Microseismic Activity beneath Active Fumarolic Areas

A Case Study at Kuju-iwoyama in Central Kyushu, Japan

Abstract

Kuju-iwoyama, the most active fumarolic area in central Kyushu, Japan is situated in an explosive crater of Kuju Volcano. The natural heat discharge is estimated as about 100 MW and most of it (more than 95%) are from steaming grounds and fumaroles. The temperature of the fumaroles exceeds 200°C and the maximum temperature is 480°C. A temporary seismic observation system was installed around the fumarolic area in order to investigate the relation between seismic activity and the thermal process beneath it. As a result, high microearthquake activities were detected just beneath the fumarolic area down to about 1.5 km below the surface. Lower P wave velocity and lower Poisson's ratio were also obtained just beneath the fumarolic area. The origin of such high microearthquake activity just beneath the fumarolic area is considered to be high pore fluid pressure in the two-phase reservoir. Microseismic observation is a useful technique to delineate geothermal reservoirs with high reservoir pressure.