II. Eruptive Events

1. General Statement

Volcanic eruptions for the period 1995-1998 were characterized by the finale of the lava dome eruption at Unzen and small-scale phreatic eruptions at Kuju, Hokkaido-Komagatake, Meakandake and Akita-Yakeyama volcanoes (Table 1; Fig. 1). Two most active volcanoes in Japan, Sakurajima and Suwanose-jima, also erupted but the degrees of activity were very low, being compared with the previous 4 years (see II. 2). Seismic swarms accompanied by intensive crustal deformation occurred in Izu-Tobe Volcano Groups several times and at Iwate Volcano in 1998. The activity in the latter was intensive enough for us to start the comprehensive volcanological study of the volcano (e.g., Hagmaguchi, 1999), though any eruption had not occurred at least as of writing (see II. 4).

Lava dome eruption at Unzen that started in May 1991, following phreatic to phreatomagmatic eruptions since November 1990, came to an end in early February 1995. The total amount of magma erupted reached 0.2 km**3 as the dense-rock-equivalent value (e.g., Ohta, 1997; Nakada et al. eds., 1999). Phreatic eruptions that began at Kuju Volcano in October 1995 called the comprehensive volcanological study of this volcano (Sudo ed., 1996). The products of this eruption contained small amounts of juvenile materials in December 1995 (Hatae et al., 1997), although the activity declined after January 1996. Emission of steam from the craters had continued, being accompanied by shrinkage of the crater area (II. 3). Phreatic eruptions occurred also at Hokkaido-Komagatake Volcano in May 1997 and October 1998, where a large-scale explosive eruption took place in the early 20th Century (II. 5). At Meakandake Volcano small-scale phreatic eruptions occurred twice during these four years.

One of old craters at the summit of Akita-Yakeyama Volcano puffed small amount of ballistics and debris flowed within the crater area in August 1997. On February 11, 1995 the steam explosion killed four workers in the hot-spring area, Nakanoyu, about 3 km south of Yakedake Volcano (Miyake and Ossaka, 1998). This was the accident when they were unloading debris from the ground surface to construct a road for the Nagano Olympic Game. Fatalities due to volcanic gas occurred at three volcanoes only in 1997 (also see II. 4; Hirabayashi, 1998). Three members of the Ground Self-Defense Force lost their lives during a training mission due to inhaling CO2 on the northern flank of Hakkoda Volcano. Four hikers losing their way winthin the crater area at Adatara Volcano and two tourists visiting one of the active craters (Nakadake) at Aso Volcano died due to inhaling H2S and SO2 respectively.

Preliminary reports on current eruptions in Japan were pasted in the internet web site; "Current Eruptions in Japan";


Additional information on eruptions in Japan was available in the following home pages;

Geological Survey of Japan; http://www.aist.go.jp/GSJ/dEG/sVOLC/Volc_E.Home.htm,

Tatsuro Chiba; http://www.geo.chs.nihon-u.ac.jp/tchiba/chibah.html, and

Yukio Hayakawa; http://www.edu.gunma-u.ac.jp/~hayakawa/English.html/.

The information on two decade volcanoes in Japan, Unzen and Sakurajima, was given in the site of Volcano Research Center, ERI, University of Tokyo;


Internet web sites of volcano observatories in Japan are as follows;

Usu (UVO), Hokkaido Univ.; http://uvo.sci.hokudai.ac.jp/,

Res. Cent. Pred. Earthq. Volc. Erup., Tohoku Univ.#; http://www.aob.geophys.tohoku.ac.jp/,

VRC, Earthq. Res. Inst., Univ. Tokyo; http://hakone.eri.u-tokyo.ac.jp/vrc/VRC.html,

Res. Cent. Seism. Volcanol., Nagoya Univ.; http://www.seis.nagoya-u.ac.jp/RCSVNU-E.html,

Aso (AVL)-Kyoto Univ.#; http://w3.vgs.kyoto-u.ac.jp/,

Sakurajima (SVO)-Kyoto Univ.; http://www.dpri.kyoto-u.ac.jp/sakurajima/sakurajima.html,

Unzen (SEVO)-Kyusu Univ#; http://www.sevo.kyushu-u.ac.jp/.

(# open in Japanese)

(Setsuya Nakada)


*Hatae, K., Watanabe, Koichiro, Watanabe, Kazunori, Tsutsui, T. and Motomura, Y. (1997) Variation in content of vesiculated glasses in volcanic ash erupted from Kuju Volcano in 1995-96, Japan. Bull. Volcanol. Soc. Japan, 42, 345-353.

**Hamaguchi, H. (1999) Eruption crisis at Iwate Volcano in 1998. Gekkan-Chikyu, 21, 255-256.

*Hirabayashi, J. (1998) Volcanic gas disaster in Japan. Japan. Jour. Disaster Medicine, 3, 11-17.

*Miyake, Y. and Ossaka, J. (1998) Steam explosion of February 11th, 1995 at Nakanoyu Hot Spring, Nagano Prefecture, Central Japan. Bull. Volcanol. Soc. Japan, 43, 113-121.

Nakada, S., Eichelberger, J.C. and Shimizu, H. (Eds.) (1999) Unzen eruption: magma ascent and dome growth. Journal of Volcanology and Geothermal Research, 89, 315pp.

*Ohta, K. (1997) Reviews on the prediction of the 1990-1995 eruption of Unzen Volcano and Supporting system for risk management. Bull. Volcanol. Soc. Japan, 42, 61-74.

**Sudo, Y. ed. (1996) Research on Oct.1995 Phreatic Eruption at Kuju Volcano. Rep. Grant-in-Aid for Sci. Res. (leaded by Kobayashi, Y.), 171p.

(* In Japanese with English abstract. ** In Japanese)

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