[1] To better constrain fault slip rates and patterns of interseismic deformation in the western Transverse Ranges of southern California, we present results from analysis of GPS and interferometric synthetic aperture radar (InSAR) data and three-dimensional mechanical and kinematic models of active faulting. Anthropogenic motions are detected in several localized zones but do not significantly affect the vast majority of continuous GPS site locations. GPS measures contraction rates across the Ventura Basin of~7 mm/yr oriented west-northwest with rates decreasing to the west and east. The Santa Barbara channel is accommodating~6.5 mm/yr in the east and~2.5 mm/yr in the western portions of N/S contraction. Inversion of horizontal GPS velocities highlights a zone of localized fast contraction rates following the Ventura Basin. Using a mechanical model driven by geodetically calculated strain rates, we show that there are no significant discrepancies between short-term slip rates captured by geodesy and longer-term slip rates measured by geology. Mechanical models reproduce the first-order interseismic velocity and strain rate patterns but fail to reproduce strongly localized contraction in the Ventura Basin due to the inadequate homogeneous elastic properties of the model. Existing two-dimensional models match horizontal rates but predict significant uplift gradients that are not observed in the GPS data. Mechanical models predict zones of fast contraction in the Santa Barbara channel and offshore near Malibu, suggesting that offshore faults represent a significant seismic hazard to the region. Furthermore, many active faults throughout the region may produce little to no interseismic deformation, making accurate seismic hazard assessment challenging.

[1] Existing interseismic models are not well-suited to simulate deformation within the network of finite, intersecting, nonplanar faults observed in the greater Los Angeles region. Instead of applying fault slip rates to a model a priori, we allow three-dimensional fault surfaces to interact and accumulate mechanically viable slip distributions and then use the deep nonseismogenic portion of slip to calculate interseismic deformation. We apply this approach to the Los Angeles region and find that the geologic timescale model results match well geologic slip rate data and the interseismic timescale model results match well the heterogeneous GPS velocity pattern in the Los Angeles region. Model results suggest that localized geodetic convergence in the San Gabriel basin can be achieved with slip on multiple active fault surfaces in the Los Angeles region including relatively fast slip on the Sierra Madre fault and slow slip on the Puente Hills thrusts, in agreement with geologic data. The ability of the three-dimensional model to reproduce well both geologic slip rates and interseismic geodetic velocity patterns suggests that current day contraction rates in the greater Los Angeles region are compatible with long-term geologic deformation rates and disputes suggestions of significant temporal variations in fault slip rates inferred from existing investigations.

We present surface evidence and displacement rates for a young, active, low-angle (~20°) reverse thrust fault in close proximity to major population centers in southern California (U.S.A.), the Southern San Cayetano fault (SSCF). Active faulting along the northern flank of the Santa Clara River Valley displaces young landforms, such as late Quaternary river terraces and alluvial fans. Geomorphic strain markers are examined using field mapping, high-resolution lidar topographic data, 10Be surface exposure dating, and subsurface well data to provide evidence for a young, active SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over 1,000-10,000 year timescales with maximum slip rates for the central SSCF of 1.9 +1.0/-0.5 mm/yr between ~19-7 ka and minimum slip rates of 1.3 +0.5/-0.3 mm/yr since ~7 ka. Uplift rates for the central SSCF have not varied significantly over the last ~58 ka, with a maximum value of 1.7 +0.9/-0.6 mm/yr for the interval ~58-19 ka, and a minimum value 1.2 +/-0.3 mm/yr since ~7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some point after ~58 ka. The geometry for the SSCF presented here, with a ~20° north-dip in the subsurface, is the first interpretation of the SSCF based on geological field data. Our new interpretation is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45-60° and intersects the surface in the middle of the Santa Clara River Valley. We suggest that the SSCF may rupture in tandem with the main San Cayetano fault. Additionally, the SSCF could potentially act as a rupture pathway between the Ventura and San Cayetano faults in large-magnitude, multifault earthquakes in southern California. However, given structural complexities, including significant changes in dip and varying Holocene displacement rates along strike, further work is required to examine the possible mechanism, likelihood, and frequency of potential through-going ruptures between the Ventura and San Cayetano faults. Confirmation of the SSCF in a previously well-studied area, such as the southern California, Highlights • Young faults often undetected but potentially key for seismic hazard assessments. • First geomorphic evidence for the Southern San Cayetano fault (SSCF). • 10 Be dating on offset terraces records Holocene slip rate of 1. 3 +0.5 /-0.3 mm yr-1. • SSCF has major implications for seismic hazard in southern California.

To assess the control of fault geometry and mechanical interactions on fault-slip distributions in a complex natural system, we present results from threedimensional mechanical models incorporating both nonplanar and rectangular planar representations of active faults within the Ventura basin region of southern California. We find that the incorporation of geologically constrained nonplanar fault surfaces into numerical models of active deformation results in a better match to available geologic slip-rate data than models utilizing rectangular planar fault surfaces. The model results demonstrate that nonplanar fault geometry and mechanical interactions exert a strong control on resultant slip distributions. Additionally, we find that slip rates at most locations along the surface trace of Ventura faults are not likely to represent average values for the entire fault surface. We propose that results from threedimensional mechanical models using realistic (i.e., nonplanar) fault geometry can be used to both predict slip rates at specific locations and determine whether existing sitespecific slip-rate estimates are representative of average fault-slip rates. Although geometric irregularities along-fault surfaces should resist slip, planar faults can have lesser slip than nonplanar faults due to the differing mechanical interactions among nearby faults in the two representations. This suggests that models using simplified or planar fault geometry are likely to inaccurately simulate regional deformation. We assert that detailed knowledge of three-dimensional fault shape as well as the geometry and configuration of deep fault intersections is essential for accurate seismic hazard characterization of regions of complex faulting such as the Ventura basin of southern California.Online Material: Three-dimensional interactive fault mesh of Ventura region faults.

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