Overview

Precariously balanced rocks (PBRs) – a subtype of fragile geologic features (FGFs) – are naturally occurring geological features that could be easily toppled by strong earthquake shaking. They provide several forms of information valuable for understanding seismic hazard. In the specific regions where PBRs occur, they provide direct information about strong ground motions not exceeded during their lifetimes. For many PBRs this time is at least thousands of years – much longer than any instrumental record at a point. Their more general application is to provide useful limits on ground motion variability. For example, PBRs near high slip rate faults can be used to evaluate the variability of ground motion from earthquake to earthquake. Near low slip rate faults, PBRs can provide limits on past earthquake size and frequency. Far from active faults, PBRs can contribute to models of background seismicity and spatial smoothing for hazard map applications.

Specific applications of PBR in southern California include evaluation of the National Seismic Hazard Map (Grant-Ludwig et al., 2015), testing the ergodic assumption in ground motion prediction applications (Anderson and Brune, 1999), testing ground motion shaking intensities near fault step overs (Lozos et al., 2015), and calibration of synthetic seismograms for a magnitude and distance range not well represented by strong motion data (e.g., Anderson et al., 2017). PBRs have also been applied outside of Southern California to constrain seismic hazard estimates on the central California coast (Stirling et al. 2016) and Clyde Dam in New Zealand  (Stirling et al. 2015), and at Yucca Mountain in southern Nevada to provide "point-in-hazard-space" constraints on likely unexceeded limits of ground motion intensity (SCEC Extreme Ground Motion Project).

Research Priorities

The PBR archive and ongoing PBR research support SCEC goals for modeling earthquake ground motions and understanding California faults:

PBR Data Archive

Archive of Precarious Rock Photos and Metadata

Locations and photographs of precarious rocks and fragile geological features have been archived for access and use by SCEC researchers. The archive currently includes over 1900 located objects, a few hundred of which are PBRs. Each object has a permanent identifier called a "rockid". Every object has a short metadata summary table and one or more accompanying photographs. Photos are linked in the table, so views associated with fragility measurements are readily accessed. Location and photographic documentation quality varies over time, reflecting the evolutionary nature of the project.  Planned improvements include new PBRs, improved photos for selected existing entries, and improved representation of contributions from New Zealand and elsewhere.

Rock toppling is governed by the angles from vertical to the center of mass, and by rock size (Purvance et al., 2008; Anderson et al., 2014). Measurements of these properties has been done by many methods, including photo-modeling of the rock and pedestal, field testing, professional estimation from multiple photos, and by 2-D methods from photographs. Rocks can have more than one measurement. Where they do, the rock metadata table indicates the measurement considered most reliable. Summary data from the PBR archive include .csv format tables of all located objects, and subsets with "best" measurements. Improvements are pending for some rock location and photomodeling data. Please let us know if you spot errors or see opportunities for improvement.

Few PBRs have measured ages. Approximate ages have been developed using cosmogenic, U-series, varnish layering, or quantitative geomorphic methods. Pedestal height has been suggested by Brune et al. (2012) as a proxy age based on the time required to lower a landscape around the rock. Pedestal height has been estimated for all PBRs and is included in the metadata tables. Fragilities of rocks with pedestals greater than 50 cm have been found to be more consistent with Ground Motion Prediction Equations (GMPEs) (Biasi and Anderson, 2014).

Use of the PBR Data Archive

Use of the PBR archive is encouraged and open to researchers of the SCEC community who have an appreciation of their unique nature and a legitimate working use for the data. PBRs are subject to vandalism, and sadly, some with published locations have been toppled using jacks and timbers. Researchers are asked to provide their name, institution, and a brief description of their application(s). Requests will be reviewed and responded to within 3-5 business days.

GO TO PBR ARCHIVE Request access

Select Publications

Background

• Grant Ludwig, L., Brune, J. N., Anooshehpoor, A., Purvance, M. D., Brune, R. J., and Lozos, J. C. (2015) Reconciling precariously balanced rocks (PBRs) with large earthquakes on the San Andreas fault system, Seismological Research Letters, 85, 1345-1353. SCEC Contribution 1545
• Anderson, J. G., Biasi, G. P., & Brune, J. N. (2014). Precarious rocks: providing upper limits on past ground shaking from earthquakes. In Wyss, M. (Ed.), Earthquake Hazard, Risk and Disasters, Elsevier, Inc., (pp. 377-403). SCEC Contribution 1910
• Biasi, G. P. and J. G. Anderson (2014). Empirical hazard curve assessment using the precarious rock archive, Final Technical Report, Award G12AP2086, USGS National Earthquake Hazard Reduction Plan External Grant. NEHRP_FTR.
• Anderson, J.G., Brune, J.N., Biasi, G.P., Anooshehpoor, A., and Purvance, P. (2011). Workshop Report: Applications of Precarious Rocks and Related Fragile Geological Features to U.S. National Hazard Maps, Seismological Research Letters, v. 82, no. 3, p. 431-441, doi:10.1785/gssrl.82.3.431.

Related Publications

• Anderson, J. G., Brune, R. J., Brune, J. N., & Biasi, G. P. (2017, 01). Wave Propagation and Source Models Compatible with Strong Motion Applications. Oral Presentation at 16 World Conference on Earthquake Engineering. SCEC Contribution 6266
• Lozos, J. C., Olsen, K. B., Brune, J. N., Takedatsu, R., Brune, R. J., & Oglesby, D. D. (2015). Broadband Ground Motions from Dynamic Models of Rupture on the Northern San Jacinto Fault, and Comparison with Precariously Balanced Rocks. Bulletin of the Seismological Society of America, 105(4), 1947-1960. doi: 10.1785/0120140328. SCEC Contribution 6072
• Brune, R. J., L. Grant-Ludwig, K. Kendrick, and J. N. Brune (2012). Geomorphic erosional models for estimating ages of precariously balanced rocks from cosmogenic isotope data, Proceedings and Abstracts 2012 SCEC Annual Meeting, 93.
• Veeraraghavan, S. and S. Krishnan (2012). 3-D dynamic analysis of precariously balanced rocks under earthquake excitation, 15th World Conference in Earthquake Engineering, Lisbon, 2012.
• Balco, G. A., Purvance, M. D., & Rood, D. H. (2011). Exposure dating of precariously balanced rocks. Quaternary Geochronology, 6, 295-303. doi: 10.1016/j.quageo.2011.03.007. SCEC Contribution 1480
• Purvance, M. D., A. Anooshehpoor, and J. N. Brune (2008a). Freestanding block overturning fragilities: numerical simulations and experimental validation, Earthq. Eng. Struct. Dyn. 37, 791–808. SCEC Contribution 1038
• Stirling, M.W.; Anooshehpoor, R. 2006 Constraints on probabilistic seismic-hazard models from unstable landform features in New Zealand. Bulletin of the Seismological Society of America, 96(2): 404-414.
• Stirling, M. W., Anooshehpoor, A., Brune, J. N., Biasi, G. P., & Wesnousky, S. G. (2002). Assessment of the Site Conditions of Precariously Balanced Rocks in the Mojave Desert, Southern California. Bulletin of the Seismological Society of America, 92(6), 2139-2144. SCEC Contribution 694
• Imbler, S (2020). Why Scientists Fall for Precariously Balanced Rocks, Atlas Obscura, https://www.atlasobscura.com/articles/precariously-balanced-rocks.