Reexamination of Eruptive Activity of Akanfuji in the Me-Akan Volcano, Eastern Hokkaido, Japan

Abstract

Eruption history of Akanfuji in the Me-akan volcano, eastern Hokkaido, has been examined by comparing multiple natural outcrops. In a previous study, at least 17 layers of scoria fall deposits were recognized. To obtain more detailed geological information and reexamine the eruptive activity of Akanfuji, we conducted trench surveys. At each survey site, the scoria fall deposits from Akanfuji are layered with a total thickness of several tens of centimeters to 1 m. A light brown volcanic ash layer is deposited just under the lowest layer of Akanfuji, and the fresh glass in the light brown volcanic ash shows the glass composition of the volcanic ash that erupted at Tarumai volcano 2500 BP. In addition, volcanic ash ejected from Ponmachineshiri of the Me-akan volcano is deposited on top of the newest deposits in the Akanfuji series with a thin soil layer in between. The volcanic ash ejected from Ponmachineshiri contains patches of volcanic ash from the Mashu volcano that erupted 1000 years ago. Therefore, Akanfuji was active from about 2500 to 1000 years ago. Akanfuji ejected scoria fall deposits at least 17 times (Akf-1 to Akf-17), and assuming that it erupted an average number of times during the activity period, it would be once every 90 years. The eruption rate was estimated to be 0.12 km3 DRE/ka.