SCEC Award Number 11108 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title SoSAFE- San Jacinto: Chronologic support to refine the paleoseismic record for the northern San Jacinto fault zone at Mystic Lake.
Investigator(s)
Name Organization
Nate Onderdonk California State University, Long Beach Tom Rockwell San Diego State University Sally McGill California State University, San Bernardino
Other Participants Marliyani, Gayatri (SDSU grad student)
1 other graduate student
3 to 4 undergraduate students
SCEC Priorities A1, A9, C SCEC Groups SoSAFE, Geology, EFP
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
 We have investigated a new paleoseismic site located within a sag formed between two fault strands that pass through the eastern side of Mystic Lake in the San Jacinto Valley. Trenches excavated across the sag exposed faulted and folded lacutrine and alluvial strata that record at least 7 ground-rupturing earthquakes over the past 1600 years. Evidence for past surface deformation includes upward terminating faults with associated fissure fills, folding, angular unconformities, pinching of strata against a paleoscarp, and a stratigraphic record of repeated subsidence of the sag during earthquake events. The ages of the events are constrained by 55 radiocarbon dates determined from detrital charcoal. The recurrence interval for the past 7 events ranges from 159 to 210 years and the most recent event occurred sometime between AD 1738 and AD 1850 based on radiocarbon ages trimmed by historical data. Some of the event ages at Mystic Lake overlap in time with events recorded at Hog Lake on the Clark strand of the San Jacinto fault zone to the south, suggesting that these events may have jumped the San Jacinto Valley releasing step-over, or that events on one fault triggered closely timed events on the adjacent fault.
Intellectual Merit The project produced new paleoseismic, slip rate, and slip per event data from an area where none existed previously. Our new interpretations regarding the influence of fault zone complexity on rupture patterns contributes to an understanding of the southern California fault system and fault systems in general. This study also adds to the growing paleoseismic database in southern California, which will allow for better modeling and evaluations of the distribution of coseismic strain across the fault system both temporally and spatially.
Broader Impacts This project has benefited society by adding new data that will be used to make better evaluations of fault hazards in southern California. These data and our new interpretations have been disseminated through presentations and lectures to the general public, professional geologic societies, university geology departments, and through interviews with the media. Such presentations have raised awareness and increased public understanding of earthquake hazards in southern California. Several graduate and undergraduate students have been involved with the project, as well as SCEC interns (associated with collaborator Sally McGill). The results of this project have also been incorporated into classes taught at CSULB in lectures, labs, and field trips to the study sites.
Exemplary Figure Figure 1. Event history from the Mystic Lake Paleoseismic site.