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Report on Unzendake (Japan) — December 1990

Bulletin of the Global Volcanism Network, vol. 15, no. 12 (December 1990)
Managing Editor: Lindsay McClelland.

Unzendake (Japan) Weak steam emissions from 17 November vent; decreased seismicity

Please cite this report as:

Global Volcanism Program, 1990. Report on Unzendake (Japan) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 15:12. Smithsonian Institution.

Volcano Profile |  Complete Bulletin


Unzendake

Japan

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

All times are local (unless otherwise noted)


Steam emission continued in December from the new vent in the E flank's Tsukuno-jima crater . . . . The emissions were 50-100 m high and included no ash. The monthly number of recorded earthquakes was 340 (no swarms were recorded), a drop from 843 in November (figure 9). Three shocks were felt, on 9 and 29 December. Epicenter distributions were unchanged from previous months, mainly concentrated on the W flank of the volcano.

Figure (see Caption) Figure 9. Daily number of earthquakes (top) and tremor episodes (bottom) at Unzen, 1990. An arrow marks the 17 November eruption. Courtesy of JMA.

Geologic Background. 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.