Report on Asosan (Japan) — August 1979
Scientific Event Alert Network Bulletin, vol. 4, no. 8 (August 1979)
Managing Editor: David Squires.
Asosan (Japan) Mid-June eruption successfully predicted, but activity continues, killing two in September
Please cite this report as:
Global Volcanism Program, 1979. Report on Asosan (Japan). In: Squires, D. (ed.), Scientific Event Alert Network Bulletin, 4:8. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN197908-282110.
32.884°N, 131.104°E; summit elev. 1592 m
All times are local (unless otherwise noted)
"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).
|late May-early Jun||Intermittent phreatic eruptions; wet ash and blocks fell on crater floor, common activity at Aso.|
| Jun||Felt earthquake [at 0904].|
|11 Jun||Continuous tremor amplitude increased during the morning; [zone 1 km from crater closed] at 1310.|
|12 Jun||Further tremor amplitude increase in the morning but sudden decrease in amplitude at 1821; ash eruption began in the evening; a few incandescent blocks seen at night.|
|13 Jun||Further tremor amplitude decrease at 0336, then steep increase at 1457; large explosion at 1510 produced a 2-km-high ash cloud; blocks fell 350 m from the vent.|
|14 Jun-15 Jun||Intermittent ash eruptions; incandescent blocks; very loud rumbling ([people said] the strongest in 50 years) began at 2340 on 15 June and lasted until the next morning, rattling doors an windows.|
|16 Jun||Activity decreased briefly in the morning, but loud rumbling resumed at 1110 and explosions started again, ejecting incandescent scoria.|
|17 Jun-26 Jun||Ash ejection every day; 25 cm deposited in the summit area 12-23 June; rumbling declined during the morning of the 18th, then resumed at 0950 on the 20th, continuing through 27 June; block ejection was observed on 3 days, and reflected glow could be seen on 22 and 23 June; lightning was seen on 19 June.|
|27 Jun-30 Jun||Poor weather prevented observations; estimated volume of ejecta in June, 1.4 million tons.|
|01 Jul-05 Jul||Ejection of ash and incandescent blocks, accompanied by rumbling; largest scoria bombs measured at 86 cm in diameter.|
|06 Jul-19 Jul||Ash ejection and weak rumbling; no blocks observed.|
|20 Jul-31 Jul||Ash and incandescent blocks ejected; lightning and reflected glow seen; continuous weak rumbling, punctuated by occasional louder periods; July ejecta volume estimated at 1.64 million tons.|
|01 Aug||Ejection of ash and incandescent blocks; reflected glow; lightning.|
|02 Aug-03 Aug||Ejection of ash and incandescent blocks; lightning.|
|04 Aug||Ash ejection; lightning.|
|05 Aug-06 Aug||Ejection of ash and incandescent blocks.|
|07 Aug||Bad weather -- no observations.|
|08 Aug-09 Aug||Ejection of ash and incandescent blocks; lightning.|
|10 Aug||Ejection of ash and incandescent blocks stopped about 1300, then loud rumbling began.|
|11 Aug-26 Aug||Loud rumbling; white steam emission, a few ash ejections.|
|27 Aug||161 mm of rainfall; rumbling stopped; steep decline in continuous tremor amplitude at 0900.|
|28 Aug-05 Sep||No eruptive activity [but slight ashfall 5 September]; tremor remained weak.|
|06 Sep||Large explosion at 1306, killing three persons; tremor amplitude increased sharply at 1350 then decreased gradually over the next 14 hours.|
|07 Sep-12 Sep||White steam emission; weak tremor.|
|13 Sep-22 Sep||Quiet; no ashfalls observed.|
|23 Sep||Ash eruption, lasting about 10 minutes.|
|24 Sep||10-minute ash eruption in the morning; continuous ashfall 1450 until night; accumulation less than 0.1 mm.|
|25 Sep-26 Sep||No ashfalls observed.|
|27 Sep||Continuous ashfall from 0540 to 1730, accumulating about 0.2 mm.|
|28 Sep||No ashfalls observed.|
|29 Sep||About 0.1 mm of continuous ashfall between 1510 and 1645.|
|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  injured by blocks ejected at about 1300 on 6 September. The area within 1 km of the active vent remained off limits.
Geologic Background. 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.