史料解読による浅間山天明三年（１７８３年）噴火推移の再構築

Abstract

An immense quantity of the ancient literature exists on the great eruption of Asama in 1783. They were written on the northern flank of the volcano, Joshu, and also on the southern flank, Shinshu, including some diaries written in Kanazawa, Edo (present Tokyo), Nagoya and as far as Kyoto (300km SW). They have been compiled by Susumu Hagiwara exhaustively for forty years and published recently. We read them critically to reconstruct the sequence of the eruption. The first explosion was noticed on May 8, by the Gregorian Calendar. The volcano had been erupting at the summit crater intermittently since. On August 2, the tephrafall from a Plinian eruption column became so intense and continuous in Shinshu that the inhabitants prepared for flight. In the afternoon of August 4, the Agatsuma pyroclastic flow spread northward up to 8 km from the summit. Coincidentally, hot lahars, caused by slumping of steep slopes thickly covered with fallout pumice, had successively surged along the Yukawa river southward to Kutsukake (present Naka Karuizawa), where a house was overwhelmed and another flooded under floor. The climactic phase attained late at the night and maintained until the morning of August 5, during which the Oni Oshidashi lava overflowed from the northerm lowest rim of the summit crater. At 10 a.m. of August 5, a part of the northern flank suddenly collapsed, probably triggered by a strong earthquake. It gave rise to the generation of debris avalanche which destroyed Kambara village. At the same time, the flank failure resulted in an explosive decompression of the inner massive part of Oni Oshidashi lava flow to produce a Pelean nuee ardente. The debris avalanche turned into a hot lahar after it cascaded into the Agatsuma river. It caused a disastrous flood on the lower part of the river and reached Edo at 2 p.m. of the same day. A large-scale collapse occurred reportedly at 8 a.m. of August 1 at Kananuma village may not be a fact.