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Report on Unzendake (Japan) — October 1992


Bulletin of the Global Volcanism Network, vol. 17, no. 10 (October 1992)
Managing Editor: Lindsay McClelland.

Unzendake (Japan) Continued dome growth, at about half of last year's rate; dome collapses generate pyroclastic flows

Please cite this report as:

Global Volcanism Program, 1992. Report on Unzendake (Japan) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 17:10. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199210-282100



32.761°N, 130.299°E; summit elev. 1483 m

All times are local (unless otherwise noted)

Lava domes 5 and 8 continued to grow through October and mid-November. The dome-growth rate was estimated at ~ 0.1-2.0 x 106 m3/day in September-October, approximately half of last year's rate. Pyroclastic flows were frequently generated by partial collapse of the dome complex, mainly towards the SE . . ., but also towards the E . . . and NE . . . . Ash clouds from the flows rose ~ 0.5-1 km. Most of the pyroclastic flows traveled 1-2 km. Two large flows were generated on 3 and 10 October. The 3 October flow (at 1549) traveled 3.5 km E along the Mizunashi Valley with a seismic duration of 260 seconds. On 10 October (at 1944) the second flow moved 3.5 km SE along the Akamatsu Valley, continuing for 130 seconds on seismic instruments. Both were smaller than the 27 September pyroclastic flow.

The monthly total of seismically detected flows reached 284, with a daily rate of 3-16. A total of 2,948 earthquakes occurred in October, the fewest in a month since October 1991. Daily frequency ranged from 50 to 150. The number of evacuees from Shimabara city and Fukae town has remained unchanged since being reduced to 3,017 on 9 September.

Geological Summary. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: JMA.