SCEC Award Number 16066 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Testing for slip rate changes on the Sierra Madre fault
Name Organization
Reed Burgette New Mexico State University Nathaniel Lifton Purdue University Katherine Scharer United States Geological Survey
Other Participants Austin Hanson, NMSU
Incoming Purdue student
SCEC Priorities 1a, 4b, 4c SCEC Groups WGCEP, SoSAFE, Geology
Report Due Date 03/15/2017 Date Report Submitted 04/27/2017
Project Abstract
The Sierra Madre fault (SMF) system uplifts the San Gabriel Mountains along the northern LA metropolitan area. This project targets a prominent geomorphic surface (Gould Mesa), preserved locally on the hanging wall of the SMF. This surface is capped by a thick, deep red soil, and thought to be ca. 200-500 ka based on regional soil correlations. Revised interpretation of well data in the SMF footwall in the vicinity of JPL constrain the vertical separation of this geologic marker across the frontal strands of the fault zone to be ca. 260 m.

We attempted to date the upper part of the Gould Mesa alluvial deposit using in situ cosmogenic nuclide isochron burial dating. Two suites of quartz-rich cobbles from natural and road-cut exposures were collected from ca. 8 m below the upper surface of Gould Mesa and analyzed for 10Be and 26Al, but low nuclide concentrations prevented reliable isochron results. Instead we used the muon production dominant at those depths to estimate exposure ages for the deposits from the samples with the lowest 10Be concentrations, yielding a mean exposure age estimate of 171±32 ka. This reduces age uncertainties at this site by a factor of ≥2 compared to the current level used by UCERF3, and yields a slip rate of 2.16 +1.06/-0.55 mm/yr. This rate is higher than the rate our group has determined for the latest Quaternary, which may reflect recent earthquake recurrence behavior or secular transfer of strain rate southward to other structures in the LA basin.