EVIDENCE OF TIME-TRANSGRESSIVE DISPLACEMENT ON THE RODGERS CREEK FAULT, WINDSOR, CA

The Rodgers Creek fault is a major strand of the San Andreas fault system in the San Francisco Bay Area, California. The Rodgers Creek fault connects with the Hayward fault under San Pablo Bay, producing a continuous fault trace of ~190 km. Together, these faults have the highest probability in the Bay Area of producing a damaging earthquake in the next 30 years [13-16%; UCERF3]. Despite this fact, the fault's slip rate is poorly constrained. The Santa Rosa pull-apart basin is an overlapping right-stepover between the Maacama and Rodgers Creek fault. To the north of this pull-apart basin the geologic slip rate for the Rodgers Creek fault is currently unconstrained post-1 Ma, leading to uncertainty in the distribution of strain over this structure during the Quaternary. To assess the seismic hazard potential of the Rodgers Creek fault, we employed geomorphic field mapping, provenance studies, and examination of high-resolution digital topography data to reconstruct fault displacement over multiple timescales near Windsor, CA. We have identified fault displacements at a variety of scales (~4 km, ~165 m and ~5 m) through realigning beheaded and displaced drainages across the fault. The largest offset of 4 km is based on realignment of a 1 km wide beheaded paleochannel west of the fault with a geomorphically similar channel to the east of the fault. This reconstruction also restores two pairs of 1 km wide paleochannels and two similar fan units across the fault. The second-largest offset of ~165 m realigns a beheaded channel with the modern channel of Windsor Creek. This reconstruction also realigns three pairs of channels along the fault. The smallest offset of ~5 m restores a deflected stream and realigns displaced stream terraces across the fault. The range in displacement observed along this section of the Rodgers Creek fault suggests that slip rates over multiple timescales (mid-Quaternary, late Pleistocene, and Holocene) are preserved here.