2008 年 60 巻 5 号 p. 23-30
A volcanic mudflow chronology spanning the past 40, 000 years was developed for the Furano River, which is located on the foot of Mt. Tokachi, using stratigraphic analysis, radiocarbon dating, dendrochronology and historical archives. Mt. Tokachi is one of the most active volcanoes in Japan, and its volcanic eruptions have often caused large-scale mudflows like the 1926 event that caused 144 deaths along the Furano and Biei Rivers. To develop local programs for preventing volcanic disasters, knowledge of past disasters accompanied with volcanic mudflow chronologies are required. We have identified 14 total mudflows during the past 40, 000 years that occurred along the Furano River, including events in A.D. 1926, AD. 1857, AD. 1740, A.D. 1150-1690, A.D. 600-900, 1, 900 YBP, two in 2, 000-2, 300 YBP, 3, 200 YBP, 3, 500 YBP, 3, 700 YBP, 7, 500 YBP, 13, 500 YBP, and 39, 000 YBP. Some of them were also crosschecked with historical archives. Of the fourteen mudflows, a total of 11 occurred during the past 4, 000 years, and 8 occurred during the past 2, 000 years. These high-frequency mudflows aggraded the valley channel and riverbed to the elevation of Kami-Furano City's central part, which was not damaged by the 1926 mudflow. Therefore, the next mudflow will easily reach the city's center and increase overall damage. It is generally suggested that the frequency and magnitude of volcanic mudflows will work synergistically in increasing the risk of sediment disasters.