Results from the Chalk Hill Paleoseismic Site, Northern Rodgers Creek Fault, San Francisco Bay Region

The Rodgers Creek fault is a principal strand of the northern San Andreas fault system and, in combination with the Hayward fault to the south, has the highest 30-year probability of generating a large, damaging earthquake in the San Francisco Bay region. Data on paleoearthquake timing and slip rates are limited, however, and are mostly absent north of Santa Rosa, where recent geodetic observations indicate that the fault is shallowly creeping at rates up to ~7 mm/yr. We conducted a paleoseismic trench investigation across the main trace of the northern Rodgers Creek fault at a site on the Chalk Hill Winery east of the town of Windsor. The trench was excavated across the base of a scarp developed on Plio-Pleistocene gravels faulted against Holocene alluvial deposits from nearby Windsor Creek. Geomorphic and structural relations indicate that surface deformation within a 25-m wide zone of faulting has varied across short spatial (between trench walls) and temporal (1000s of years) scales. The development of a pressure ridge along one strand of the fault was followed by creation of a pull-apart or tension gash, while an asymmetric sag formed along another strand ~10 m to the east and created a small depocenter within the basin fill. This eastern strand extends to the ground surface, through deposits that bury a soil A-horizon and that likely reflect historical aggradation due to land-use change. Because no large earthquakes are known from the historical record, this observation of recent faulting is interpreted as evidence of contemporary fault creep. Earthquake-event horizons were not identified in the trench, likely due to the poorly stratified nature of the fine-grained alluvium. We hope to derive a minimum slip rate at this site from age constraints on a faulted debris-flow deposit beneath the base of the scarp. We tentatively interpret this deposit as offset from a broad debris-flow landform ~35 m to the south, where overbank discharge from Windsor Creek appears to have dammed against the scarp.