PRECARIOUS ROCKS AND SEISMIC RISK IN THE LAS VEGAS REGION
 
                James N. Brune and John G. Anderson 
                      Seismological Laboratory
                     University of Nevada, Reno
 
 ABSTRACT
 
      In some types of terrain, groups of precariously balanced rocks
 evolve naturally unless shaken down by earthquakes.  The time scale
 for the evolution and stability of these rocks is on the order of
 thousands of years.  Therefore groups of precariously balanced
 rocks are effectively earthquake strong ground motion seismoscopes. 
 Zones of precarious rocks are direct evidence that no strong ground
 accelerations have occurred in the zone for thousands of years.
 They thus have the potential to provide important information about
 seismic risk. 
 
      Precarious rocks have been found at more than a dozen sites in
 Nevada.  Although we have not been able to do any accurate age
 dating on these rocks, we can make rough estimates of their age
 based on geomorphic conditions.  Most of the spectacular precarious
 rocks near Las Vegas are of the hoodoo type, i.e., a hard
 protective boulder on top of a less weather-resistant pillar. 
 Because the pillars may erode relatively rapidly we cannot
 establish as long existence times as we can for more resistant
 rocks.  A rough estimate is that these hoodoos have been in
 existence for at least a thousand years.  They have been documented
 at Nelson Landing and in Kyle Canyon.  Precarious rocks in more
 resistant formations near Beatty and Yucca mountain have probably
 been in existence for more than ten thousand years.
 
      Indirect positive evidence of strong ground shaking may be
 provided by the lack of precarious rocks in the Red Rock area.  The
 formations there appear to be of the type that should create
 numerous precarious rocks, yet none have been found.  This suggests
 that there has been relatively strong ground motion in the Red Rock
 area in the last ten thousand years.
 
      We plan to present updated evidence from precarious rocks in
 the Las Vegas area.  We will use the evidence to provide
 constraints on the probability of strong ground motion for the
 area.