Migration of seismic activity during the 1998 volcanic unrest at Iwate volcano, northeastern Japan, with reference to P and S wave velocity anomaly and crustal deformation

We describe the seismicity at Iwate volcano, northeastern Japan, during the volcanic unrest of 1998 with reference to a three-dimensional P and S wave velocity model from tomographic analysis. The abnormal seismic activity beneath Iwate volcano started under the caldera in February, 1998 and migrated westward in the period February to August, 1998. Previous geodetic modeling [Sato and Hamaguchi, Chikyu Monthly 21 (1999) 312-317] suggested the growth of a dike in the time of the seismic activity. Comparing the seismicity and dike extension with the tomographic images of the P and S wave velocity structure, we find that the trace of the growing dike coincides with the region of the high V_p and high V_p/V_s ratio beneath the volcano. The seismic and geodetic data are consistent with an intrusion of magma or other fluid under the caldera in 1998. Another pressure source causing the predominant crustal deformation at Iwate volcano was detected from geodetic data, which was located in the region with high V_p/V_s ratio under the western end of the volcano through the period from February to August. It is suggested that the activation of the point pressure source probably associated with the inflation of a hot fluid reservoir relate to a geothermal region adjacent to the western edge of the volcano.