SCEC Award Number 14049 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title A High Resolution Lake Cahuilla Chronology to Constrain Earthquakes on the Southern San Andreas System
Investigator(s)
Name Organization
Thomas Rockwell San Diego State University Ray Weldon University of Oregon
Other Participants Graduate student Andy Jerrett
SCEC Priorities 1a, 2a, 4a SCEC Groups Geology, SoSAFE, EFP
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
 This project is to continue to improve the age control for Lake Cahuilla sediments and develop a cheap and efficient method for correlating individual lakes across the Salton Depression for the purpose of placing the past 1500 years of earthquakes in the southern San Andreas Fault system into a common chronology. Efforts focused on three areas: dating of in situ stumps that indicate when a lake was absent, direct dating of fossil mollusks, and further analysis of Melanoides as a correlation index fossil. Dating of stumps indicates that the most recent full lake occurred in the early to mid AD 1720’s. The penultimate lake likely dates to the period between AD 1640 and 1655, with a best estimate of AD 1650. The average δ18O/δ13C ratios for Melanoides samples from Carrizo Wash (Car) and Coachella (CH) for the past six lakes are shown in the associated figure. With the exception of two outlier groups, the various lakes appear to group very well. Of importance in paleoseismic studies is that some of the lakes appear to be easily distinguished. The outliers are from the very first set of data that were analyzed, and they are so far removed from the rest of the data that we question whether the results are valid, so more work is planned. If additional results are consistent with the new data, it appears that isotopic ratios may well be useful in distinguishing and correlating individual lake stands, which will be important for paleoseismic studies in the basin.
Intellectual Merit This project will lead to much greater precision in dating earthquakes of the southern San Andreas System in the Salton Trough, which will better constrain long-term earthquake recurrence patterns and statistics, and potentially allow for sequencing of the past 1500 years of large earthquakes at a systems-level approach.
Broader Impacts This project has supported two MS students (one male, one female), and was used as a teaching laboratory for a graduate class at SDSU (Geol 633). The class involved multiple field trips to Lake Cahuilla: students collected samples to be analyzed, and one student was directly involved in the analyses.
Exemplary Figure Figure 4. Average isotopic ratios determined for samples of Melanoides sp., a small, high-spired gastropod found with Lake Cahuilla sediments at many localities. L1 corresponds to Lake 1, L2 to Lake 2, etc. CH are samples from the Coachella site of Philibosian et al. (2011), whereas Car samples are collected from Carrizo Wash.