Stratigraphy, chronology, and depositional processes of lahar deposits during the last 2500 years at the northern base of Chokai Volcano, northeast Japan

Minami, Yusuke*; Oba, Tsukasa*; Hayashi, Shintaro*; Kokubu, Yoko   ; Kataoka, Kyoko S.*

A volcanic-fan developed at the northern base of Chokai Volcano in northeast Japan. To understand the flow characteristics and history of far-reaching lahars occurring in the volcanic fan, we carried out excavation survey at the distal part of the volcanic fan, sedimentary facies analysis, and radiocarbon dating. Geological observation revealed that at least five large lahar events occurred. The C ages of the lahar deposits and their correlation with historical documents indicate that the large lahars travelled down the volcanic fan at: the 2nd - 5th century BC; 2nd to 4th century BC; AD 5th to 7th century; AD 871; and AD 1801 after the Kisakata debris avalanche (2.5 ka). These events can be classified into three types: (1) a cohesive debris flow sourced by the debris avalanche deposit; (2) a non-cohesive lahar by a magmatic eruption; and (3) a cohesive debris flow by a phreatic eruption.