Oral Presentation

The location and geometry of faults (fault models) and the rate of which a fault slips (deformation models) are first order inputs for earthquake probability models. However, these data can be challenging to obtain in northern California because steep terrain, mass wasting, vegetation, and urban development have generally made it difficult to identify and map continuous traces of faults and offset landforms to estimate slip rate. Yet, obtaining these data are important because fault and deformation models directly feed into estimates of earthquake probability in a region (e.g. UCERF3, Fields et al., 2015). This presentation highlights recent field and geochronologic investigations on 4 main faults of the northern San Andreas Fault System: San Gregorio, northern San Andreas, Rodger’s Creek and Maacama faults. The combination of mapping offset features in the field and on high-resolution digital topography data (LiDAR) on these faults, along with the application of different geochronologic techniques yield newly identified active fault strands and slip rates for faults of the northern San Andreas Fault System. These data suggest a slip rate of 20-24 mm/yr for the northern San Andreas Fault, supporting a kinematic model of constant fault slip along the length of the fault in the San Francisco Bay Area. Moreover, comparison of our results with previous work indicates that this slip rate has remained constant over Holocene timescales. This constancy in slip rate on the northern San Andreas Fault suggests that seismic hazard on faults west of the northern San Andreas Fault System, specifically the San Gregorio Fault, are likely lower over this Holocene time scale. These results combined with more recent investigations on the San Gregorio, Rodger’s Creek and Maacama faults illustrate opportunities to improve earthquake science and seismic hazard by expanding the geographic scope of SCEC.