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Reconciling precariously balanced rocks with large earthquakes on the San Andreas fault system

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

Published 2015, SCEC Contribution #1545

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 are 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 USA and New Zealand (Anderson et al., 2011; Stirling and Anooshehpoor, 2006). 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 et al., 2008a; Andersen et al., 2011, 2014). Neither PBRs, nor paleoseismic data, nor historic damage reports allow unique inversion for discerning patterns of fault rupture and possible earthquake triggering, but together they can be used to identify the most likely scenarios among infinite possibilities. Since the recurrence interval for fault ruptures exceed the modern instrumental period, there are few options for understanding fault behavior over multiple seismic cycles prior to the instrumental period. Longstanding practice has been to evaluate rupture potential for individual faults in isolation, even though it is clear that earthquakes can be triggered by movement of nearby faults, and thus faults are not isolated. The only way we can reconcile PBRs with large earthquakes on the San Andreas fault, is to consider the SAF and its branches, such as the SJF, as a fault system, and that ground motions from large surface rupturing earthquakes on that system are low enough to allow the PBRs survival. PBRs, together with other observations, suggest that complex ruptures and low ground motions are viable characteristics of the San Andreas / San Jacinto fault system.

Grant Ludwig, L. B., Brune, J. N., Anooshehpoor, A., Purvance, M. D., Brune, R. J., & Lozos, J. C. (2015). Reconciling precariously balanced rocks with large earthquakes on the San Andreas fault system. Seismological Research Letters, 86(5), 1345-1353.

Related Projects & Working Groups
Earthquake Geology, Ground Motion Prediction