SCEC Award Number 18143 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Resolving the late Quaternary slip rate of the Santa Susana fault with in situ cosmogenic 36Cl profile dating
Investigator(s)
Name Organization
Reed Burgette New Mexico State University Nathaniel Lifton Purdue University Katherine Scharer United States Geological Survey
Other Participants Michael Reed, NMSU M.S. student
Incoming Purdue graduate student
Devin McPhillips, USGS Pasadena
SCEC Priorities 1a, 5b, 2e SCEC Groups Geology, SAFS, WGCEP
Report Due Date 03/15/2019 Date Report Submitted 11/15/2019
Project Abstract
 The Santa Susana fault (SSF), located in the Western Transverse Ranges, is a 38-km-long active thrust fault. The trace of the SSF is near the northern margin of the Los Angeles metropolitan area and connects other active faults along the Ventura and San Fernando basins. The reported slip rates for the SSF in the UCERF-3 earthquake hazard forecast range from 10 mm/yr as determined from balanced cross-sections to as low as 0.5 mm/yr based on a subdued geomorphic expression. We evaluated the SSF slip rate in the late Quaternary using geologic mapping, 36Cl cosmogenic nuclide depth profile dating, and analysis of lidar topography. Results from two depth profiles show the downward decreasing cosmogenic nuclide concentrations expected for a stable surface. Preliminary analysis of the data suggests the offset surface is between ~12 and 30 ka in age, consistent with a late Quaternary slip rate in the lower portion of the range currently considered in hazard forecasts. Further work in progress will provide better resolved ages from the depth profile data.
Intellectual Merit This project has generated new geologic observations and geochronologic data that will better resolve the slip history on the Santa Susana fault in the late Quaternary. The preliminary slip rate suggests that the deformation rate of the SSF has slowed, and other structures in the western Transverse Ranges are accommodating more of the contraction budget. This project has also produced new age information for the timing of aggradation and incision of geomorphic surfaces in the late Quaternary that can be used regionally for assessing deformation. The new 36Cl cosmogenic nuclide data demonstrate the power of this nuclide to date surfaces underlain by sediment that would not be possible to date with more commonly used nuclides like 10Be. Analysis of the 36Cl data from this study has prompted development of new code that will useful for similar data collected elsewhere.
Broader Impacts This project has supported an M.S. student at New Mexico State University. He was able to travel to the PRIME Lab at Purdue University and conduct the chemical sample preparation for this study. This opportunity provided valuable experience, and strengthening ties between two universities and the USGS in southern California. The data generated from this study better resolve the late Quaternary slip rate for a fault with a previously poorly-constrained slip rate, which will improve future seismic hazard forecasts, and ultimately contribute to mitigation of earthquake hazard in one of the most populous areas of the country.
Exemplary Figure Figure 2. Detailed map of the study area along the western Santa Susana fault. The two depth profile samples collected and analyzed for this project come from both the hanging wall and footwall portions of the Qoafs surface. The black rectangle indicates the extent of the swath profile used for slip estimation. Geologic mapping modified from previous work (Yeats, 1987; Dibblee and Ehrenspeck, 1992; Campbell et al., 2014)using our observations. Shaded relief basemap is derived from a 1 m resolution DEM (Ventura County, CA).