Clarification of volcanic history and structure by flank-drilling

Evaluation of structural model

The seismic experiment using artificial explosions conducted in November 1995 showed the velocity structure beneath the volcano as below. The structure model can be checked directly by drillings that penetrate the boundaries between layers with different seismic velocities. Physical measurements using the drill holes or recovered rock samples make us possible to evaluate the data from the seismic experiment and to get new image of the subsurface structure of Unzen Volcano.

Figure explanation: Survey line extended from the northwestern coast of the Shimabara Peninsula (north of the Chijiwa active fault) through the central part of the volcano (just west of the summit) to the eastern coast of the peninsula (south of Shimabara City). Graben structure that is geographically clear was not identified by this experiment; rather, the basement rocks with high velocities are uplifted upward in the center of the volcano. Sites of the scientific drilling are close to the survey line.