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We installed five broadband seismometers around Mt. Usu, Hokkaido, Japan, just before the first surface eruption on March 31, 2000. By using these broadband data with short-period seismograms recorded by Japan Meteorological Agency and Institute of Seismology and Volcanology of Hokkaido University, we located 590 earthquakes associated with the 2000 eruption of Mt. Usu from March 31, 12:58 to April 5, 2000, including 41 low-frequency earthquakes. Low-frequency earthquakes have clear predominant frequencies of about 1.0–1.5 Hz. While the seismicity of tectonic earthquakes was still active on April 5, there were almost no low-frequency events after April 2. All the low-frequency earthquakes occurred within a vertical and narrow zone near the craters of eruption, and they are shallower than 4 km. In contrast, the active area of tectonic earthquakes spreads out in a large area, particularly deep in the south of the craters. From these temporal and spatial patterns, together with GPS and gravity measurements that support the termination of magma activities before April 2, we suggest that low-frequency earthquakes were caused by magma or hydrothermal activities beneath the volcano while tectonic ones were caused by the regional tectonic stress affected by the intrusion of magma body near the craters. Related to long-term magma activities of this eruption, four deep crustal low-frequency events were identified on October 17, 1998 just in the south of Mt. Usu in the depth range of 20–30 km. As proposed for other active volcanoes, these deep crustal events may represent a part of deep magma activities beneath Mt. Usu. [Copyright &y& Elsevier]

We installed five broadband seismometers around Mt. Usu just before the first eruption in 2000 and the observation continued over 8 months. Among the total of 590 relocated earthquakes that occurred from March 31 12:58 p.m. to April 5, 2000, 41 low-frequency earthquakes were identified. Low-frequency earthquakes, based on waveforms and S-wave spectra, can be classified into several families. We investigated the source process of the low-frequency earthquakes classified as “type 1” for which the similarity among events was the highest. The character of waveforms and S-wave spectra of type 1 earthquakes at Mt. Usu are quite similar to the “hybrid” and “LP” earthquakes at Mt. Redoubt, Alaska, whose source process was explained well by the resonance of a fluid-filled crack. Assuming that they have similar source processes, we estimated several parameters such as crack size by forward modeling. We obtained the following parameters that explain main features (e.g., the frequencies of major peaks) of the observed S-wave spectra at three stations (HGSN, SHNZN and ZNKJI) in the frequency range of 0.5–5 Hz, assuming a vertical crack. Crack length is 200 m, 100 m for its width, 0.14 m for its thickness, 100 for its stiffness, 0.5 for the ratio of bulk modulus and rigidity, and 0.07 for viscous damping loss. The pressure pulse triggering the crack oscillation has a rise time of 0.25 s. From the difference in characteristics of waveforms among stations of various azimuths, the strike of the crack was likely to be northeast-to-southwest. The use of the site-corrected spectra enables us to pin-point the value of viscous damping loss, so that we can conclude that water-steam mixture was responsible for the low-frequency events at Mt. Usu. [Copyright &y& Elsevier]