1993 Volume 45 Issue 6 Pages 3-12
In 1783, the eruption of Mt. Asama buried Kambara Village, which lay at the northern flank of the mountain, with earth and sand. There was a major mudflow along the Agatsuma and Tone Rivers which are located further down. Thus, it was a ruinous disaster involving more than 1, 400 deaths.
The realities of the transport, descent and deposition of sediments that caused it have been clarified to some extent from, the positions of deposits left there of that time. Soil tests and the grasping of sedimentary structures by test pits and boring excavation, along with the analysis of old records was performed.
The many gigantic essential blocks, each being scores of meters across, that accumulated on the slope at the northern flank of Mt. Asama were inferred from the results of paleomagnetic measurements to have been effused from the crater in the eruption of 1783 and deposited in the high-temperature state of more than the Curie temperature. Also, aerial photograph analysis and field survey have revealed that these essential blocks are distributed from the lava sedimentary area of Onioshidashi to Koyado river area and that large blocks, in particular, a concentration immediately below the lava sedimentary area of Onioshidashi where there is a changing point of gradient. From the test pits we dug at several points along this direction of flow, it was inferred that the flow that transported these essential blocks was a mudflow.
Regarding the origin of the mudflow, it was considered from the analysis of old records and the reading of aerial photographs, that the mudflow originated in or about the hollow located in the lava sedimentary area of Onioshidashi. So we carried out a boring excavation in this area and found that the old ground of the time immediately before the eruption exists about 65 m under the present ground. Assuming that the hollow was then a swamp with a water depth of 50 m and that water in the swamp completely overflowed in the eruption and became a mudflow, this can explain the hydrological balance of the mudflow along the Agatsuma and Tone Rivers. As to what caused the overflow of water from the hollow, many things remain to be seen but, for the present, there can be two alternatives.
(1) A phyroclastic flow started as the result of a summit eruption, which penetrated into the swamp and displaced water from it. or,
(2) water overflowed because a lateral eruption occurred from the hollow.
