Anomalous Uplift at Pitas Point and the 2019 Ventura River Earthquake Swarm: Whose Fault is it Anyway?

Abstract

Based on four Recent ~8-m uplift events at Pitas Point, many believe these represent earthquakes near M8 on the N-dipping Pitas Point-Ventura fault (PPVF), part of the larger primarily offshore North Channel-Pitas Point-Red Mountain fault system. However, this model of multiple Holocene M8 events on the PPVF has major problems, not the least of which are: failure of the 2D fold model used to properly infer subsurface 3D fault geometry, an implied Holocene slip rate for the blind PPVF that is inconsistent with observations, and the marked lack of seafloor offset or widespread tsunami deposits expected from such shallow M8 events that would extend offshore. The reason for these discrepancies may be that uplift at Pitas Point is driven primarily by slip on the S-dipping, listric Padre Juan fault (PJF), not the PPVF. The PJF juxtaposes the strongly N-verging San Miguelito anticline in its hanging wall above the more symmetric Ventura Avenue-Rincon anticline in its footwall. Fault and fold geometry are well determined by industry wells that produce from the distinctly different San Miguelito and Rincon oilfields, and by imaging offshore with seismic reflection data. The PJF exhibits up to 2.6 km of dip separation, in contrast to ~200 m of inferred dip separation on the Ventura fault at similar depths. The timing and slip involved for San Miguelito fold growth, and the specifics of 3D fault geometry require that much of this fault slip occurred while the PJF acted independently and syntectonically with growth of the Rincon anticline, as PJF splays are folded by this lower fold. Relocation of the 2019 Ventura River earthquake swarm indicates most events occurred on a tear fault below the PPVF, but at or above the PJF, while deep focal mechanisms also indicate slip on a separate S-dipping nodal plane, suggesting the PJF is still Holocene active. The PJF soles into the weak Rincon Shale at a depth of ~7 km and may thus represent a classic out-of-syncline thrust. Such faults are known to generate anomalously large slip for their size and may explain why the uplift at Pitas Point is so anomalous, localized, and not necessarily indicative of the average slip at depth during past large earthquakes. Rather, this uplift at Pitas Point is probably localized to where slip on the PJF predominates, where the PJF and PPVF strongly interact, or where the tear fault concentrates strain, which limits the possible seismic & tsunami potential of the active fault(s) involved.

AAPG Datapages/Search and Discovery Article #90372 © 2020 AAPG Pacific Section Convention, 2020 Vision: Producing the Future, Mandalay Beach, Oxnard, CA, April 4-8, 2020 (Cancelled)