A MASSIVE rockslide in California鈥檚 Yosemite National Park last month
should help efforts to monitor a future nuclear test ban treaty. Seismologists
say that data collected from the rockslide provide a unique opportunity to test
current monitoring efforts. They want to find out whether these can accurately
distinguish the seismic signatures of underground nuclear blasts from those of
similar natural events.
The slide began when a granite slab weighing 30 000 tonnes broke loose from
Yosemite鈥檚 Glacier Point on 10 July and tumbled almost 700 metres to the valley
floor. The resulting shock wave toppled trees and created a dust cloud
that rose
a kilometre into the air. The impact was also registered by three seismometers
operating within 240 kilometres of the park, which recorded it as an earthquake
with a magnitude of 2.15.
The rockslide鈥檚 shock wave travelled closer to the surface than that of an
earthquake, however. Another distinguishing feature was that the ground motion
built to a peak over 12 seconds, as the huge slab bounced down the cliff on its
way to the valley. 鈥淭hat鈥檚 what really gave it away,鈥� says Robert Uhrhammer of
the University of California at Berkeley鈥檚 seismographic station. 鈥淲e knew it
couldn鈥檛 be an earthquake, because that would peak in about one-tenth of a
蝉别肠辞苍诲.鈥�
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Using the same data, Bill Walter of the Lawrence Livermore National
Laboratory in California hopes to test his skill at detecting nuclear
tests. The
challenge is to rule out all other causes for the motion. Walter鈥檚 team
has been
studying the seismic character of earthquakes and the collapse of deep mines.
Using this information, the researchers have developed mathematical models that
they believe can accurately calculate the size and type of event that triggered
a particular seismic disturbance.
The problem they have had so far, however, is that earthquakes and mine
collapses are triggered by mysterious events located deep
underground鈥攚hich makes checking the accuracy of the models very
difficult. But this is not the case with the new data from Yosemite, which
Walter鈥檚 group began to analyse last week. 鈥淭he exciting thing here is that we
know so many details about what really happened,鈥� says Walter. 鈥淲e鈥檒l get
to see
how good our models really are.鈥�