霧島火山下部のP波速度構造

抄録

The underground P-wave velocity structure in the Kirishima volcanic area was first proposed by Minakami et al. in 1969. Their structure, having two (parallel) layers of which velocities 4.3km/sec. (upper) and 6.1km/sec. (lower) respectively, was derived from the analysis of the seismometrical data observed during the period of the Ebino earthquake swarms in 1968. From 1972 to 1975, explosion earthquake surveys in the southern Kyushu district were carried out by the research group of the GDP. We observed these explosion earthquakes with the routine and temporal seismometrical networks of the Kirishima volcano observatory. Combining the observational data of the 1968 Ebino earthquakes with newly observed data of explosion earthquakes, the P-wave velocity structure of the Kirishima volcanic area was obtained. The results are as follows. 1) The true P-wave velocity of the second layer of the Kirishima volcanic area is 5.9km/sec. 2) The depth of the second layer is 5.Okm from the ground surface in the Okamoto area. and the upper surface of 5.9km/sec. layer has an inclination of about 2.1°dipping toward the directiod N35°W. 3) It is supposed from the observed apparent velocities of the explosion earthquakes that the surface of the 5.9km/sec. layer is slightly domed up beneath the Kirishima volcanoes, except the eastern side of the volcanoes, where the data are lacking. 4) The distribution of the hypocenters of the earthquakes which occured just beneath the area of active cones of the Kirishima volcanoes were restricted with in the upper layer (4.3km/sec.) of the newly decided velocity structure.