SCEC Award Number 16130 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Testing model predictions of large tsunamis associated with great earthquakes on the Pitas Point Thrust using ground-penetrating radar
Investigator(s)
Name Organization
Alexander Simms University of California, Santa Barbara
Other Participants Julie Zurbuchen, Regina DeWitt
SCEC Priorities 1a, 2a, 4a SCEC Groups Geology
Report Due Date 03/15/2017 Date Report Submitted 03/13/2017
Project Abstract
 The objective of this project is to determine if the sandy beaches of the Oxnard and Ventura coastal plains contain evidence of erosional scour from past tsunamis created by great earthquakes along the Pitas Point Thrust-Ventura Avenue Anticline (PPT-VAA). Published tsunami models suggest earthquakes along the PPT-VAA resulted in tsunamis as large as 6-9 meters. However, tsunami deposits have yet to be identified within the Santa Barbara Channel. Part of this conspicuous absence of deposits may be in part due to the few suitable archives of muddy marsh sediments. This proposal seeks to use a much more common archive along the Santa Barbara Channel, sandy beach deposits, to look for evidence of past tsunami scour. To date, we have collected over 12 km of ground-penetrating radar (GPR) data across the beaches of the Ventura and Oxnard plains looking for erosional scour and have found only 2 candidate tsunami surfaces. One is found in a dip-oriented GPR profile that appears to record a landward-shallowing scoured surface extending to the toe of a former beach face and the second is what appears to be a series of shore-normal return flow-channels of unknown origin. Age constraints for these features are lacking. Depending on the age of these features and the beach surveyed, our results may confirm studies from Carpinteria Slough suggesting that motion along the PPT-VAA did not produce as large of tsunamis as originally modeled and thus the fault-geometry or stress drops used in the original models should be reevaluated.
Intellectual Merit This grant directly addresses SCEC priorities by corroborating the record of large earthquakes on the Pitas Point Thrust, and by determining whether the southern California coast may be struck by locally-generated large tsunamis, as has been inferred to result from uplift events on the PPT-VAA during large earthquakes on the underlying PPT and reported in many modeling studies (Borrero et al., 2001; Ryan et al., 2015). The PPT is one of the largest faults in the Transverse Ranges, falling in the former Ventura Special Fault Study Area (VSFSA), and documenting evidence for tsunami deposits will demonstrate linkage of most elements of this major thrust system and provide a test for the opposing structural models of the PPT-VAA. In conjunction with the results of the Ryan et al. (2015) model, this work explicitly tests the Hubbard et al. (2014) structural model for the PPT-VAA. This work also contributes to a better understanding of how to identify tsunami deposits in areas without muddy marsh systems and may provide a template for searching for other locally-generated tsunamis in southern California.
Broader Impacts This grant supported the collection of field data for the PhD dissertation of Julie Zurbuchen. Six other graduate/undergraduate students also helped in the collection of the GPR data including three from groups traditionally underrepresented in STEM fields. The project also led to greater networking with individuals in two governmental agencies (USGS and Navy).
Exemplary Figure Figure 2. GPR profiles from Ventura-Oxnard illustrating A.) the two major facies assemblages within the Ventura-Oxnard Plain, B.) the potential tsunami erosional surface in a shore-normal profile as well as the other minor erosional surfaces, and C.) the potential tsunami erosional surface in a shore-parallel profile resembling return-flow channels.
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