Seismic Wave Attenuation Beneath Geothermal Areas in Central Kyushu, Japan

Abstract

Three tripartite and one linear array were installed at geothermal areas in central Kyushu, where many active an do the quaternary volcanoes exist and many fumarolic activities and hot springs also exist in connection with the volcanoes. Seismograms were analyzed using the spectral ratio technique to determine the seismic wave attenuation factor Q. Seismic waves through the crust and upper mantle of central Kyushu attenuate more than those through its surroundings. This fact may be due to many water filled fractures in the upper crust (surface to c.a. 15 km deep) and partial melting of the upper mantel. Local anomalies of seismic wave attenuation were also detected beneath the Kuju area in the eastern half of central Kyushu. This face may also show that there exist not only much more water filled structures in the shallower curst (surface to 4.2 km) but also heat sources in the lower part (4.2 km to 14.2 km) in the Kuju area.