nevada, including river basins above Oroville Dam and Shasta Dam. This study highlights differences between rainfall and AR characteristics in coastal versus inland northern California, differences that largely determine the regional geography of flood risks and water-reliability. These analyses support water resource, flood, levee, wetland, and ecosystem management within the catchment of the San Francisco estuary system by describing regional characteristics of ARs and their influence on rainfall on an hourly timescale.

Seismic Imaging of the Southern California Plate Boundary around the South-Central Transverse Ranges Using Double-Difference Tomography

We derive high-resolution P and S seismic velocities (VP and VS) within the South-Central Transverse Ranges section of the San Andreas Fault (SAF), using a new double-difference tomography algorithm incorporating both event-pair and station-pair differential times. The addition of station-pair data allows for better absolute event locations and higher model resolution at shallow depths. Velocities within a 222 km × 164 km region are inverted using > 1,000,000 P and S arrival (picked with an automatic detection algorithm) and differential times from > 10,000 local events recorded by > 250 stations. Similarly large P and S datasets lead to high-quality VP/VS estimates of the region. The resulting models include low velocities along major fault segments and across-fault velocity contrasts. They also show very high VP/VS anomalies near shallow damaged rock, whereas fault zones exhibit either low ( 1.73) VP/VS characteristics at greater depth. The variations in amplitude of these anomalies along the SAF through San Gorgonio Pass (SGP) suggest abrupt west-to-east changes in elastic properties. Moreover, their geometries imply near-vertical SAF segments northwest of SGP and northeast dipping faults southeast of that area. The SAF near Coachella Valley is estimated to dip by 57°. Regional-scale low and high VP/VS values are related to relative abundances of crystalline or metamorphic rocks. Near-fault VP/VS anomalies at depth are likely associated with changes in wet crack geometries. The obtained results can improve future calculations of seismic motion from large earthquakes in the area and related seismic hazard estimates.

Genetic structure and environmental niche modeling confirm two evolutionary and conservation units within the western spadefoot (Spea hammondii)

The western spadefoot (Spea hammondii) is a Species of Special Concern in California and is now under review by the U.S. Fish and Wildlife Service for listing under the Endangered Species Act. We delineated potential conservation units within S. hammondii by analyzing spatial genetic structure across the species’ range using five nuclear and one mitochondrial loci. For both nuclear and mitochondrial markers we found that S. hammondii consists of two genetically distinct, allopatric clusters divided by the Transverse Ranges. To corroborate the northern and southern genetic clusters as conservation units from an ecological perspective, we applied a niche identity test to environmental niche models of the two groups. We found that the niche models of the northern and southern clusters were significantly different, suggesting they may be ecologically non-exchangeable. Given our demonstration of significant genetic and ecological differentiation between allopatric clusters of S. hammondii, we recommend that ongoing conservation efforts consider each as a separate unit with potentially unique management needs.

Sycamore Knoll: A wave-planed pop-up structure in a sinistral-oblique thrust system, Southern California Continental Borderland

At the boundary between the Western Transverse Ranges province and Inner Continental Borderland of Southern California, strain is partitioned across the sinistral-oblique Anacapa-Dume Fault system. Sycamore Knoll, a pop-up structure 20 km west of Point Dume in the hanging wall of the north-dipping Anacapa-Dume Fault, stands out as an anomalous bathymetric high along the Southern California coast. We synthesize new and existing geophysical datasets, including multibeam bathymetry/backscatter and single/multichannel seismic, to understand the morphology and tectonic implications of Sycamore Knoll. We identify a complete eustatic low-stand submerged marine-terrace package encircling Sycamore Knoll platform that corresponds to the Last Glacial Maximum (LGM). The marine-terrace deposits and wave-planed surface of Sycamore Knoll require a late Quaternary uplift rate of 0.55 (+0.5/−0.3) mm/yr relative to sea-level. By combining our uplift rate with previous estimates of basin subsidence and a revised structural model of the Anacapa-Dume Fault, we determine a maximum slip rate in latest Quaternary time of 1.50 (+0.5/−0.3) mm/yr and hence a post-LGM rate of convergence across the Anacapa-Dume Fault of 0.60 (+0.2/−0.1) mm/yr.

The Paleoproterozoic Vishnu basin in southwestern Laurentia: Implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave crustal province

The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8–1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ∼1.87 and 2.4–2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ∼1.8 to 1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ∼1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791 ± 15 to 1691 ± 15 Ma. Plutons show a systematic change in Hf-isotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8–2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7–1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.

Uplift of the Western Transverse Ranges and Ventura Area of Southern California: A Four‐Technique Geodetic Study Combining GPS, InSAR, Leveling, and Tide Gauges

AbstractWe estimate the rate of vertical land motion (VLM) in the region around the Western Transverse Ranges (WTR), Ventura, and Big Bend of the San Andreas Fault (SAF) of southern California using data from four geodetic techniques: GPS, interferometric synthetic aperture radar (InSAR), leveling, and tide gauges. We use a new analysis technique called GPS Imaging to combine the techniques and leverage the synergy between (1) high geographic resolution of InSAR, (2) precision, stability, and geocentric reference frame of GPS, (3) decades long observation of VLM with respect to the sea surface from tide gauges, and (4) relative VLM along dense leveling lines. The uncertainty in the overall rate field is ~1 mm/yr, though some individual techniques have uncertainties as small as 0.2 mm/yr. The most rapid signals are attributable to subsidence in aquifers and groundwater changes. Uplift of the WTR is geographically continuous, adjacent to the SAF and appears related to active crustal contraction across Pacific/North America plate boundary fault system. Uplift of the WTR and San Gabriel Mountains is ~2 mm/yr and is asymmetrically focused west of the SAF, consistent with interseismic strain accumulation across thrust faults in the Ventura area and Santa Barbara channel that accommodate contraction against the near vertical SAF.

Rayleigh‐Wave H/V via Noise Cross Correlation in Southern California

We study the crustal structure of southern California by inverting horizontal‐to‐vertical (H/V) amplitudes of Rayleigh waves observed in noise cross‐correlation signals. This study constitutes a useful addition to traditional phase‐velocity‐based tomographic inversions due to the localized sensitivity of H/V measurements to the near surface of the measurement station site. The continuous data of 222 permanent broadband stations of the Southern California Seismic Network (SCSN) were used in production of noise cross‐correlation waveforms, resulting in a spatially dense set of measurements for the southern California region in the 1–15 s period band. The fine interstation spacing of the SCSN allows retrieval of high signal‐to‐noise ratio Rayleigh waves at periods as low as 1 s, significantly improving the vertical resolution of the resulting tomographic image, compared to previous studies with minimum periods of 5–10 s. In addition, horizontal resolution is naturally improved by increased station density. Tectonic subregions including the Los Angeles basin and Salton trough are clearly visible due to their high short‐period H/V ratios, whereas the Transverse and Peninsular Ranges exhibit low H/V at all periods.

Transrotation Induced by Crustal Blocks Moving through Restraining Double Bends, with Southern California Examples

AbstractWithin the San Andreas system of southern California, vertical-axis clockwise rotations have been attributed to (1) capture of the Monterey microplate, during which oceanic lithosphere beneath North America was transferred to the Pacific plate imparting basal traction and inducing northwestward motion of overlying North American crust, and (2) torsional force couples imposed at the ends of rotating panels. In contrast, some crustal blocks that are smaller and/or weaker than crust on opposite sides of faults have experienced clockwise transrotation as they have moved through restraining double bends: (1) rapid 40° clockwise rotation of the eastern Transverse Ranges (6–0 Ma), starting with the southernmost parts and progressing northward; (2) rapid 23° clockwise rotation of the Morales Formation in the Cuyama Valley (3–0 Ma); and (3) continuing clockwise rotation of the westernmost Transverse Ranges. The magnitude of rotation matches the angle between the strikes of fault segments on either side of ...

How fault evolution changes strain partitioning and fault slip rates in Southern California: Results from geodynamic modeling

AbstractIn Southern California, the Pacific‐North America relative plate motion is accommodated by the complex southern San Andreas Fault system that includes many young faults (&lt;2 Ma). The initiation of these young faults and their impact on strain partitioning and fault slip rates are important for understanding the evolution of this plate boundary zone and assessing earthquake hazard in Southern California. Using a three‐dimensional viscoelastoplastic finite element model, we have investigated how this plate boundary fault system has evolved to accommodate the relative plate motion in Southern California. Our results show that when the plate boundary faults are not optimally configured to accommodate the relative plate motion, strain is localized in places where new faults would initiate to improve the mechanical efficiency of the fault system. In particular, the Eastern California Shear Zone, the San Jacinto Fault, the Elsinore Fault, and the offshore dextral faults all developed in places of highly localized strain. These younger faults compensate for the reduced fault slip on the San Andreas Fault proper because of the Big Bend, a major restraining bend. The evolution of the fault system changes the apportionment of fault slip rates over time, which may explain some of the slip rate discrepancy between geological and geodetic measurements in Southern California. For the present fault configuration, our model predicts localized strain in western Transverse Ranges and along the dextral faults across the Mojave Desert, where numerous damaging earthquakes occurred in recent years.

Synoptic conditions associated with cool season post-fire debris flows in the Transverse Ranges of southern California

The Transverse Ranges of southern California often experience fire followed by flood. This sequence sometimes causes post-fire debris flows (PFDFs) that threaten life and property situated on alluvial fans. The combination of steep topography, highly erodible rock and soil, and wildfire, coupled with intense rainfall, can initiate PFDFs even in cases of relatively small storm rainfall totals. This study identifies common atmospheric conditions during which damaging PFDFs occur in the Transverse Ranges during the cool season, defined here as November–March. A compilation of 93 PFDF events during 1980–2014 triggered by 19 precipitation events is compared against previous studies of the events, reanalysis, precipitation, and radar data to estimate PFDF trigger times. Each event was analyzed to determine common atmospheric features and their range of values present at and preceding the trigger time. Results show atmospheric rivers are a dominant feature, observed in 13 of the 19 events. Other common features include low-level winds orthogonal to the Transverse Ranges and other conditions favorable for orographic forcing, a strong upper level jet south of the region, and moist-neutral static stability. Several events included closed low-pressure systems or narrow cold frontal rain bands. These findings can help forecasters identify more precisely the synoptic-scale atmospheric conditions required to produce PFDF-triggering rainfall and thus reduce uncertainty when issuing warnings.

Large Transverse Range Earthquakes Cause Coastal Upheaval near Ventura, Southern California

Abstract Slip on the Pitas Point thrust produces uplift and folding of the Ventura Avenue anticlinal (VAA) trend in the western Transverse Ranges of southern California. Rapid convergence has resulted in 2.7 km of uplift on the VAA over the past 200–300 ka, with as much as 320 m in the past 45–50 ka, indicating a long‐term uplift rate of 6–7 mm/yr. Four Holocene emergent marine terraces are present between Carpinteria and Ventura, with the fourth terrace uplifted a minimum of 38 m. The most recent emergence event uplifted a Chumash Indian village at Pitas Point by 5–6 m about 950 yrs ago. The first terrace, which formed when the sea level was 0.5–1 m lower than at present, is nearly continuously preserved from east of Pitas Point toward the fold crest of the VAA, where its elevation increases, reaching an altitude of 7–8 m; this implies a large amount of slip on the causative fault at depth. Radiocarbon dates on marine shells and culturally derived charcoal indicate terrace emergence events occurred at about 0.95, 2.09, 4.4, and 6.7 ka.

A molecular biogeography of the New World cypresses (Callitropsis, Hesperocyparis; Cupressaceae)

Previous studies of phylogenetic relationships among cypresses have recovered separate Old and New World lineages, resolved a Southeast Asian species sister to the New World clade, and split the New World group into two principal lineages (i.e., the Macrocarpa and Arizonica clades) having fundamentally different geographic distributions. Collectively, these observations suggested a number of intriguing hypotheses regarding the origin and biogeographic history of the New World cypresses (NWC). In this study, we use DNA sequence data to examine the historical biogeography of NWC. Divergence times are estimated in BEAST using fossil-calibrated minimum age constraints, and a spatial history of the group reconstructed using Statistical Dispersal-Vicariance Analysis. Results presented here suggest ancestral NWC colonized the New World from Asia, perhaps by a more widespread trans-Beringian ancestor, in the late Cretaceous or early Cenozoic. Strong positive correlations between species age and geographic location (i.e., latitude and longitude) suggest current distributions were influenced by directional migration (northwest to southeast) as climates cooled and became increasingly arid in the latter half of the Cenozoic. Although our data support a middle Eocene (45 Mya) origin for NWC, nearly all species are apparently no more than late Miocene (6 Mya) in age, and net diversification rates in the group are among the highest reported to date for gymnosperms. Key coastal to interior migrations underlie the fundamentally different biogeographies of the Macrocarpa and Arizonica clades, with the Transverse Ranges of southern California being a barrier to north–south migration of certain species from these two lineages.

A Fault‐Based Model for Crustal Deformation, Fault Slip Rates, and Off‐Fault Strain Rate in California

We invert Global Positioning System (GPS) velocity data to estimate fault slip rates in California using a fault‐based crustal deformation model with geologic constraints. The model assumes buried elastic dislocations across the region using Uniform California Earthquake Rupture Forecast Version 3 (UCERF3) fault geometries. New GPS velocity and geologic slip‐rate data were compiled by the UCERF3 deformation working group. The result of least‐squares inversion shows that the San Andreas fault slips at 19–22 mm/yr along Santa Cruz to the North Coast, 25–28 mm/yr along the central California creeping segment to the Carrizo Plain, 20–22 mm/yr along the Mojave, and 20–24 mm/yr along the Coachella to the Imperial Valley. Modeled slip rates are 7–16 mm/yr lower than the preferred geologic rates from the central California creeping section to the San Bernardino North section. For the Bartlett Springs section, fault slip rates of 7–9 mm/yr fall within the geologic bounds but are twice the preferred geologic rates. For the central and eastern Garlock, inverted slip rates of 7.5 and 4.9 mm/yr, respectively, match closely with the geologic rates. For the western Garlock, however, our result suggests a low slip rate of 1.7 mm/yr. Along the eastern California shear zone and southern Walker Lane, our model shows a cumulative slip rate of 6.2–6.9 mm/yr across its east–west transects, which is ∼1 mm/yr increase of the geologic estimates. For the off‐coast faults of central California, from Hosgri to San Gregorio, fault slips are modeled at 1–5 mm/yr, similar to the lower geologic bounds. For the off‐fault deformation, the total moment rate amounts to 0.88×1019 N·m/yr, with fast straining regions found around the Mendocino triple junction, Transverse Ranges and Garlock fault zones, Landers and Brawley seismic zones, and farther south. The overall California moment rate is 2.76×1019 N·m/yr, which is a 16% increase compared with the UCERF2 model. Online Material: Table of geological slip rates.

From the mountains to the coast and back again: Ancient biogeography in a radiation of short-range endemic harvestmen from California

The harvestmen genus Calicina is represented by 25 short-range endemic species occurring in the western Sierra Nevada, Transverse and Coast Ranges of California. Our principal aim was to reconstruct the temporal and spatial biogeographic history of this arachnid lineage. We inferred a time-calibrated species tree for 21 of 25 described Calicina species using multiple genes and multilocus coalescent-based methods. This species tree was used as a framework for algorithmic biogeographic and divergence time analyses, and a phylogenetic canonical correlation analysis (CCA) was used to examine the relationship between morphological evolution and environmental variables. Species tree and biogeographic analyses indicate that high-elevation Sierran taxa are early-diverging in Calicina, with subsequent biogeographic “criss-crossing” of lineages from the Sierra Nevada to the Coast Ranges, back to the Sierra Nevada, then back to Coast Ranges. In both the Sierra Nevada and Coast Ranges, distantly-related parapatric lineages essentially never occur in sympatry. CCA reveals that in both the Coast Ranges and the Sierra Nevada, distant phylogenetic relatives evolve convergent morphologies. Our evidence shows that Calicina is clearly dispersal-limited, with an ancient biogeographic history that provides unique insight into the complex geologic evolution of California since the mid-Paleogene.

Geochemical and tectonic uplift controls on rock nitrogen inputs across terrestrial ecosystems

AbstractRock contains &gt; 99% of Earth's reactive nitrogen (N), but questions remain over the direct importance of rock N weathering inputs to terrestrial biogeochemical cycling. Here we investigate the factors that regulate rock N abundance and develop a new model for quantifying rock N mobilization fluxes across desert to temperate rainforest ecosystems in California, USA. We analyzed the N content of 968 rock samples from 531 locations and compiled 178 cosmogenically derived denudation estimates from across the region to identify landscapes and ecosystems where rocks account for a significant fraction of terrestrial N inputs. Strong coherence between rock N content and geophysical factors, such as protolith, (i.e. parent rock), grain size, and thermal history, are observed. A spatial model that combines rock geochemistry with lithology and topography demonstrates that average rock N reservoirs range from 0.18 to 1.2 kg N m−3 (80 to 534 mg N kg−1) across the nine geomorphic provinces of California and estimates a rock N denudation flux of 20–92 Gg yr−1 across the entire study area (natural atmospheric inputs ~ 140 Gg yr−1). The model highlights regional differences in rock N mobilization and points to the Coast Ranges, Transverse Ranges, and the Klamath Mountains as regions where rock N could contribute meaningfully to ecosystem N cycling. Contrasting these data to global compilations suggests that our findings are broadly applicable beyond California and that the N abundance and variability in rock are well constrained across most of the Earth system.

Landscape genetics and population structure in Valley Oak (Quercus lobata Née).

Although long-distance pollen movement is common in wind-pollinated trees, barriers to gene flow may occur in species that have discontinuous ranges or are confined to certain habitat types. We investigated the genetic structure of Quercus lobata Née populations throughout much of their range in California. We assessed the connectivity of populations and determined if barriers to gene flow occurred, and if so, if they corresponded to landscape features. We collected leaf samples from 270 trees from 12 stands of Quercus lobata and genotyped these trees using eight polymorphic microsatellite loci. Genetic structure and clustering was evaluated using genetic distance methods, Bayesian clustering approaches, and network analysis of spatial genetic structure. The southernmost population of Quercus lobata sampled from the Santa Monica area comprised a separate genetic cluster from the rest of the species, suggesting that Transverse Ranges such as the San Gabriel Mountains limit gene flow. Population differentiation among the other sites was small but significant. Network analysis reflected higher connectivity among populations along the Central Coast range, with few connections spanning the dry, low Central Valley. While long distance pollen movement has been shown to be common in oaks, on larger spatial scales, topographic features such as mountain ranges and the large, flat Central Valley of California limit gene flow. Such landscape features explain gene flow patterns much better than geographic distance alone.

Paleoseismologic evidence for large-magnitude (Mw7.5–8.0) earthquakes on the Ventura blind thrust fault: Implications for multifault ruptures in the Transverse Ranges of southern California

Detailed analysis of continuously cored boreholes and cone penetrometer tests (CPTs), high-resolution seismic-reflection data, and luminescence and 14C dates from Holocene strata folded above the tip of the Ventura blind thrust fault constrain the ages and displacements of the two (or more) most recent earthquakes. These two earthquakes, which are identified by a prominent surface fold scarp and a stratigraphic sequence that thickens across an older buried fold scarp, occurred before the 235-yr-long historic era and after 805 ± 75 yr ago (most recent folding event[s]) and between 4065 and 4665 yr ago (previous folding event[s]). Minimum uplift in these two scarp-forming events was ∼6 m for the most recent earthquake(s) and ∼5.2 m for the previous event(s). Large uplifts such as these typically occur in large-magnitude earthquakes in the range of Mw 7.5–8.0. Any such events along the Ventura fault would likely involve rupture of other Transverse Ranges faults to the east and west and/or rupture downward onto the deep, low-angle décollements that underlie these faults. The proximity of this large reverse-fault system to major population centers, including the greater Los Angeles region, and the potential for tsunami generation during ruptures extending offshore along the western parts of the system highlight the importance of understanding the complex behavior of these faults for probabilistic seismic hazard assessment.

High‐resolution mapping of two large‐scale transpressional fault zones in the California Continental Borderland: Santa Cruz‐Catalina Ridge and Ferrelo faults

AbstractNew mapping of two active transpressional fault zones in the California Continental Borderland, the Santa Cruz‐Catalina Ridge fault and the Ferrelo fault, was carried out to characterize their geometries, using over 4500 line‐km of new multibeam bathymetry data collected in 2010 combined with existing data. Faults identified from seafloor morphology were verified in the subsurface using existing seismic reflection data including single‐channel and multichannel seismic profiles compiled over the past three decades. The two fault systems are parallel and are capable of large lateral offsets and reverse slip during earthquakes. The geometry of the fault systems shows evidence of multiple segments that could experience throughgoing rupture over distances exceeding 100 km. Published earthquake hypocenters from regional seismicity studies further define the lateral and depth extent of the historic fault ruptures. Historical and recent focal mechanisms obtained from first‐motion and moment tensor studies confirm regional strain partitioning dominated by right slip on major throughgoing faults with reverse‐oblique mechanisms on adjacent structures. Transpression on west and northwest trending structures persists as far as 270 km south of the Transverse Ranges; extension persists in the southern Borderland. A logjam model describes the tectonic evolution of crustal blocks bounded by strike‐slip and reverse faults which are restrained from northwest displacement by the Transverse Ranges and the southern San Andreas fault big bend. Because of their potential for dip‐slip rupture, the faults may also be capable of generating local tsunamis that would impact Southern California coastlines, including populated regions in the Channel Islands.