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.
Intellectual Merit This project uses a novel approach to surface exposure dating using in situ cosmogenic nuclides to constrain ages of mid-late Quaternary (1,000,000 – 100,000 y.b.p.) surfaces in southern California. These surfaces are omnipresent along the southern San Andreas Fault system and Transverse Ranges, but our inability to accurately date them has hindered interpretation of long-term slip rates in southern California. Our research builds on advances in isochron burial dating with in situ cosmogenic 10Be and 26Al resulting from dramatically improved detection limits at PRIME Lab, pushing the potential lower age limits of the method to well below 100 ka. Lifton had recent success in dating a buried paleosol at Whitewater Wash to 390 ka with prior SCEC funding (Lifton et al., 2016) using this method. This suggested that a similar approach, applied to dating a potentially correlative surface offset by the Sierra Madre fault (SMF) on the southern boundary of the San Gabriel Mountains, could help constrain long-term slip rates on that feature.

This study focused on Gould Mesa - the oldest geomorphic surface in a large, well preserved flight of fan terraces in the Arroyo Seco area of Altadena/Pasadena. The Gould Mesa surface is capped by a thick, deep red soil, and is thought to be ~200-500 ka based on regional soil correlations. Wells in the footwall of the SMF in the vicinity of JPL constrain the vertical separation of the top of the Gould Mesa surface across the frontal strands of the fault zone to be ~260 m.

Unfortunately, the deposit lacked exposures of a buried paleosol that were appropriate for burial dating. We thus sampled cobbles from a range of lithologies in fluvial/debris flow deposits buried ca. 8 m below well-preserved portions of the upper Gould Mesa surface at two locations. The sampling strategy was based on the idea that the different lithologies would reflect differing hillslope/fluvial pre-burial exposure histories with enough spread in concentrations to yield a robust isochron. However, resulting concentrations of 10Be and 26Al were all too low for this purpose. To determine an age from these non-ideal results, we reasoned that the samples from each site exhibiting the lowest 10Be concentrations would be least likely to contain an inherited signal and would thus reflect production by the deeply penetrating muon portion of the cosmic-ray flux. Utilizing recent work by Lifton et al. (2014), Phillips et al. (2016), and Balco (2017) on muon production systematics we calculated exposure ages at 8 m depth – results from both sites overlapped at 1 sigma, and averaged 171±32 ka (including both production rate and AMS measurement uncertainties). Given the long penetration length of muons, ages derived using this approach should be relatively insensitive to erosion assumptions. This approach should be applicable to similar deposits elsewhere in southern California and other tectonically active environments.

When integrated with our revised interpretation of the displacement across the frontal strands of the SMF, we calculate a slip rate averaged over the period since the late-middle Pleistocene. The ca. 2 mm/yr slip rate is faster than that estimated for the same area over the latest Quaternary (Hanson et al., 2016). This result complements other SCEC-funded work to characterize fault slip histories and slip rates elsewhere in the metropolitan LA area, and is consistent with recent work suggesting faults in the LA basin have accelerated over a similar time span (Bergen et al., 2017).
Broader Impacts This project has involved 1 MS student from New Mexico State University. Moreover, this project has created a new collaboration between New Mexico State, Purdue University, and the USGS that will continue for many years. The ultimate product from this project will be a more accurate slip rate for a major fault bounding the LA metropolitan area. Our results will complement the work of other members of the SCEC community investigating the middle to late Quaternary temporal and spatial distribution of deformation and those modeling fault slip rates from geodetic data. Our results will contribute to hazard mitigation when incorporated into future seismic hazard analyses.
Exemplary Figure Figure 1. Geologic map of the Arroyo Seco area showing sampling locations. Geology modified from Crook et al. (1987) with our observations, draped over shaded relief derived from 0.5 m bare-earth USGS lidar data (Hanson et al., 2016). Well data in the footwall of the CSMF constrain the subsurface positions of the upper and bottom contacts of the Middle Pleistocene Gould Mesa Qal4 deposit (Crook et al., 1987).