A New 3D Fault Model of the Southern San Joaquin Fold and Thrust Belt, California, and its implications for Regional Earthquake Hazards

Robert M. Welch, John H. Shaw, Chris Anthonissen, James F. Dolan, & Benjamin Kargère

Submitted September 8, 2024, SCEC Contribution #13651, 2024 SCEC Annual Meeting Poster #092

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 the underlying 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 within the Temblor Range. This model is based on 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, CPT data, as well as a relocated seismicity catalog and focal mechanism solutions. To constrain activity in the Pleistocene and Holocene section, we leverage our near-surface well data and portions of our seismic data. Our model reveals that along strike, the system changes from a structural wedge to a composite growth fault-bend fold. Additionally, where the structural styles change along strike, we observe distinct variations in the easternmost 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 that have been active through the Late Pleistocene. At depth, these fault segments sole to two major basal detachment levels that extend west beneath the Temblor Ranges. Folding of Pleistocene strata along axial surfaces that define the transition from ramps to detachments suggests that past earthquakes were not limited to the fault segments that ruptured in the 1980s but rather may have involved detachments and other thrust ramps in the Temblor Range and San Joaquin basin. Such multi-segment/multi-fault events in the broader San Joaquin Fold and Thrust Belt would pose considerable seismic hazards.

Key Words
Faulting, Active Folding, Thrust-Fault

Citation
Welch, R. M., Shaw, J. H., Anthonissen, C., Dolan, J. F., & Kargère, B. (2024, 09). A New 3D Fault Model of the Southern San Joaquin Fold and Thrust Belt, California, and its implications for Regional Earthquake Hazards. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology