SCEC Award Number 09199 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Establishing a Chronostratigraphic Framework for Large-offset Faults Imaged in the Salton Sea: Unifying Onshore and Offshore Records of Lake Cahuilla Stratigraphy
Name Organization
Neal Driscoll University of California, San Diego Graham Kent University of Nevada, Reno
Other Participants
SCEC Priorities A1, A6, A9 SCEC Groups Geology, EFP, SoSAFE
Report Due Date 02/28/2010 Date Report Submitted N/A
Project Abstract
The importance of large magnitude ruptures within the Brawley Seismic Zone (BSZ), near the terminus of the southern San Andreas Fault (SAF), is underscored by (1) the extended period of dormancy of the southern SAF that is nearly twice as long as the average recurrence interval, and (2) recent numerical models that show enhanced shaking within the LA Basin for northward propagating ruptures along the southern SAF. These concerns bring into sharp focus the relationship between large BSZ ruptures and
the potential initiation of slip along the southern SAF. However, much of the BSZ is covered by the Salton Sea requiring a different set of tools to conduct paleoseismic studies. We have been using an arsenal of marine imaging techniques (i.e., seismic CHIRP profiling, side-scan imagery and high-resolution bathymetry) to constrain the fault architecture in the Salton Sea. A series of predominately NE-SW trending, dip-slip faults spanning the central and southern sea have been imaged and they provide a high fidelity record of rupture. To date, over 1000 line-km of CHIRP profiles have been collected throughout the sea, with some 700+ km of new profiles collected in October 2008. Funding sources for seismic acquisition and interpretation over the past 3 years includes: Scripps/UCSD (2007-8), SCEC (2008), and the Department of Water Resources (DWR), State of California (2008-09). High amplitude reflectors observed in seismic profiles correlate with coarse silty-sand layers deposited during low-stands of paleo-Lake Cahuilla. Information gleaned from past engineering studies (both CPT and standard cores) provides an ideal stratigraphic framework from which to compare our seismic results to on-shore trenching, where chronologic constraints for past lake-stands are more robust. The acoustic packages offshore correlate well with Cahuilla lake-stands observed at onshore trench sites. Unfortunately, the sediment cores collected by URS Corporation have either been destroyed or were poorly cataloged—so we are unable to test our offshore-onshore correlation with radiocarbon dating.
Intellectual Merit The importance of cross faults beneath the Salton Sea and their role in potential triggering scenarios with the southern San Andreas Fault highlight the need for better chronostratigraphy of Lake Cauhilla sediments. This project was able to collect nearly 2-m-long cores from the Salton Sea, thereby providing full sampling of the most recent lake stand, but also highlights why technologies beyond piston and/or gravity coring is needed to fully recovery the last 14 stands of Lake Cauhilla (i.e., Glad 800 drilling platform). Age-dating of these sediments is ongoing.
Broader Impacts Core sampling, splitting and XRF scanning of sediments therein have given graduate students at Scripps Institution of Oceanography an opportunity to learn these important techniques.
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