Exogenous growth of lava dome 11 continued from mid-June to mid-July, with relatively large-scale collapses resulting in pyroclastic flows in late June that burned houses and killed one man. Dome 11 has grown to 600 m long, 500 m wide, and 500 m high, and has overridden both parts of dome 4, which had been pushed and separated during endogenous growth in early 1992. This is now the longest dome since 1991, with an estimated volume of 8 x 10⁶ m³. Such enlargement may have been possible because the dome 4 remnants provided support and prevented major collapses.

Heavy rainfall caused large debris flows along the Nakao and Mizunashi rivers on 2, 12-13, 18-19, and 22 June. Temporary evacuation recommendations were issued to 5,000-9,000 residents along those rivers. The largest debris flow (18 June) destroyed a large concrete bridge over the Mizunashi River, burying many roads and damaging 207 houses. Debris flows damaged 391 houses in June and the first half of July, bringing the total to 1,326 since May 1991 (table 12). The rainy season is expected to be over by the end of July.

Table 12. Summary of damage from debris flows and pyroclastic flows at Unzen, 15 May 1991-15 July 1993. Courtesy of JMA.

The S part of dome 11 began to collapse on 21 June (table 13), and the N side on 23 June. The pyroclastic flows descended 4.5 km E along the Mizunashi River and 4.0 km NE along the Nakao River. Large pyroclastic flows that descended the Mizunashi River at 0421 on 21 June, and at 0503 the next day, removed ~ 500,000 m³ of material but caused no damage.

Table 13. Larger pyroclastic flows at Unzen, June 1993. Courtesy of JMA.

Pyroclastic flows at 0252 and 1114 on 23 June were accompanied by pyroclastic surges that exited a narrow gorge of the Nakao River and swept ~ 1 km into the residential Senbongi district of Shimabara City. Houses were damaged and burned, and trees were toppled. The area had been evacuated since August 1991, and the restricted zone was enlarged on 24 May. Most of the block-and-ash deposits were near the exit from the river. Three people entered the evacuated area after the first pyroclastic flow to watch their houses burn; one man was killed by the second pyroclastic flow. This was the first death at Unzen since 43 people were killed by a pyroclastic flow on 3 June 1991. Additional pyroclastic flows at 0525 on 24 June traveled 4 km down the Nakao River and were observed by S. Nakada from a helicopter. The pyroclastic flow, moving against the wind, included pyroclastic ground surges with powerful brown clouds that changed to white soon after they stopped moving. When the clouds blew away, it was revealed that houses enveloped by the clouds were burning. These three pyroclastic flows of 23-24 June burned a total of 187 houses.

By 24 June, an estimated 1.5 x 10⁶ m³ of material had been removed from the S side of dome 11, and 2.0 x 10⁶ m³ from the N side, leaving horseshoe-shaped collapse craters on each side of dome 11 and remnants of dome 4. Another pyroclastic flow at 0115 on 26 June traveled 5.2 km along the Mizunashi River, crossed a road (closed at night), and extended beyond the evacuated zone to within ~ 2.5 km of the beach. Field inspections by local authorities showed that the temperatures at the front of the block-and-ash flow deposits were not high enough to melt vinyl ropes along the road or burn nearby houses.

Seismicity at the dome complex remained unchanged before and during the pyroclastic flows. There were 295 seismically detected pyroclastic flows in June . . . . A total of 506 microearthquakes was recorded at the lava-dome complex in June, greatly reduced from the 3,037 recorded in May.

Evacuated areas along the Nakao and Mizunashi Rivers were enlarged between 24 June and 6 July because of the pyroclastic-flow activity, increasing the number of evacuees to 3,614.

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; S. Nakada, Kyushu Univ.