Precise Hypocenter Distribution of Seismic Signals Associated with the 2000 Volcanic Unrest of Mount Bandai, Northeastern Japan

We precisely determine hypocenters of volcanic earthquakes and tremor and compare their distribution with 3-D seismic structure and low resisitivity zones of the volcano to understand the 2000 volcanic unrest of Mount Bandai, Japan. Many volcano tectonic earthquakes (VT), volcanic tremor and very long period events (VLP) have been recorded by our broad band seismic network closely deployed around the volcano since April 2000. VT earthquakes are characterized by high frequency (>10Hz) signals with clear onsets of P and S phases, and volcanic tremor by long coda having a frequency ranging from a few Hz to more than 10 Hz. Waveforms of VLP consist of very long period (about 10s) signals preceded by short period ( less than 1 s ) signals. Absolute locations of their hypocenters are precisely determined by using the 3-D seismic structure that is obtained from P-wave arrival times of about 290 seismic stations for eight active sources that was conducted in 1997 as a national project of the program of prediction for volcanic eruptions. The results show that: hypocenters of VT earthquakes are concentrated in two clusters at a depth of 0-2 km beneath the summit and the northwestern area of the volcano; hypocenters of the volcanic tremor and the source of the short period signals of VLPs are located in a gap of the two clusters of volcano tectonic earthquakes. Since the gap of VT earthquakes is very close to a low resistivity zone detected by magnetotelluric soundings, we infer that volcanic tremor and the short period signals of VLPs are very closely related with magmatic fluid (hot water and/or magma) activity at the shallow part of the volcano. On the other hand, very long period signals of the VLPs are located at 5 km beneath the north-northwestern flank, which is more than 2 km far from the short period source of VLPs. These results imply that the activity of the shallow part of the volcano is dynamically related with inflation and deflation motions of the VLP source.