Abstract
Explosion seismic refraction experiments were carried out in 1988 for the investigation of the shallow crustal structure in the volcanic island of Nii-jima, northern part of Izu-Bonin island arc, Japan. One hundred and eight seismograms were obtained at 54 temporary stations aligned on the three profiles arranged like a triangle on the island. The main features of the shallow crustal velocity structure revealed by the travel time analysis are as follows:
1) Surfaces of high velocity layers are raised beneath volcanos in the island of Nii-jima.
2) The structure mostly consists of three layers with P-wave velocities of 1.9km/s, 2.8km/s and 4.5km/s, respectively.
3) A layer with a velocity of 1.5km/s exists over the 1.9km/s layer at two explosion sites in shore.
4) There is no 1.9km/s layer beneath Mt. Acchi volcano in the northern part of the island.
5) The top of the 2.8km/s layer is much shallower beneath Mt. Mineji volcano in the central part of the island and beneath Ajiaiso beach in the northwestern part of the island.
6) Both top of the 2.8km/s layer and that of the 4.5km/s layer are much shallower beneath Mt. Acchi volcano in the northern part of the island.
7) The 1.9km/s layer is thicker beneath the small plain in the central part of the inland, beneath the place between Mt. Mineji and Ajiaiso beach, and beneath Wakago village in the northern part of the island.
