浅間山2004年噴火 : 噴出物調査とインターネット掲示板によるリスク・コミュニケーション
2006 年 115 巻 2 号 p. 149-171
詳細
2006 年 115 巻 2 号 p. 149-171
Asama Volcano repeatedly erupted in 2004. Six major eruptions were identified, each of which discharged more than 10, 000 tons of ash, lapilli, and bombs from the summit crater. Five were single Vulcanian explosions including the first eruption on 1 September. They are common in Asama, as thousands of Vulcanian explosions occurred in the last century. However, the series of eruptions from 14 to 18 September was unusual for Asama. It was an ash eruption. An eruption plume of a few kilometers high intermittently rose above the summit crater for the first two days. It was sustained by the morning of 16 September. From the plume, sand-sized micropumice and free-crystals continuously showered down at Karuizawa, 10 km SE from the source. It was observed for the first time since the notorious 1783 eruption that the juvenile was dominant among fallout. This makes a remarkable contrast to preexistent material by Vulcanian explosions. In the evening, a small amount of ash (1-0.1 g/m2) reached as far as Tokyo, 150 km from source. Incandescent bombs ejected from the summit crater were seen from the slopes until midnight. The maximum range traveled by the bombs in 2004 was 2.2 km. It was measured at the NW flank for the 1 September explosion and also at the W flank for the 23 September explosion. In total, 3.7 × 108 kg of ash, lapilli, and bombs were ejected from the crater in 2004. This is no more than 8 % of new lava, which was emplaced on the crater floor in the middle of September. An Internet forum is a useful tool for risk communication during a volcanic crisis. Thousands of messages were posted on three boards of a forum during the 2004 crisis. They described detonations, ash fallout, and other unrests of Asama. Some of them were posted ahead of not only television news but also official Volcano Information issued by Japan Meteorological Agency. Although Internet forums are very helpful for volcano risk communication, readers must use caution to distinguish the false from the true.
