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.