Seismic observations from the August 28, 1997, ML=2.6, suspected
underground cavity collapse at the Nevada Test Site

Smith, K.D., D. von Seggern UNR, K. Meyeda, W.R. Walter LLNL

At 14:06 UTC on August 28, 1997 an unusual seismic event was detected
near the Pahute Mesa area of the Nevada Test Site (NTS) that appears to
be associated with the cavity collapse from a much earlier nuclear test
(nuclear testing ended in the US in 1992). The event shows all
impulsive dilatational first arrivals at all 23 stations of the
University of Nevada Reno Digital Network, operated for the Yucca
Mountain Project, at distances from 8 to 100 km. These first motions
are not consistent with an explosion source nor most earthquake source
mechanisms except for some reverse mechanisms which in turn are
inconsistent with the regional tectonics. Analysis of the regional coda
and the presence of short period Rg waves are consistent with a very
shallow source depth. The event appears to be relatively depleted in
high frequency energy as compared with earthquakes in this region of
the NTS. It was followed by a least two additional smaller events of
similar waveform character at 01:18 and 02:22 UTC on August 29 that
appear to have been at the same location.  These events are located
near the sites of several prior underground nuclear explosions
(1965-1991) that did not collpase to the surface to form craters.  Our
working hypothesis is that the event resulted from a subsurface
collapse of roof material associated with the cavities from these prior
tests. We are currently investigating previous cavity collpase data,
modeling local waveform data, testing discriminants and refining
preliminary locations to learn more about these interesting events.