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The Tipping Point

To the Editors:

In "Gauging Earthquake Hazards with Precariously Balanced Rocks," by James N. Brune, Matthew D. Purvance and Abdolrasool Anooshepoor (January-February), I was surprised that the analysis did not include any "prior" environmental density of these easily toppled boulders. Wouldn't you draw different conclusions if someone were to inform you that 1,000 years ago, say, there had been millions of such formations in the area, so that the ones surviving today are the rare exception?

John Freidenfelds
Chester, NJ

Dr. Purvance responds:

Dr. Freidenfelds's question is one that we have been pondering for quite some time: Are these precariously balanced rocks remnants of some enormous original distribution? Should this be the case, we would expect to find some geologically similar areas with no earthquakes that are just littered with precarious rocks. The precarious rocks used in our analyses are primarily granite boulders formed as the soil horizon lowers, unlike the sedimentary hoodoos in Bryce Canyon Nation Park, which are formed by differential erosion (e.g., strong rock overlying weaker rock that is preferentially eaten away, leaving spires). Hoodoos can be distributed very densely indeed. To date, however, we have been unable to find sites with enormous numbers of granite precarious rocks.

We have found areas in the Mojave National Preserve in California that are very similar to our study sites near the San Andreas Fault, but where the earthquake hazard is very low. Preliminary reconnaissance surveys find that the spatial densities of difficult-to-topple precarious rocks are roughly the same for sites about 30 kilometers from the San Andreas Fault and in the Mojave National Preserve. On the other hand, our preliminary studies suggest that there are large numbers of rocks in the Mojave National Preserve that would be toppled by weak ground motions that undoubtedly have occurred thousands of times near the San Andreas Fault and are also absent from sites closer to the fault.

Uncertainties in the precarious-rock densities and in the completeness of our reconnaissance surveys for precarious rocks in general have led us towards finding upper bounds on ground shaking. This is accomplished by taking each rock as a piece of data unto itself, analyzing it with the knowledge that it alone has survived for the last 10,000 years without toppling. As we refine our surveys, we hope to discern the precarious rock distributions more clearly and incorporate them into our future analyses.

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