Fig. 1

Map view of the profile in the seismic reflection survey. The solid line in the middle of the figure represents the survey line. The active faults (The Research Group for Active Faults of Japan, 1991) are denoted by dashed lines. The Unzen graben is the area sandwiched between two active faults (thick dashed lines). The solid triangle represents the lava dome formed during the last eruption. The light grey lines are the contours of the topography with 0.1-km-height intervals. The open circles denote the locations of the pressure sources, as estimated by Kohno et al. (2008). Their depths are listed at the bottom of the figure. The solid rectangle shows the area whose geological map is shown in Fig. 3.