On 10 February, a pyroclastic flow took place that was caused by collapse of the dome's lobe 7 or 8. No pyroclastic flows had been observed since 11 February 1995. In the next five days, six more small block-and-ash flows occurred; the highest plume reached 500 m.

According to the Shimabara Earthquake and Volcano Observatory, the pyroclastic flows descended SE, traveling ~1 km from the source. The resulting deposits were reddish brown and based on infrared camera measurements hosted lava blocks with temperatures > 60°C. The ash-clouds accompanied by these flows were similar to those of pyroclastic (block-and-ash) flows that took place frequently during 1991-94. Simple rockfalls (without ash-clouds) also occurred simultaneously and reached ~1.5 km from the source, beyond the front of the block-and-ash flows.

Neither volcanic earthquakes nor near-dome tiltmeter perturbations occurred before or after the pyroclastic flows. The collapses may have been due to stresses from either cooling-related or seasonal temperature changes.

Unzen is a large volcanic complex that covers much of the Shimabara Peninsula E of Nagasaki. The Mayu-yama lava dome was the source of a devastating 1792 avalanche and tsunami. Partial dome collapses have continued following Unzen's 1990-93 eruption.

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: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan; Shimbara Earthquake and Volcano Observatory (SEVO), Kyushu University, Shimabara-shi, Nagasaki-ken 855 Japan; Setsuya Nakada, Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).