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NEXT6: Recent activities
6.1 Eruptions
After the 1884-85 eruption, strombolian, small vulcanian, and ash eruptions occurred at Otake crater. The frequency and intensity of eruption have changed temporally so that the recent activity of Suwanosejima Volcano can be subdivided into active and quiescent periods. From at least 1976, when continuous observation records began, the active periods have been identified as around 1980 to 1984, September 1989 to August 1994, and December 2000 to 2011, with relatively high-frequency of explosive activities. In these active periods, fluctuations in the intensity of activity were observed. In Otake crater, incandescent lava has been reported during active periods.
6.2 Earthquakes
The seismicity in Suwanosejima has been classified into A-type, B-type, explosion-type, and tremor-type earthquakes (Iguchi, 2000). The sequence of these seismic activity exhibits fluctuations in various time scales, from a few weeks or months to several years, at least since 1989, when the continuous observation began. The waxing of A-type and B-type earthquakes is well correlated with activation of surface phenomena.
6.3 Fumaroles
The amount and composition of volcanic gas and of leachates on the surface of volcanic ash are analyzed during joint observations carried out every several years. Hirabayashi et al. (1993) indicated the emission of high-temperature gas based on the F/Cl ratio of leachate on the vulcanian ash sample from October 1989. Hirabahashi et al. (2005) estimated the total SO2 gas release rate to be a few hundred to a thousand tons/day. They also reported the amount just before and after the explosion and indicated a larger amount of gas release in case of a more intense explosion.
6.4 Geothermal activity and hot springs
There are some geothermal spots around Otake crater, but such activity is not observed anywhere else in the island. The temperature in the crater changes largely with time and varies among different points in the crater (Oshima and Tamekuri, 2000). On the basis of remote visual observation, Kagiyama and Masutani (1993) proposed the idea that an aquifer layer exists at a shallow level beneath the crater, and that the activity of fumaroles waxes or wanes with the ascent or descent of a magmatic body into the aquifer.
At a locality 1 km upward along a river from the shore of Sakuchi, there is a hot spring with reported temperature ranging from 43-49.2 degrees C in 1952, 1980, and 1984 (Ohta, 1988). There is also a spot of hot spring around the shore of Sakuchi.