Anomalous Uplift at Pitas Point: Localized Phenomena or Indicator of Tsunamigenic M8 Earthquakes Along Coastal California?

Recently, some investigators have proposed that large earthquakes near M8 may have occurred in the western Transverse Ranges based on repeated 8-m uplift events of coastal marine terraces at Pitas Point near Ventura. Many believe the locus for these proposed events is 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, large Holocene events on the PPVF has some fundamental inconsistencies, not the least of which are: the appropriateness of the 2D fold model used to infer oblique subsurface 3D fault geometry, the implied Holocene slip rate for the blind PPVF, and the lack of seafloor offset or widespread tsunami deposits from such expected shallow M8 uplift events that would extend 10's of km offshore. The reason for these discrepancies may be that uplift at Pitas Point is primarily driven by slip on the S-dipping listric Padre Juan fault (PJF) [Grigsby, 1988], not the PPVF. The PJF juxtaposes the N-verging asymmetric San Miguelito anticline in its hanging wall above the more symmetric Ventura Avenue-Rincon anticline in its footwall. Fault and fold geometry is well determined by industry wells that produce from the distinctly different San Miguelito and Rincon oilfields, and by imaging offshore with seismic reflection data. In places, the PJF exhibits up to 2.6 km of dip separation, with much of this slip likely occurring in the last 200 kyr. This fault soles into the weak Rincon Shale at a depth of 7 km and thus represents a classic out-of-syncline thrust. Such faults are known to generate anomalously large slip for their size and may explain why the uplift rate at Pitas Point is so much higher than elsewhere along the coast. The presence of the localized listric PJF and San Miguelito anticline strongly suggests that the uplift events at Pitas Point are anomalous, and not necessarily indicative of the expected slip at depth either along strike of the PPVF, or even the average slip expected during large earthquakes that continue offshore. Rather, these uplift events at Pitas Point are probably localized to where slip on the PJF predominates, or where the PJF and PPVF strongly interact, which limits the length & depth of possible seismic ruptures and the geohazard & tsunami potential of the active fault(s) involved.