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Gauging Earthquake Hazards with Precariously Balanced Rocks

Finding easily toppled boulders that are still standing provides a way to test models of seismic hazard

James Brune, Matthew Purvance, Abdolrasool Anooshehpoor

Figure 5. Most previous studies...Click to Enlarge Image


Science has so far failed to provide any way to predict earthquakes on a timescale of days, weeks or months. But geologists are able to determine, at least roughly, the probability that an earthquake of a certain size might strike a given area within, say, decades to centuries. Such predictions are useful for establishing seismic-safety requirements in building codes, for example. The problem is that there are few ways to test such evaluations of local seismic hazard. One novel system uses precariously balanced boulders. If one can be found, it serves as an indicator that an earthquake of sufficient size to topple the tippy rock has not occurred—at least not for a very long time. This commonsensical observation has blossomed into a quantitative technique for testing more conventional seismic-hazard assessments.

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