Geomorphic evidence for the geometry and slip rate of a young, low-angle thrust fault: Implications for hazard assessment and fault interaction in complex tectonic environments

摘要

We present surface evidence and displacement rates for a young, active, low-angle (similar to 20 degrees) reverse thrust fault in close proximity to major population centers in southern California (USA), 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, Be-10 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 10(3)-10(4) yr timescales with maximum slip rates for the central SSCF of 1.9 +10/-0.5 mm yr(-1) between similar to 19-7 ka and minimum slip rates of 1.3 +0.5/-0.3 mm yr(-1) since similar to 7 ka. Uplift rates for the central SSCF have not varied significantly over the last similar to 58 ka, with a maximum value of 1.7 +0.9/-0.6 mm yr(-1) for the interval similar to 58-19 ka, and a minimum value of 1.2 +/- 0.3 mm yr(-1) since similar to 7 ka. The SSCF is interpreted as a young, active structure with onset of activity at some time after similar to 58 ka. The geometry for the SSCF presented here, with a similar to 20 degrees 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 degrees 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, multi-fault earthquakes in southern California. However, given structural complexities, including significant changes in dip a