SCEC Award Number 13069 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title PBR Studies Important for Testing Cybershake, NGA and Hazard Maps
Name Organization
Lisa Grant Ludwig University of California, Irvine Thomas Jordan University of Southern California
Other Participants James Brune, Richard Brune, Jessica Donovan
SCEC Priorities 6, 6, 6 SCEC Groups GMP, Geology, GMSV
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
We have been conducting research to develop, refine, and implement the use of precariously balanced rocks (PBRs) for validation of ground motion studies and seismic hazard analysis. Currently, the only tool available to empirically test unexceeded earthquake ground motions over timescales of 10 ky-1 My is the use of fragile geologic features, including PBRs. The age of PBRs together with their aerial distribution and mechanical stability (“fragility”) provide constraints for probabilistic seismic hazard analysis (PSHA) over long timescales, including USGS National Seismic Hazard Maps (NSHM), and validation of ground motion models such as Cybershake. SCEC has previously supported research to identify and document locations of PBRs in S. California, and to develop methods of analyzing fragility and estimating ages. A database at UNR contains locations of thousands of PBRs in S. California. With prior funding, we identified important PBRs, collected samples for a feasibility dating study, and used the samples to develop a numerical modeling method of dating PBRs in the granitic terrains of southern California with cosmogenic 10Be. In FY2013, we focused on measuring fragility of PBRs that are important for validation of Cybershake calculations. We used photogrammetry to construct 3D models of 17 PBRs at 16 distinct sites, and measured fragility parameters such as rocking angles and azimuths, to derive quasi-static toppling acceleration and toppling direction at each location. Fragilities range from 0.1 g to > 0.5 g.
Intellectual Merit Earthquake rupture forecasting methodology has improved significantly in recent years (e.g. WGCEP, 2008, 2013). Concurrently, the numerical power for calculating seismic ground motion, based on various types of modeling and assumed input parameters, has also improved. The weak link in proceeding to estimates of seismic hazard is validation of inputs and modeling procedures. Current models require assumptions about source parameters (e.g., rupture rise time, slip weakening distance, rupture velocity, direction of rupture, background stress, frictional stress, dynamic stress history) but there are not enough near-source data from large earthquakes to validate or constrain the programs and assumed source parameterizations. The study of precariously balanced rocks (PBRs) provides critical insights. Analysis of apparent discrepancies between PBRs and seismic hazard maps (Brune et al., 2010a, 2010b, 2011), suggests that PBRs may be important in testing the ergodic assumption, attenuation relationships, random background earthquake assumptions, directions of rupture propagation (Weiser et al., 2007), relative hanging wall-foot wall ground motions, step-over ground motions, frequency of supersonic ruptures, and other assumptions such as fault activity and fault geometry (Anderson et al., 2011). For example, orientations of PBRs between the Elsinore an San Jacinto faults suggest supersonic rupture velocities (Brune et al., 2006 ). PBRs at Silverwood Lake and Grass valley in the San Bernardino mountains, seem incompatible with the NW to SE rupture commonly assumed for the 1857 Fort Tejon earthquake, raise questions about whether ruptures have propagated between the San Andreas and San Jacinto fault in Cajon Pass, and suggest that the Cleghorn fault and Pinto Mountain faults are not as active as assumed in UCERF2 (Schlom et al., 2007; Grant Ludwig and Brune, 2010; Brune et al., 2010a).
Broader Impacts Seismic hazard maps are essential tools for regional planning and development of earthquake resilient infrastructure in seismically active regions. Seismic hazard maps are derived from two basic components: fault rupture probability and ground motion probability. Precariously balanced rocks (PBRs) provide important insights and data for each component. PBRs provide the only source of data for constraining maximum ground motions over long periods of time needed for seismic hazard assessment. By providing constraints on maximum ground motions over hundreds to thousands of years, PBR data also provide implicit constraints on rupture probability of nearby faults. Thus PBR data are important for assessing seismic hazard and designing earthquake-safe buildings.
Exemplary Figure Figure 3. Photo by R. Brune.