THE WILMINGTON ANTICLINE: AN ACTIVE FAULT-PROPAGATION FOLD WITH A MULTI-PHASE GROWTH HISTORY

We define the Wilmington anticline and underlying blind-thrust fault as a tectonically active structure capable of generating large, damaging earthquakes, through analysis of 2D and 3D seismic reflection surveys, petroleum and water wells, and recent mapping of groundwater aquifers in the southwestern Los Angeles basin. Our analysis suggests that the Wilmington anticline formed as a fault-propagation fold above the Wilmington Blind-Thrust fault. Based on geometric and kinematic modeling using Trishear, the fault underwent multiple stages of growth history, including a normal-slip phase in the Miocene and a reverse-slip phase in the Pliocene to recent times. This most recent phase of reverse motion includes a period of tectonic quiescence, in which the fault appears to have shut off for ~1.5 my. This is indicated by upper Pliocene-Pleistocene strata that maintain thickness across the forelimb of the Wilmington anticline, but are uplifted and folded by later fault slip. Kinematic modeling results suggest that total reverse slip on the fault is ~1575 meters; however, the basement shows only ~300 m of reverse vertical separation due to the earlier phase of normal separation. We also demonstrate a direct linkage between subsurface folding in the forelimb of the Wilmington anticline and deformation of shallow Pleistocene growth strata above the fault. This overturns the long-held view that the fault became dormant in the Late Pliocene, barring its inclusion in state-of-the-art regional earthquake hazard assessments. Our analysis of uplift of upper Pliocene to recent strata, including sequential backstripping and decompaction considerations, indicates that the dip slip rate is 0.14-0.18 mm yr-1, averaged over the most recent phase of growth on the fault. The size of the fault suggests that it can generate moderate-magnitude earthquakes (M 6.3-6.4), and potential linkages with other nearby faults (e.g., Huntington Beach, Torrance, Compton) pose the threat of larger, multi-segment events (M > 7). These earthquakes would directly impact the overlying Ports of Los Angeles and Long Beach, as well as the broader Los Angeles metropolitan area.