爆破地震動から求めた九重火山群の深部構造 : Time-Term 法による
1982 年 27 巻 2 号 p. 81-95
詳細
1982 年 27 巻 2 号 p. 81-95
Artificial explosions were carried out at 6 sites in March, 1979 and 5 sites in March, 1980 for the purpose of the refraction seismic explorations, one of the geothermal surveys on the Hohi geothermal area surrounding the Kuju volcano group, central Kyushu. Seismic waves from these explosions were clearly recorded at the 6 short-period seismograph stations forming the permanent seismic network (Kyushu Electric Power K.K.) in and around the Kuju volcano. Time term analyses were applied to the travel time data, in order to determine delay times under each recording site and P wave velocity in the basement layer. From the time term analysis, P wave velocity of 5.8km/sec was obtained in the basement layer and the time terms (delay times) ranged from 0.07 to 0.73 sec. The south eastern part of the surveyed area where granitic or metamorphic rocks crop out is a region of small time terms. On the contrary the central part of the Kuju volcano where low gravity anomaly is observed, is a region of large time terms, implying a large thickness of low-velocity rocks overlying the basement and the time term pattern of this region shows an oval-shaped. This region is considered to represent the Kuju caldera proposed by Matumoto, T. The time term pattern is easily convertible into the thickness pattern of the upper layer overlying the basement when introducing an assumption of the P wave velocity of the upper layer is constant. The mean velocity of this layer is estimated to be 3.3km/sec from travel time-distance curves. Then the depth of basement in the caldera is estimated at about 3km from the ground surface. Under the geothermal active region, basement is uplifted. Its depth is estimated to be about 2km from the ground surface and is entirely identical with the recent report of a deep well drilled in this region in the search for geothermal heat source. Generally, the time term patterm or thickness pattern of the upper layer is clealy supported by the gravity data in the surveyed area except the northern part.
