IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4P1_2I-O16

Room A5

Date/Time: July 24 15:30-15:45

Seismic and geodetic precursors of small vulcanian eruptions at Suwanose-jima
volcano, JapanTakeshi Nishimura1 , Masato Iguchi2 , Hiroshi Yakiwara3 , Jun Okiawa4 , Yu’ichi Hirai1 , Hiroshi Aoyama5 ,Haruhisa Nakamichi6 , Ryohei Kawaguchi1 , Yusaku Ohta1 , Takeshi Tamaguri21Science, Tohoku Univ., Japan, 2 DPRI, Kyoto Univ., Japan, 3 Science and Engineering, Kagoshima
Univ., Japan, 4 ERI, Univ. of Tokyo, Japan, 5 Science, Hokkaido Univ., Japan, 6 Environmental Studies,Nagoya Univ., Japan
E-mail: nishi@zisin.gp.tohoku.ac.jp

Suwanosejima is a small volcano island south of Kyushu on the Pacific Ocean. The volcanic eruption of theisland is characterized by small vulcanian explosions that repeatedly occur within a short interval of a few tens ofminutes every a few months. We deploy 4 broad-band seismometers, 3 short-period seismometers and 5 boreholetilt meters at distances of 0.4-1.5 km. In the present study, we summarize precursor signals that are caused bymagma behavior beneath active crater before explosions to understand the mechanism of vulcanian eruptions at
Suwanosejima volcano for the period from September 2009 to 2011. Vulcanian explosions often occur during
successive ash emissions for a few hours to days that generate continuous tremor. The explosions follow a sudden
stop for about one or two minutes of continuous tremor. Uplift toward the crater also start at the same time whencontinuous tremor amplitude stop. Such coincidence suggests that the pressurization in the shallow magma iscaused by accumulation of volcanic gas or ash due to a formation of a cap just beneath the active crater. It isnoted that the inflations and explosions are also observed during a quiet period when no ash emission is observed.
The cap is considered to be formed in a short time because the continuous tremor amplitude generally decreases
for a few tens of seconds. Amplitude and duration of the inflation signals tend to become larger and longer,respectively, for explosion earthquakes with large maximum amplitude. The tilt signals are about a few tens of nanoradian, which are detected at only station located close the active crater. The inflation amplitude is attributed tothe pressurization at a depth of a few hundred kilometers beneath the active crater, and the maximum amplitude
of explosion earthquakes are also explained by a release of the pressure stored before explosion. About 0.5 to
1 s before the onset of explosions, a tiny deflation with duration of about 0.6 s is observed in the seismogram ofexplosion earthquake. Our seismic waveform inversion shows that the tiny deflations are explained by contractionof volcanic conduit at a few hundred meter depth beneath the active crater. Pressure decreases are estimated to
be about 0.03-1 MPa for a cylindrical source with a length of 500 m and a radius of 10 m. The pressure decrease
may be a small leakage of gas, but no significant signal is detected by the acoustic sensors at stations: acoustic
signal gradually and slightly sometimes increase its amplitude just before an onset of explosion, but the duration is
not matched with that of the contraction seismic source. Although the mechanism of the contraction is still underinvestigation, it is noteworthy to mention that the maximum amplitude of explosion increases with the amplitude ofcontraction source.

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