SCEC Award Number 21009 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Developing comprehensive alternative CFM representations of active thrust systems in the Santa Barbara Channel and Ventura basins, Western Transverse Ranges
Name Organization
John Shaw Harvard University
Other Participants Andreas Plesch
Jessica Don
SCEC Priorities 1a, 3a, 2e SCEC Groups Geology, CXM
Report Due Date 03/15/2022 Date Report Submitted 03/14/2022
Project Abstract
We developed a comprehensive 3D representation of the active blind-thrust and reverse fault systems in the Santa Barbara Channel and Ventura basin for inclusion in the Community Fault Model (CFM). In particular, emphasis has been placed on the Ventura-Pitas Point thrust system, and its interaction with the Red Mountain, San Cayetano, and other active structures in the southern Transverse Ranges. Our SCEC sponsored work in 2021 involved new analyses of geologic and geophysical datasets to refine our understanding of the 3D geometry and slip rates on these faults. We have integrated these results along with that of other SCEC investigators into a new, geometrically and kinematically viable 3D representation for the Ventura-Pitas Point, Midchannel, Red Mountain, Lion, Cayetano, and Southern San Cayetano faults. Collectively, these faults pose one of the largest deterministic seismic hazards in southern California, with some of the fastest Holocene uplift and slip rates in the Transverse Ranges, and evidence that they sourced large (M ≥ 7.5) paleoearthquakes. Our new fault model is currently under review as part of the CFM 5.3 evaluation, and based on the outcome of this assessment will be considered as the preferred or an alternative representation in CFM 6.0. This reflects the final culmination of our SCEC sponsored research on this fault system.
Intellectual Merit We define the Late Pleistocene slip rate on the Ventura-Pitas Point and Mid-Channel blind thrusts in the Santa Barbara Channel, California. This fault system is tectonically active and through its potential linkage with other large fault systems in the Transverse Ranges, including the Ventura-Pitas Point fault, poses significant earthquake (and potential tsunami) hazards to the coastal population of southern California. We use high resolution industry seismic reflection data to characterize the 3D geometry of the faults and define their interactions with other structures in the region. We use a detailed chronostratigraphic sequence of nine Pleistocene horizons ranging in age from 120-975ka to interpret the Mid-Channel anticline, which overlies the blind thrust. These precisely dated horizons provide unique constraints on the Pleistocene activity of the Mid-Channel fault because of their near complete preservation on the backlimb of the fold. We calculate the fault’s late Pleistocene slip rate of 2.1 ± 0.2mm/yr, suggesting that it may accommodate about one third of the observed geodetic contraction across the basin. Our analysis also suggests that the structure became active between 790 and 710ka in the eastern part of the Channel and subsequently propagated west along strike. We propose that this fault links directly with the mid-crustal detachment in the Ventura-Pitas Point fault system, which we have also mapped using extensive seismic reflection surveys (2- and 3-D) and well control. Linkage of these faults suggest that slip on the deep ramp shared by these structures is partitioned in the shallow crust to motion on the Ventura-Pitas Point and Mid-Channel blind thrusts. This explains the apparent westward decrease in slip from the Onshore Ventura to the Offshore Pitas Point fault segments, and suggest the possibility of large, multi-segment rupture scenarios on this fault system. Our work quantifies these earthquake scenarios including potential magnitudes and recurrence intervals.
Broader Impacts This research documents the activity and slip rate of a major blind thrust fault in southern California. Our results provide information about the fault, including its geometry and slip rate, that can be used to assess its earthquake hazards. Moreover, we developed a new method to calculate fault slip rate for blind thrust faults that that can be applied to study other earthquake sources in the region and worldwide.

The new fault representation is included in the SCEC Community Fault Model (CFM), thus contributing to improved regional earthquake hazards assessment. The project involved the training of one graduate and one undergraduate student.
Exemplary Figure Figure 3: 3D view of the Ventura-Pitas Point fault system. (A) View showing linkage of Venutra and Pitas Point faults with the ramp-flat-ramp geometry and the Mid-Channel fault extending south into the Santa Barbara Channel. (B) View showing linkage of the Ventura-Pitas Point fault system with other regional fault systems. Red Mountain, Lion/Southern San Cayetano backthrust, and San Cayetano faults after Hubbard et al. (2014). View is approximately to the east. Contour interval = 1km. California coast show in black.