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.

Understanding how tectonics and erosion shape the landscape is fundamental to infer deformation from geomorphic observables (e.g., Crosby & Whipple, 2006;Kirby & Whipple, 2012;Wobus et al., 2006). This is well-documented in extensional settings where slip on normal faults uplifts the surface, which is simultaneously or subsequently reworked by various erosive and sediment transport processes (e.g.,

To investigate the subsurface geometry of a recently discovered, seismically active fault in the Ventura basin, southern California, USA, we present a series of cross sections and a new three-dimensional fault model across the Southern San Cayetano fault (SSCF) based on integration of surface data with petroleum industry well log data. Additionally, the fault model for the SSCF, along with models of other regional faults extracted from the Southern California Earthquake Center three-dimensional Community Fault Model, are incorporated in static Coulomb stress modeling to investigate static Coulomb stress transfer between thrust faults with complex geometry and to further our understanding of stress transfer in the Ventura basin. The results of the subsurface well investigation provide evidence for a low-angle SSCF that dips~15°north and connects with the western section of the San Cayetano fault around 1.5-3.5 km depth. We interpret the results of static Coulomb stress models to partly explain contrasting geomorphic expression between different sections of the San Cayetano fault and a potential mismatch in timings between large-magnitude uplift events suggested by paleoseismic studies on the Pitas Point, Ventura, and San Cayetano faults. In addition to new insights into the structure and potential rupture hazard of a recently discovered active reverse fault in a highly populated area of southern California, this study provides a simple method to model static Coulomb stress transfer on complex geometry faults in fold and thrust belts.