SCEC Award Number 11070 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Evolution of precariously balanced rocks (PBRs) in the Mojave desert: Constraints on San Andreas fault rupture and hazard models
Investigator(s)
Name Organization
Lisa Grant Ludwig University of California, Irvine Dylan Rood University of California, Santa Barbara
Other Participants James Brune, Richard Brune
SCEC Priorities B4, A9, B2 SCEC Groups Geology, GMP, SHRA
Report Due Date 02/29/2012 Date Report Submitted N/A
Project Abstract
The primary goal of this study was to constrain the evolution (timing and formation rate) and comparative geomorphic development of precariously balanced rocks (PBRs) at two well-characterized, but contrasting, sites in the Mojave Desert: one in a region of low seismicity (Granite Pediment) and another (Lovejoy Buttes) with high hazard from numerous large (~M7.5) San Andreas fault (SAF) ruptures (Fig. 2). With 2007-2011 SCEC3 funding, we developed a robust method to date corestone PBRs in the granitic terrains of southern California using cosmogenic 10Be with a numerical model (Balco et al., 2011). We made significant progress in 2011 in the refinement and implementation of 1) photogrammetry and shape modeling, 2) fragility modeling, and 3) numerical age model calculations. We have refined our dating method, and are applying it to numerous PBRs throughout the southern California, including measuring 10Be concentrations in 58 samples (out of 58 funded) from 7 corestone PBRs (Lovejoy Buttes, Pacifico, Beaumont South, Benton Road, Perris, Round Top, and Grass Valley sites) that provide constraints on ground motions associated with the San Andreas, San Jacinto, and Elsinore faults in southern California.
Intellectual Merit Results from studied PBRs (ages and fragilities) will directly test seismicity models, ground motion prediction equations, and hazard estimates associated with the 2008 USGS National Seismic Hazard Maps (NSHM) and Cybershake. We developed a robust method to date corestone PBRs in the granitic terrains of southern California using cosmogenic 10Be with a numerical model (Balco et al., 2011); our methods provide a tool to place quantitative constrains on ground motions associated with the San Andreas, San Jacinto, and Elsinore faults in southern California.
Broader Impacts SCEC3 funds in 2011 supported a graduate student (Jonathan Harvey, UCSB) and technician (Inyo Saleeby, UCSB), who assisted in the collection of field data, data processing, and interpretation (specifically, data collection of ground-based LiDAR, photogrammetry, DEM processing, and morphometric analysis). Both Rood (UCSB PI) and graduate student (Harvey) attended the SCEC Annual Meeting, where they presented posters (Rood et al., 2011; Harvey & Rood, 2011). Our SCEC3 2011 project also facilitated participation of a science journalist in field work, which resulted in an article published online about PBR research for earthquake hazards in Science News for Kids (http://www.sciencenewsforkids.org/2011/10/big-rocks’-balancing-acts/).

SCEC3 funds also facilitated critical knowledge transfer from experts in the fragility estimation methods (collaborators J. Brune, R. Anooshehpoor, and M. Purvance) to the PIs. Our SCEC3 work has also continued our collaboration with USGS collaborator T. Hanks and K. Kendrick to improve the utility of fragile geologic structures for PSHA at long timescales and earthquake hazard studies (e.g., Extreme Ground Motions project at Yucca Mountain).
Exemplary Figure Fig. 6 in progress report.