Issues

The cover image shows a precariously balanced rock in northern Vermont, USA. The rock is an erratic set in place as the continental ice sheet retreated ∼13,000 years ago. Its survival in a precarious position can be used to estimate the maximum ground motions that could have occurred from nearby earthquakes during that time. Precariously balanced rocks and in general, other fragile geological features (FGFs), are useful negative indicators of strong ground motion. By evaluating the stability of FGF’s and determining their age, it is possible to constrain the local maximum ground acceleration that has occurred during their lifetime. The paper by McPhillips and Pratt in this issue (“Precariously Balanced Rocks in Northern New York and Vermont, U.S.A.: Ground-Motion Constraints and Implications for Fault Sources”) describes analyses of a number of such precarious rocks in the region to test assumptions in the U.S. Geological Survey’s National Seismic Hazard Model.

Image credit: Noah Lindberg/U.S. Geological Survey