MECHANISM OF PHREATIC ERUPTIONS AT ASO VOLCANO INFERRED FROM NEAR-FIELD BROAD-BAND SEISMIC OBSERVATIONS

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that i s continually resonating with periods as long as 15 seconds. When phre atic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the condui t: gradual pressurization and long-period (similar to 20 seconds) puls ations of the reservoir during the 100 to 200 seconds before the initi ation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, whe re water and heat from the deeper magma chamber probably interact, app ears to help control the surface activity at Aso volcano.