Broadband seismic signals associated with the 2000 volcanic unrest of Mount Bandai, northeastern Japan

We have observed volcano-tectonic (VT) earthquakes, volcanic tremor and very long-period events (VLPEs) associated with the volcanic unrest of Mount Bandai since 2000 using a broadband seismic network closely deployed around the volcano. VT earthquakes are characterized by high-frequency (>10 Hz) signals with clear onsets of P and S phases, whereas volcanic tremor is characterized by a long coda comprising frequencies ranging from a few hertz to more than 10 Hz. In contrast, waveforms of VLPEs consist of very long-period (∼10 s) signals preceded by short-period (<1 s) components. Hypocenters of the VT earthquakes are concentrated in two clusters at depths of 0-2 km beneath the summit of Mount Bandai and to the northwest of the volcano. Conversely, the hypocenters of the volcanic tremor and the source of the short-period signals associated with VLPEs are located in the gap between the two clusters of VT earthquakes. Since this gap lies near a low-resistivity zone detected by magnetotelluric soundings, the source mechanisms of the tremor and short-period VLPE precursors are considered to be related to magmatic fluids consisting of hot water and/or magma. The short-period source of VLPEs is inferred to trigger inflation and deflation at a depth of about 5 km beneath the volcano's north-northwestern flank and to thereby excite the subsequent very long period signals. Our analysis of these broadband seismic signals clarifies the recent activity of magmatic fluid beneath Mount Bandai, a volcano previously quiescent for more than 110 years.