SEISMIC IMAGE OF A CO2 RESERVOIR BENEATH A SEISMICALLY ACTIVE VOLCANO

Mammoth Mountain is a seismically active volcano 200 000 to 50 000 yea rs old, situated on the southwestern rim of Long Valley caldera, Calif ornia. Since 1989 it has shown evidence of unrest in the form of earth quake swarms (Hill et al. 1990), volcanic 'long-period' earthquakes (P itt & Hill 1994), increased output of magmatic He-3 (Sorey et al. 1993 ) and the emission of about 500 tonnes day(-1) of CO2 (Farrar et al. 1 995; Hill 1996; M. Sorey, personal communication, 1997), which has kil led trees and poses a threat to human safety. Local-earthquake tomogra phy shows that in mid-1989 areas of subsequent tree-kill were underlai n by extensive regions where the ratio of the compressional and shear elastic-wave speeds V-P/V-S was about 9 per cent lower than in the sur rounding rocks. Theory (Mavko & Mukerji 1995), experiment (Ito, DeVilb iss & Nur 1979), and experience at other geothermal/volcanic areas (Ju lian et al. 1996) and at petroleum reservoirs (Harris et al. 1996) ind icate that V-P/V-S is sensitive to pore-fluid compressibility, through its effect on Vp. The observed V-P/V-S anomaly is probably caused dir ectly by CO2, and seismic V-P/V-S tomography is thus a promising tool for monitoring gas concentration and movement in volcanoes, which may in turn be related to volcanic activity.