Forty-three deaths were caused and 129 houses were destroyed or burned in the pyroclastic flows that generated at Mt. Unzen Fugendake on 3 and 8 June 1991. Generally, a pyroclastic flow is divided into a main body (bottom layer port) composed of lava blocks, lava pieces, etc. and an hot ash cloud (top layer part) composed of hot and fast blasts containing volcanic ash. The hot ash cloud burned 120 houses and brought down or scorched trees in a forest area about 2. 9km
2. The damage caused to houses and forest trees by hot ash cloud of the proclastic flows amounted, respectively, to about 90% and about 70% of the damage by the whole of the pyroclastic flows. Thus, clarifying the mechanism of damage caused by hot ash cloud is important to prevent pyroclastic flow damage. So we inquired into the forms of damage by, mainly, the hot ash cloud of the pyroclastic flows of June 3 and 8 and figured out the temperature and wind velocity of the hot ash cloud of the pyroclastic flows from the state of damage by high temperture and the state of damage by blasts.
From such conditions as the state of damage to vehicles, the temperature of the hot ash clouds is estimated at 300-400°C or more. Also, from the fall or break of concrete electric poles and standing tree, the wind velocity of the hot ash pyroclastic flows is dynamically estimated at more than 20m/s.
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