浅間火山天明噴火 : 遠隔地の史料から明らかになった降灰分布と活動推移

概要

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The 1783 activity of Asama volcano was reviewed from May to the end of the year based on 166 old documents, including those recorded at a distance. 1. Prior to the 1783 Asama eruption, the level of magma head ascended at Kama-yama crater-pit. Moderately explosive eruptions commenced on May 9, and repeatedly blew off the plugged magma. 2. Depending on the wind direction, ash fell N, NNE, and NE of the crater including Sado Island, Tohoku and Kanto districts. From August 3 to 5, climactic plinian eruption dispersed pyroclastic materials. Distributions of 8 tephra- fall units were presented. 3. The timing of rumbling and quakes at distant places farther than 100km from the crater well correspond with explosive events witnessed by neighbors of the volcano. 4. Duration of a single eruptive event rarely exceed 6 hours. It was true even during the culminating plinian stage from Aug. 3 to 5, 1783. The eruption was so violent in this stage that huge blocks larger than 10m were thrown from Kama-yama crater. 5. Documents concerning with Kambara pyroclastic flow and subsequent debris avalanche occurred on August 5, suggested that an explosion on the northern flank triggered collapse of northern sector. The event occurred at about 08:00 to 08:30am, which is 90 to 120 minutes earlier than estimations appear in previous work. 6. Small and less frequent eruptions continued until January 15, 1784.
2011-06-30