凝集粒子を用いた噴煙高度の推定 : 浅間火山2004年9月23日噴火に伴う降下火砕物の堆積様式(<特集>2004年浅間山噴火(2))

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A cloud height generated by a volcanic eruption reflects the immensity and/or magnitude of the eruption; thus a measuring of the height's temporal variation during the event is very significant in judging whether the activity will become violent or decline. However, when a volcanic eruption occurs during bad weather, we must take information about the cloud's height by means of the pyroclastic deposits. In general, the total time taken for pyroclastic materials to be ejected and deposited at a given distance from the source vent can be divided into three parts as follows : the time for the eruption cloud to ascend and reach its neutral buoyancy level (T_1); the time for the pyroclastic materials to be transported laterally by the eruption cloud (T_2); and the time for pyroclastic materials to fall and be deposited on the ground (T_3). Since T_3 can be calculated from the settling velocity of pyroclastic materials, if the time that the pyroclastic materials fell at a given locality was observed and a given value for T_1 is assumed, the most suitable wind velocity to explain T_2 can be determined. Thus the height at which pyroclastic materials separate from the eruption cloud can be determined by using the vertical profile of wind velocity around the volcano. These ideas were applied to the eruption occurred at 19:44 (JST) on September 23, 2004, at the Asama volcano, which produced a pyroclastic fall deposit with a minimum weight of 7.2×10^6kg. Because this eruption occurred in bad weather, the pyroclastic materials fell as mud raindrops that were aggregate particles saturated by the rainwater. Based on the depositional mass, the number of impact marks of the mud raindrops in the unit area, and the apparent density and the equivalent diameter of these drops during their fall was estimated to be 2.2-3.1mm, which is consistent with the grain-size distribution of pyroclastic materials. According to some experienced accounts, mud raindrops several millimeters in diameter fell at 20:03 in the Kitakaruizawa area (about 9km north-northeast from the source). Assuming 2-5 minutes for T_1 and 11.5-12.0m/s of average lateral wind velocity, the height at which the mud raindrops separated from the eruption cloud can be estimated at 3,430-3,860m (3,610m on average). From this conclusion, the transportation and depositional process of the pyroclastic materials generated on September 23, 2004, at the Asama volcano can summarized as follows : the explosion occurred at 19:44 and the eruption cloud rose to 3,610m while blowing 2.49km downwind from the source. The cloud moved laterally for 4.51km with generating raindrops. At 19:54, mud raindrops separated from the cloud 7.0km north-northeast from the source, then fell to the ground at 20:03 after being blown 2.0km downwind by a lateral wind.
2005-12-30

凝集粒子を用いた噴煙高度の推定 : 浅間火山2004年9月23日噴火に伴う降下火砕物の堆積様式(<特集>2004年浅間山噴火(2))

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