吉松地震観測所における地震観測 (1) -2つの3点観測網からえられた震源位置の比較-

Abstract

Tripartite observation at two sites was carried out at Kakuto caldera in Southern Kyushu, Japan. The one site is situated inside the caldera, at Nakatsugawa on the northwesternflank of Kirishima volcanoes and the other is nearly 4km apart westward from the former, locating at Kawanishi on the western rim of the caldera. For the events occurred in theeastern part of the caldera, the locations of hypocenters obtained from the tripartite observa-tion at the two sites differ each other 5 to 8km in the horizontal position, nearly 5km indepth and 25° to 30° in azimuthal angle. This differences could be explained by assumingthat P-wave velocity in the surface layer at Nakatsugawa is 1.0 to 1.5km/sec, nearly onethird of the value assumed at Kawanishi.Existence of the surface layer having the low velocity of P-wave (about 1.4km/sec) atNakatsugawa is also derived from the differences of arrival time between Kawanishi stationsand Nakatsugawa stations for the events occurred in Kumamoto prefecture and the thicknessof the layer is estimated to be a few hundreds meters. Existence of the surface layer witha low velocity of seismic waves at Nakatsugawa is reasonable, considering the geologicalstructure in this area.