TOPOGRAPHIC EXPRESSION OF ACTIVE FAULTING AND FOLDING IN THE SACRAMENTO/SAN JOAQUIN DELTA, CALIFORNIA

The Sacramento-San Joaquin Delta region of north-central California (‘the Delta’) is home to significant infrastructure, including water projects that supply two-thirds of the state’s population. Even so, the seismic source model and hazards within the Delta region remain relatively poorly constrained, due in large part to variable fault low slip rates and limited preservation potential of geomorphic features. In addition, both blind and surface-rupturing faults have been reported in the region, and structures have an array of kinematics, from dextral to reverse and oblique transpression. To address these challenges, we use analysis of high-resolution (1 m) topographic datasets to calculate tectonic geomorphology metrics that constrain subsurface fault geometry and activity.

To identify and describe locations of local fault-related uplift and deformation within the Delta region, we use these high-resolution topographic data to calculate geomorphic indices including normalized channel steepness (k_sn), concavity, and local relief. In addition to locating active deformation, these metrics also help constrain subsurface geometry and kinematics of active structures. In the central Delta, k_sn values are consistently higher on the southern (as compared to the northern) side of the Montezuma Hills, suggesting that topographic growth is asymmetric, with implications for subsurface fault geometry. In the northwestern part of the Delta region, topographic analyses of the Coast Ranges foothills suggest that slip may be accommodated on both a blind fault beneath the Delta and a west-vergent backthrust near the topographic rangefront.

These insights provide a framework for future investigations of deformation rates and paleoseismic history on these structures, which will serve as constraints on location and activity of potential seismic sources within the Delta region.