Precarious Rock Evidence for Low Ground Shaking on the Footwall of Major Normal Faults

Abstract

Preliminary interpretations of precarious rock observations suggest relatively low near-fault (<3 km) footwall ground motions for normal faults relative to strike-slip faults, as reported for a dynamic foam rubber normal fault model by Brune and Anooshehpoor (1999). Since for large normal fault earthquakes there are no instrumental data at such close distances from the fault trace, precarious rocks may provide important data for estimating the ground motion for these earthquakes. Use of empirical curves in the literature, based mainly on strike-slip data, may lead to overestimates of seismic hazard for the near-fault footwall of normal faults. These preliminary results may warrant more comprehensive and more quantitative followup studies.