- © 2015 by the Seismological Society of America
Online Material: Photos of precariously balance rocks (PBRs) with a regional location map, illustrations of measuring PBR fragility, and measured parameters of PBR fragility.
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; Andersen 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).
Brune and colleagues have employed multiple methods to survey, identify, and catalog the locations of PBRs in corestone‐producing granitoid outcrops in arid and semiarid areas of southern California that could be subjected to strong ground motion from known active faults, as well as areas that are more than 50 km from …