Distribution and characteristics in waveform and spectrum of
seismic events associated with the 2000 eruption of Mt. Usu
Wakana Matsubara, Kiyoshi Yomogida, Junji Koyama, Minoru Kasahara,
Makoto Ichiyanagi, Hitoshi Kawakatsu, and Mare Yamamoto
J. Vol. Geothermal Res., 136, 141-158, 2004
Abstract
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.