"The eruption of Aso continued through July (table 1). Seismicity remained low. It is empirically known for Aso volcano that the amplitude of continuous tremor becomes large before an eruption and remains large throughout the eruptive period (figure 3).

Table 1. Activity at Aso, late May-29 September 1979. Ash thicknesses, at about 1 km from the source, were estimated by weighing the small amount of material accumulated in a measured area. Courtesy of JMA.

Figure 3. Aso seismicity since 1973. Monthly number of isolated tremor events (top) plus 10-day means of continuous tremor anplitudes during the same period (bottom). Arrows show months in which Aso was erupting. Courtesy of JMA.

The local disaster control group for Aso volcano [closed the area within 1 km of the crater] at 1310 on 11 June because high-amplitude continuous tremor had begun to be recorded at JMA's Asosan Weather Station [originally referred to as Aso Observatory] during the early morning. Civil Defense personnel kept people 1 km from the crater, visited by many persons when the volcano is inactive. The eruption began during the evening of 12 June. No casualties have occurred.

"The second characteristic event of this eruption was the decrease in the amplitude of the continuous tremor just before the largest explosion, on 13 June. The extraordinary decrease in amplitude was observed for 11 and 1/2 hours, from 0336 to 1457 on the 13th. The explosion occurred at 1510, after a steep increase in tremor amplitude for 13 minutes. Many cases of a decrease in tremor amplitude before a larger explosion are known for past eruptions at Izu-Oshima and Aso. For example, a decrease lasting four days was recorded before Aso's large explosion of 31 October 1965."

Kyodo radio reported that [three] persons were killed and [11] injured by blocks ejected at about 1300 on 6 September. The area within 1 km of the active vent remained off limits.

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; Kyodo radio.