DESCRIPTION:
Unzen Volcano, Japan

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The massive Unzen volcanic complex comprises much of the Shimabara Peninsula east of Nagasaki. A 30-40-kilometer-long, E-W-trending graben 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 peninsula. Fugen-dake and Mayu-yama volcanoes in the east-central portion of the andesitic-to-dacitic Unzen volcanic complex have been active during the Holocene. The Mayu-yama lava dome complex, which formed about 4000 years ago, was the source of a devastating 1792 AD debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugen-dake. The latest activity 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

In 1991, Japan's Unzen Volcano, located on the Island of Kyushu about 40 kilometers east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapses of this active dome generated destructive ash flows that swept down its slopes at speeds as high as 200 kilometers per hour. Unzen is one of more than 75 active volcanoes in Japan; its eruption in 1792 killed more than 15,000 people -- the worst volcanic disaster in the country's history.

Unzen is well know for Japan's greatest volcanic disaster. In 1792, about a month after lava stopped erupting from the volcano, a landslide from nearby Mount Mayuyama swept through ancient Shimabara City, entered the sea, and generated a tsunami that struck nearby areas. More than 15,000 people were killed by the landslide and tsunami. The amphitheater-shaped scar of the landslide is still clearly visible on Mount Mayuyama just above the city.

In 1991, Japan's Unzen Volcano, located on the Island of Kyushu about 40 kilometers east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapses of this active dome generated destructive ash flows that swept down its slopes at speeds as high as 200 kilometers per hour.

When pasty lava first breached the forested summit crater of Unzen in May 1991 after several months of small explosions, nearby residents may have breathed a sigh of relief. The slow extrusion of gas-poor lava quietly built a small dome that made the volcano seem less threatening and less likely to erupt explosively. But, within three days, as the margin of the growing lava dome crept toward the crater's precipitous edge and then became perched above the volcano's east flank, the first of many small pyroclastic flows swept as far as 2 kilometers down the volcano.

Suddenly on June 3, a much larger dome collapse and explosion produced a pyroclastic flow and ash-cloud surge that raced 4.5 kilometers from the crater, burning about 180 houses and killing 43 people who had ventured into a previously designated hazard zone. Subsequently, lava continued to extrude from the summit crater toward the volcano's east flank. Another collapse event on June 8 swept 5.5 kilometers down the same river valley, burning 210 additional houses. By the end of July, extruding lava had built an elongated dome-500 meters long, 150 meters wide, and 80 meters high-that generated an average of about 10 small pyroclastic flows every day.

Between June 1991 and December 1993 the pattern of eruption - extrusion of lava and frequent collapse of the dome's eastern margin - progressively increased the volcano's potential for wreaking havoc on local residents. By 1992, pyroclastic flows were rushing down a broader sector of the volcano and lahars became commonplace as heavy rains remobilized the hot pyroclastic debris. Reaching beyond areas covered by pyroclastic flows, these lahars swept away bridges and buried roads, precious farmland, and houses with boulders, gravel, and sand.

The actively collapsing margin of Unzen's growing dome changed location many times in response to where magma was rising into the dome and leaking onto the surface. Depending on which margin was active, pyroclastic flows spilled into one of four stream valleys. Scientists devoted much of their attention to monitoring the dome's active margin to identify which valley was most at risk from pyroclastic flows. For example, two years after the eruption began, pyroclastic flows started cascading northeast into the Nakao River valley for the first time. The most extensive of these flows reached a point 4.5 kilometers from the dome. Fortunately, officials had already ordered residents to evacuate this area in anticipation of these pyroclastic flows.

In addition to destroying previously inhabited areas, pyroclastic flows created an enormous apron of loose fragmental deposits on the volcano's steep east side. The apron has filled the headwaters of streams with many tens of meters of debris. Combined with destabilization of old debris on Unzen's upper slopes owing to the death of vegetation, these deposits are a ready source of loose debris for generating lahars during rainstorms. Heavy rainfall, commonly exceeding 1 centimeter per hour in this area, readily erodes this material to form destructive lahars. Between August 1992 and July 1993, lahars triggered by heavy rains damaged about 1,300 houses. Each period of heavy rain required sudden evacuation of several thousand residents along the Mizunashi and Nakao rivers.

Japanese officials have worked hard to ensure public safety by developing an efficient warning system and evacuation plan. They have also sought to minimize destruction from lahars by building "countermeasures" designed to trap sediment and channelize the flows as much as possible. Countermeasures along the Mizunashi River include three sediment basins lined with interlocking-concrete blocks and a series of discontinuous dikes along both sides of the main channel. The dikes funnel most of the flows down the main channel while allowing some material to spill around their margins. When the sediment basins and areas around the dikes fill with debris, workers excavate the debris with heavy equipment to make room for the next series of lahars.

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