Geomorphic evidence for the geometry and slip rate of the Southern San Cayetano fault: Implications for hazard assessment and fault interaction in complex tectonic environments

Alex Hughes, Dylan H. Rood, Alexander Whittaker, Rebecca Bell, Thomas K. Rockwell, Yuval Levy, Klaus Wilcken, Lee Corbett, Paul Bierman, Duane E. DeVecchio, Scott T. Marshall, Larry D. Gurrola, & Craig Nicholson

Published August 9, 2018, SCEC Contribution #8348, 2018 SCEC Annual Meeting Poster #253

We present surface evidence and displacement rates for an active, low-angle (~20°) 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, low-angle SSCF along the northern flank of the Santa Clara River Valley. Displacement rates for the SSCF are calculated over thousand to ten thousand 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 maximum values of 1.6 +0.6/-0.4 mm/yr since ~58 ka, down to 1.2 +/- 0.3 mm/yr since ~7 ka. The SSCF is interpreted as a young, active, low-angle range-front structure with onset of activity at some point after ~58 ka. The geometry for the SSCF presented here, with a ~20° north-dip, is the first interpretation of the SSCF based on geological field data and is significantly different from the previously proposed model-derived geometry, which dips more steeply at 45-60° and outcrops in 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 fault and the San Cayetano fault in large-magnitude, multi-fault earthquakes in southern California. However, given structural complexities, including significant changes in dip and varying Holocene displacement rates and slip histories along strike, further work is required to examine the possible mechanism, likelihood, and frequency of potential thorough-going ruptures between the Ventura and San Cayetano fault systems.

Key Words
Thrust faults, Be-10, surface exposure dating, lidar, southern California

Citation
Hughes, A., Rood, D. H., Whittaker, A., Bell, R., Rockwell, T. K., Levy, Y., Wilcken, K., Corbett, L., Bierman, P., DeVecchio, D. E., Marshall, S. T., Gurrola, L. D., & Nicholson, C. (2018, 08). Geomorphic evidence for the geometry and slip rate of the Southern San Cayetano fault: Implications for hazard assessment and fault interaction in complex tectonic environments. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology