Seismic image of a CO 2 reservoir beneath a seismically active volcano

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 3 He (Sorey et al. 1993) and the emission of about 500 tonnes day −1 of CO 2 (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 V P /V S 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 V P /V S is sensitive to pore‐fluid compressibility, through its effect on V P . The observed V P /V S anomaly is probably caused directly by CO 2 , 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.