SCEC Award Number 13039 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: Long term slip-rate of the Banning strand of the southern San Andreas Fault at Devers Hill, request for continued funding
Name Organization
Whitney Behr University of Texas at Austin Tom Rockwell San Diego State University Dylan Rood University of California, Santa Barbara Warren Sharp Berkeley Geochronology Center
Other Participants Collaborator: Katherine Kendrick (USGS)
Ph.D. student: Peter Gold (UT Austin)
SCEC Priorities 1, 4, 3 SCEC Groups SoSAFE, Geology, Seismology
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
The purpose of this project is to measure the Holocene to Late Pleistocene geologic slip rate of the Banning strand of the southern San Andreas Fault. Toward accomplishing this goal we have completed geomorphic mapping of distinct alluvial fan units that have been offset or truncated by the Banning Fault at its northwest end where it enters San Gorgonio Pass. Our initial 13 10Be cosmogenic exposure ages from the most prominent of these surfaces reveal a complex depositional/erosional history that precludes their use for a slip rate measurement. However, these measurements constitute an important contribution to a regional catalog of quantitatively dated alluvial fans and will form part of a comparative study of common geochronologic methods used in dating Quaternary alluvial features. Fortuitously, more detailed airborne lidar analysis and observations of grain-size distributions have revealed two additional offset fan surfaces that we are currently in the processes of dating, also with cosmogenic exposure methods. Four of these dates from one fan have returned scattered ages, while 8 more are in the process of AMS analysis. 26 additional samples from boulders, cobbles and depth profiles were in the process of being prepared for 10Be cosmogenic exposure dating at the time of submission of this progress report. These additional ages should yield a spatially dense and statistically significant number of ages with which to constrain Holocene and Late Pleistocene slip rates along the Banning Fault.
Intellectual Merit The complex faulting that defines the northern Coachella Valley and the San Bernardino Mountains also characterizes much of the Pacific-North American plate boundary, where in many places relative motion is accommodated by multiple kinematically diverse overlapping or parallel faults. Determining the spatial and temporal scales over which slip is partitioned between multiple active faults is key to understanding how complex fault systems evolve as they accommodate strain, which in turn may aid our understanding of fault structure in the upper crust. Constructing fault slip histories with which to investigate fault interaction requires fault slip rates over multiple time scales to complement Holocene paleoseismic data. This intellectual merit of this study derives from its contribution to a broader investigation of slip partitioning, complex fault zone evolution and deep fault structure in the complex San Gorgonio Pass-San Bernardino Mountains area.
Broader Impacts The most immediate impact of the Holocene slip rate we will measure for the NW end of the Banning fault is that it will allow us to directly test one of the central assumptions of the UCERF seismic hazard model: that an earthquake on southern San Andreas is most likely to propagate north along the Banning Fault into San Gorgonio Pass (Field et al., 2009). This first constraint on the proportion of slip that actually follows this rupture path has direct implications for seismic hazard in the Los Angeles Basin. In addition, a better understanding of long-term fault interaction and fault zone evolution provides context within which to interpret evidence of more recent ruptures, and thus is useful in estimating seismic hazard. This research is also supporting two early-career researchers (Behr, Rood), a Ph.D. student (Gold) and has involved an undergraduate research assistant (Salin).
Exemplary Figure Figure 2 from project report.

Figure 2 Rupture path options for an earthquake nucleated on the southern San Andreas fault (sSAF). A) The rupture follows the Banning Fault to San Gorgonio Pass, as current hazard model predict. B) The rupture follows the Mission Creek Fault, and most of the time is transferred north to the Eastern California Shear Zone. Geologic slip rates from both faults are needed to distinguish between these options.