Magma intrusion system and 3D structure of Iwate volcano, Japan revealed from active seismic survey

The three-dimensional P-wave velocity structure of the Iwate volcano, northeastern Japan, is obtained through an active seismic survey. The survey was conducted by 70 scientists from 11 Japanese universities and other national organizations in October 2000. Nine artificial explosions using dynamite charge of 200-250 kg were detonated at the bottom of 50 m borehole. We deployed vertical 330 seismometers in a 40 x 40 km² area, and obtained approximately 2700 travel-time data. We take 22 x 28 horizontal grid nodes with a grid interval of 0.02\deg for latitudinal and longitudinal directions, and 16 vertical grid nodes at 1-km intervals. After inverting the data by TOMG3D [Zhao et al., 1992] , the weighted RMS residual is reduced from 0.36 s to 0.17 after 10 iterations. The rigorously solved region confirmed by the checkerboard test is limited to the region around the volcanic edifice in a 20 x 13 km² horizontal area and 5 km vertically. The most prominent feature is a column-like high-velocity body (Vp > 5.6 km/s) that extends vertically for 2 km beneath the caldera, of which the horizontal location coincides with the high-Bouguer anomaly. Therefore this body is interpreted as a magmatic body that has intruded from a deeper region and accumulated over a long time. A moderate-velocity region (4.6 < Vp < 5.4 km/s) extends eastward from this high-velocity column, and coincides with the sites of the last flank eruption, so called the Yakebashiri lava flow. As the vents of the lava flow are oriented ENE, the magma considered to be moving approximately eastward from the caldera region along the path of the moderate-velocity body. The western part of the volcano extending from the caldera is also characterized by a moderate-velocity region, coinciding with the locations of many geophysical and geodetic phenomena in the 1998 volcanic unrest. This suggests the moderate-velocity in the western region represents a region of dike intrusion from the conduit beneath caldera.