SCEC Award Number 13008 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative research: Documentation of Tsunami Deposits in the Carpinteria Estuary: A signal of Great Earthquakes on the Pitas Point Thrust
Name Organization
Thomas Rockwell San Diego State University Robert Peters San Diego State University
Other Participants Bruce Jaffe and Bruce Richmond, both at the USGS
Graduate student assistant to be named
SCEC Priorities 1, 2, 4 SCEC Groups Geology, USR, WGCEP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
The primary objective of this work is to identify whether Carpinteria Salt Marsh contains tsunami deposits and experiences rapid subsidence events that may result from large earthquakes on the Pitas Point-Ventura thrust system. Towards that end, we recovered 12 new vibracores (4 m depth) and 7 longer Geoprobe cores (up to 15.8 m depth) to investigate the sedimentation history of the march. From 37 radiocarbon dates, a preliminary result is that the marsh is rapidly subsiding at 1.5 mm/yr. Preliminary analysis of the sedimentology suggests that aggradation proceeds until a marsh surface is established, followed by a major change in environment with a return to estuarine conditions, which we interpret as likely the result of rapid submergence. The accommodation space provided by this submergence is filled with relatively massive sand interpreted to be estuarine in nature, which is then capped by another paleo-marsh surface. There is evidence for three such submergence events in the past 6 ka or so, although more radiocarbon dates are required to more precisely date these events and determine whether they are coincident with the three uplift events at Pitas Point that have occurred in the same general timeframe. We also identify a tsunami-like sand that dates to the period between AD 1780 and 1870 and may result from the reported 1812 tsunami.
Intellectual Merit We are testing whether a number of local and regional observations on related structures all point to very large thrust events on the Pitas Point – Ventura thrust system. Understanding the linkage between coastal uplift along the Ventura coast, offshore folding as imaged in seismic reflection profiles, motion on the Rincon Creek back-thrust, rapid subsidence in Carpinteria, and the generation and preservation of tsunami deposits in Carpinteria Salt Marsh is critical to the correct assessment of seismic hazard in the Western Transverse Ranges, and towards understanding how individual structures link up and contribute to the development of M8-class earthquakes. The recognition that M8-class earthquakes may occur in the Western Transverse Ranges is a major shift in paradigm, with previous studies typically looking at individual faults as individual seismic sources.
Broader Impacts By joining forces with Alex Simms, an untenured Assistant Professor at UCSB, the project goals will now be a major focus of UCSB PhD student Laura Reynolds. The fieldwork has been accomplished by a joint effort of SDSU and UCSB graduate students. Also by joining forces and buying drilling supplies for Simms’ Geoprobe drilling rig, we have been able to collect far more core than if we had sub-contracted the coring to a commercial firm, and we were able to enhance the infrastructural aspects of Simms' drilling capacity by purchasing necessary supplies that will allow for deeper drilling. A fundamental impact to society is the recognition that the Santa Barbara coast may be struck with a tsunami within minutes of its generation. Perhaps more disturbing is that the City of Carpinteria may subside by as much as 1-2 m in the next event, placing much of downtown Carpinteria in the surf zone. This is a major unrecognized hazard to that coastal city.
Exemplary Figure Figure 3. Isopach with core locations (A) and core photograph (B) of the proposed 1812 tsunami deposit.