Report on Unzendake (Japan) — October 1993
Bulletin of the Global Volcanism Network, vol. 18, no. 10 (October 1993)
Managing Editor: Edward Venzke.
Unzendake (Japan) Lava production and pyroclastic-flow activity decrease
Please cite this report as:
Global Volcanism Program, 1993. Report on Unzendake (Japan) (Venzke, E., ed.). Bulletin of the Global Volcanism Network, 18:10. Smithsonian Institution.
32.761°N, 130.299°E; summit elev. 1483 m
All times are local (unless otherwise noted)
Lava production decreased in October, although lobe 11 continued to grow both endogenously and exogenously. The lobe, growing to the ESE, was ~800 m long and 450 m wide in mid-November, slightly wider than in mid-October. A crude peel structure formed over the lobe-11 vent, and reddish lava blocks from lobe 10, close to the vent, were uplifted. Minor inflation around lobe 10 caused small-scale collapses to the W and SW, and slight tilting of large blocks towards the W. Pyroclastic flows to the N were generated by partial collapses of lobe 10.
The Geographical Survey Institute of Japan analyzed aerial photographs taken on 13 October. The analysis indicated that the volume of the lava dome was 90 x 106 m3 and the pyroclastic-flow deposits contained 80 x 106 m3 of material, for a total of 170 x 106 m3 erupted since the beginning of the current activity in May 1991. Over the 29 months of this eruption, the average eruption rate has been ~5.9 x 106 m3/month, and the average rate of lava-dome growth has been ~2.7 x 106 m3/month. The average daily eruption rate from 5 March to 13 October was 0.16 x 106 m3/day. A rate of 0.10 x 106 m3/day was reported by the same Institute for the period from late November 1992 to early March 1993. Daily eruption rates were lower than these averages during December 1992 to January 1993, and higher during February-April and July-August 1993. The average rate in late October had decreased to 0.05 x 106 m3/day.
Partial collapses of lobe 11 generated 2-3 pyroclastic flows/day, mainly to the E and NE. There were 80 seismically counted pyroclastic flows in October, a decrease from the 138 detected in September. The longest pyroclastic flows traveled 2.5 km NE down the Oshiga and Nakao valleys on 1, 11, 16, 18, 20, and 27 October, with seismic durations of 100-140 seconds each. Some of the pyroclastic flows generated ash clouds as high as 1,200 m above the summit. The pyroclastic-surge deposits spread into a fan shape upon exiting the gorge, and extended several hundred meters beyond the block-and-ash flow deposits near the exit. Rockfalls and smaller pyroclastic flows to the E and SE were generated by partial collapses of lava from the vent area. No damage was caused by any of the pyroclastic flows. Seismicity at the lava dome remained at low levels with 1,101 small shocks, similar to September (1,032 events). The number of residents evacuated from the E slopes because of the threat of pyroclastic flows has remained unchanged at 3,617 since early July.
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; S. Nakada, Kyushu Univ.