UNVEILING A NEW 3D STRUCTURAL MODEL OF THE SOUTHERN SAN JOAQUIN FOLD AND THRUST BELT, CALIFORNIA, AND ITS IMPLICATIONS FOR REGIONAL EARTHQUAKE HAZARDS

The 1982 (M 5.5) New Idria, 1983 (M 6.5) Coalinga and 1985 (M 6.1) Kettleman Hills earthquakes demonstrated the activity of blind fault thrust ramps in the San Joaquin Fold and Thrust Belt. Yet, the activity and seismogenic potential of other faults in the system, including thrust ramps that lie along strike of the epicentral zones and basal detachments, are not known. Previously published cross-sections through the region have varied in their representation of structural geometries and styles, and the level of detail they offer in defining the late Pleistocene and Holocene activity of the various faults in the system. We present a new comprehensive and internally consistent 3D model of the structures in the region, including fault segments along strike from the 1980s earthquake sequences (Kettleman Middle and South Domes, Lost Hills) and the Temblor Range. This model is based on rigorous analysis of industry seismic data depth-converted using the SCEC Community Velocity Model and coupled with horizon picks from CalGEM Oil and Gas Well Data, Cal DWR groundwater wells, and CPT data. In addition, we incorporate a relocated seismicity catalog and focal mechanism solutions in our analysis. Our model reveals that along strike, the system changes from a structural wedge to a composite growth fault-bend fold. At depth, these fault segments sole into a larger blind fault system that underlies the Temblor Range. Additionally, where the structural styles change along strike, we observe distinct variations in the eastern extent of deformation in the basin. Specifically, those segments characterized by fault-bend folding send slip into the basin on an upper detachment, which is manifest in both detachment folds and additional thrust ramps. Near-surface well data and portions of our seismic data confirm that the kinematics of the deeper structure of this system persisted through the Pleistocene and Holocene. We suggest that future earthquakes may not be limited to the fault segments that ruptured in the 1980s, but may involve detachment and other thrust ramps in the Temblor Range and San Joaquin basin. Such multi-segment events in the broader San Joaquin Fold and Thrust Belt would pose considerable seismic hazards.