Volcanic mudflow chronology in Furano river of Mt. Tokachi, Hokkaido
Tomoyuki NANRI, Tomohiro MAKINOU, Koh YONEKAWA, Norikuni HARADA, Hiroshi
ANDO and Takashi YAMADA
Abstract
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, A.D. 1857,
A.D. 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 cross]checked 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.