Thermal Structure and Seismic Activity in Central Kyushu, Japan

Abstract

There are many active and dormant volcanoes in central Kyushu, Japan. Many fumaroles and hot springs also exist in connection with the volcanoes and they are in a higher than 2 HFU heat flow zone. The shallow seismic activity in central Kyushu is generally high than that in other parts of Kyushu. Detailed studies of epicenter and hypocenter distributions in central Kyushu were made with special reference to its thermal state of the crust. Although the shallow seismic activity is generally high in central Kyushu as mentioned above, it is rather low in thermally active areas in a low tectonic stress level and most of hypocenters are shallower than 10 km. Such a seismic activity is affected by the crustal temperature. The temperature at the depth where the seismic activity decreases rapidly is about 500 to 600℃ which was calculated based on thermal data. Another feature is that earthquakes in geothermally active areas seem to occur primarily is swarm type sequences. The origin of such swarm type events is considered to be not the geothermal origin, for example, an increase of hot water supply, but an increase of tectonic stress, from the standpoint of time variation of seismic activity.