INVESTIGATING SLOW-MOVING, COMPLEX FAULTS WITH A COMBINATION OF GEOMORPHOLOGY, BEDROCK, AND SUBSURFACE DATASETS: AN EXAMPLE FROM THE SACRAMENTO – SAN JOAQUIN DELTA, NORTHERN CALIFORNIA

Geologic slip rates are a primary data input in seismic hazard assessments. However, slip rates are not always attainable on active faults for a variety of reasons, including landform degradation and poor resolution of features that record fault offsets, age uncertainty of deformed depositional units or landforms, and unknown or complex fault geometries and kinematics that make reconstructions of offset features challenging. Uncertainty in calculated slip rates can be higher where deformation is slow, where erosion is high, and where geochronology is difficult.

In such settings, investigations of geology and geomorphology at larger spatial and temporal scales may provide insights beyond what can be gleaned from studies of discrete offset of landforms across a fault. As an example, we present results of active fault investigations in the Sacramento – San Joaquin Delta of northern California (the ‘Delta’). In this region, deformation is often kinematically complex, blind, long-wavelength and/or low-amplitude, and does not provide clear opportunities for slip rate calculations.

To address these challenges, we integrate surface geomorphic observations with subsurface, bedrock geology, and geodetic datasets, collected locally and compiled from the neighboring Coast Ranges and Diablo Range. In the northern Delta, we use bedrock structure and shallow geophysical data to identify apparent strain partitioning on segments of the Great Valley fault system. In the central Delta, the geomorphology of the Montezuma Hills records regional uplift across blind structures that show multiple episodes of deformation through the Neogene and Quaternary. Where possible, we use these diverse datasets to better identify datable landforms that are deformed across a fault. However, even where datable and clearly offset landforms are not identifiable, local bedrock structure, subsurface geometry, and uplifted and deformed sedimentary deposits and landforms provide important constraints on fault activity and long-term rates of deformation.