Аннотация
Mammoth Mountain is a seismically active volcano 200 000 to 50 000
years old, situated on the southwestern rim of Long Valley caldera,
California. Since 1989 it has shown evidence of unrest in the form
of earthquake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes
(Pitt & Hill 1994), increased output of magmatic 3He (Sorey et al.
1993) and the emission of about 500 tonnes/day of CO2 (Farrar et
al. 1995; Hill 1996; M. Sorey, personal communication, 1997), which
has killed trees and poses a threat to human safety. Local-earthquake
tomography shows that in mid-1989 areas of subsequent tree-kill were
underlain by extensive regions where the ratio of the compressional
and shear elastic-wave speeds VP/VS was about 9 per cent lower than
in the surrounding rocks. Theory (Mavko & Mukerji 1995), experiment
(Ito, DeVilbiss & Nur 1979), and experience at other geothermal/volcanic
areas (Julian et al. 1996) and at petroleum reservoirs (Harris et
al. 1996) indicate that VP/VS is sensitive to pore-fluid compressibility,
through its effect on VP. The observed VP/VS anomaly is probably
caused directly by CO2, and seismic VP/VS tomography is thus a promising
tool for monitoring gas concentration and movement in volcanoes,
which may in turn be related to volcanic activity.
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