Reconciling Precariously Balanced Rocks (PBRs) with Large Earthquakes on the San Andreas Fault System

Lisa Grant Ludwig, James N. Brune, Rasool Anooshehpoor, Matthew D. Purvance, Richard J. Brune, & Julian C. Lozos

Published September 10, 2015, SCEC Contribution #10140

A major goal of seismology is to predict earthquake shaking (Frankel, 1999). Earthquakes are generated by rupture of faults, but it is difficult to predict the endpoints of fault rupture (Wesnousky, 2006) and the ground motions that will likely be produced. Precariously balanced rocks (PBRs) are a class of fragile landforms that are susceptible to toppling by earthquakes (Brune, 1996). PBRs have been reported to exist in many areas, including seismically active regions of the United States and New Zealand (Stirling and Anooshehpoor, 2006; Anderson et al., 2011). The population of PBRs is continuously renewed through exhumation and erosion (Brune et al., 2007), such that the absence of PBRs in appropriate rock types and weathering conditions near active faults implies that prior earthquake ground motions have been sufficiently strong to topple them (Brune, 2002). The existence of PBRs therefore implies that ground motions have not exceeded the fragility of the PBRs at the locations where they are found. Thus, the distribution of PBRs can be used to set bounds on the intensity of seismic shaking integrated over the age, or time period since the rocks became precariously balanced (Brune, 1996; Purvance, Brune, et al., 2008; Anderson et al., 2011, 2014). Many PBRs in southern California have been stable for over 10,000 years (Bell et al., 1998; Balco et al., 2011).

Grant Ludwig, L., Brune, J. N., Anooshehpoor, R., Purvance, M. D., Brune, R. J., & Lozos, J. C. (2015). Reconciling Precariously Balanced Rocks (PBRs) with Large Earthquakes on the San Andreas Fault System. Seismological Research Letters, 86(5), 1345-1353. doi: 10.1785/0220140239.