Workshop Report: Applications of Precarious Rocks and Related Fragile Geological Features to U.S. National Hazard Maps

The Workshop on the Applications of Precarious Rocks and Related Fragile Geological Features to U.S. National Hazard Maps met on the University of Nevada campus in Reno (UNR), Nevada, on 4–5 October 2010. The goal of this workshop was to develop recommendations to the U.S. Geological Survey (USGS) on the use of precariously balanced rocks and related fragile geological features to improve the national seismic hazard maps (NSHM). The context of the workshop is the growing realization that it is essential to regard hazard maps as the calculated output of hazard models that should be tested, and that fragile geological features provide the only data to validate the predictions of these models at low probabilities. A fragile geological feature (FGF) was defined as a feature that might be easily destroyed by strong earthquake ground motions and is mechanically simple enough to allow for analysis of the ground motions that might cause it to be destroyed. For testing the prediction of a seismic hazard model, an FGF must also have been in essentially the present geometry for enough time to constrain past ground motions. A precariously balanced rock (PBR) is the type example of an FGF. A PBR is a rock that is balanced on, but not mechanically attached to, its pedestal, and it has a high ratio of height to width of the contact with the pedestal, so that it is easily toppled. Brune and Whitney (1992) and Brune (1996) suggested that PBRs might serve as low-resolution seismoscopes to put upper bounds on ground motions over the lifetime of the rock. FGFs were recognized by Hanks et al. (2006) for their critical contribution to evaluation of extreme ground motions at Yucca Mountain. Because PBRs have been studied more than other FGFs, most attention at the workshop focused on their distribution …