Lahar record during the last 2500 years, Chokai Volcano, northeast Japan; Flow behavior, sourced volcanic activity, and hazard implications

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

The study focuses on volcanic-fan deposits at Chokai Volcano in northeast Japan. The authors conducted sedimentary facies analysis, radiocarbon dating, and historical document analysis to understand the flow characteristics and history of far-reaching lahars. The volcanic fan is composed mainly of a series of lahar deposits formed by at least five large lahar events at: the 2nd - 5th century BC; 2nd to 4th century BC; AD 5th to 7th century; AD 871; and AD 1801 during last 2500 years. These lahar 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. The lahar, incorporating debris avalanche deposit, flowed as hazardous cohesive debris flow; lahars after magmatic eruptions tends to be streamflow through the flow transformation of sandy debris flow; and a cohesive lahar by a phreatic eruption tends to be of small scale.