Methodology for Using Precarious Rocks in Nevada to Test Seismic Hazard Models

John G. Anderson and James N. Brune

Seismological Laboratory and Department of Geological Sciences, Mackay School
of Mines, University of Nevada, Reno, Nevada 89557

Abstract

Fields of precariously balanced rocks indicate that strong earthquake motions
have not occurred at that site since the precarious rocks developed.  These
fields can be characterized with an estimate of the peak acceleration that
would be sufficient to topple the rocks, and an estimate of how long the rocks
have been precarious.  This paper uses this information to test the input to
probabilistic seismic hazard analysis.  The fundamental assumption is that the
probability of exceeding a ground motion capable of toppling a precarious rock
during a time period equal to the age of the rock is equal to the confidence
level at which the inputs to the probabilistic seismic hazard analysis can be
rejected.

We performed a probabilistic seismic hazard analysis for 26 sites of observed
precarious rocks in Nevada, using preliminary estimates of the toppling
acceleration and the age of the features.  Following standard practice, the
first probabilistic seismic hazard analysis used both faults and diffuse area
seismic sources.  The area sources had a minimum magnitude of 5.0.  The
attenuation relationship allowed ground motions of up to + 3 sigma.  Two models
of this type are rejected with over 95% confidence by most of the precarious
rock observations.  Clearly, some aspect of analysis is wrong.

We considered possible explanations for the inconsistency of the precarious
rock observations and the probabilistic seismic hazard analysis.  As in
southern California (Brune, 1996), a probabilistic seismic hazard analysis
which eliminates the area sources and only includes faults is consistent with
the precarious rock observations at essentially all of the sites.  However,
additional calculations indicate that it may not be necessary to totally reject
the inclusion of diffuse zones from the probabilistic seismic hazard analysis.
The physics of rock stability may allow increasing the minimum magnitude to 6.0
in the area sources, since the short duration of high frequency accelerations
in smaller events may not topple all precarious rocks.  Alternatively, because
the precarious rocks are generally sited on relatively good quality rock
outcrops, truncating the attenuation relationship to eliminate above-average
accelerations may be appropriate.  Individually, each of these effects allow
more of the precarious rock sites to be consistent with the area source zones,
and if both are effective only about 20% of the precarious rock sites are
inconsistent with the probabilistic seismic hazard analysis input including
diffuse zones.  Changes in diffuse source zone geometries might further reduce
the number of discrepancies.  Thus, with the present uncertainties in
interpretation of the precarious rocks, it is premature to reject the concept
of area sources in general.