SCEC Award Number 20085 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title The late Pleistocene slip rate on the Mid-Channel thrust and its implications for large, multi-segment earthhquakes on the Ventura-Pitas Point fault system
Investigator(s)
Name Organization
John Shaw Harvard University
Other Participants Andreas Plesch
Jessica Don
SCEC Priorities 1a, 3a, 2e SCEC Groups Geology, CXM, Seismology
Report Due Date 03/15/2021 Date Report Submitted 03/11/2021
Project Abstract
 We define the Late Pleistocene slip rate on the Mid-Channel blind thrust in the Santa Barbara Channel, California. This fault 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 Mid-Channel fault and define its lateral extent and 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. Using a new method of relating structural relief of these horizons to slip on the underlying Mid-Channel thrust, 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 explore alternative ways that the fault may extend to depth and interact with surrounding structures and explore the seismic hazard implications of these scenarios.
Intellectual Merit 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.
Broader Impacts The new fault representation will be 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: (top) 3-D seismic image of the Mid-Channel fault and anticline showing stratigraphic correlations of late Pleistocene horizons. The fault steeps from a lower (S1) to upper segment (S3), with its displacement consumed by the overlying fault-propagation fold. Location in Figure 1. (bottom) Plot of slip versus age for the Mid-Channel fault. Slip is calculated from uplift of stratigraphic horizons using the method described in Don et al., (2020). Points that show similar slip are pre-tectonic units. Points showing increasing slip with age are syntectonic (growth) units. The slope of the line through these points is the average fault slip rate.
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