An eruption from Crater 1 occurred at 1340 on 26 October, the fourth this year and the first such activity since 29 September. A steam plume containing ash rose 2,500 m, and wet ash fell 1.5 km S of the crater. No blocks were jected. Eruption-tremor amplitude was 20 µm at the nearest seismometer, 0.8 km W of the crater (table 7).

Table 7. Eruptions at Aso during 1992. Courtesy of JMA.

Steam was steadily emitted to a few hundred meters throughout October, and volcanic-tremor frequency was low. No changes in steam emission, tremor frequency, or earthquakes were noted before or after the eruption. Weak ejections of mud, blocks, and water to 15 m height continued in the crater lake, which ccupies half of the crater floor. Similar activity has continued through 14 November, without additional eruptions. The area within 1 km of the crater has been closed to tourists since 24 August. No damage was caused by the eruption.

Geological Summary. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic-andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.