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

Abstract

Volcanic fans and volcaniclastic aprons (ring plains) developed at the foot of a volcano preserve the products of high-discharge, sediment-laden lahar flows that originate from a volcano. Geological and geomorphological features of a volcanic fan are important for discussing the flow characteristics and history of far-reaching flows such as lahars. The present study focuses on volcanic-fan deposits at the northern base of Chokai Volcano in northeast Japan. The authors conducted geomorphological analysis, excavation survey at the distal part of the volcanic fan, sedimentary facies analysis, radiocarbon dating, and historical document analysis to understand the flow characteristics and history of far-reaching lahars in relation to changes in the depositional processes and their triggering volcanic activity. Geological observation by manual trenching and coring revealed that the volcanic fan is composed mainly of a series of lahar deposits formed by at least five large lahar events. The 14C ages of the lahar deposits and their correlation with historical documents indicate that the large lahars travelled down the volcanic fan at: the 2nd–5th century BCE (L1); 2nd to 4th century BCE (L2); 5th to 7th century CE (L3); 871 CE (L4); and 1801 CE (L5) after the Kisakata debris avalanche (2.5 ka). According to the stratigraphy, ratio of magmatic fragments and hydrothermally altered material content in the deposits, and correlation with historical documents, these lahar events can be classified into three types in terms of their material source: (1) a cohesive debris flow sourced by the debris avalanche deposit (L1); (2) a non-cohesive lahar by a magmatic eruption (L2, L3, L4, and L5–1 subunit); and (3) a cohesive debris flow by a phreatic eruption (L5–2 subunit). The facies analysis revealed that the different flow behaviors of lahars reflect their sourced volcanic activity. The oldest lahar (L1), incorporating debris avalanche deposit, flowed as hazardous cohesive debris flow; lahars after magmatic eruptions (L2, L3, L4, and L5–1 subunit) tends to be streamflow through the flow transformation of sandy debris flow; and a cohesive lahar (L5–2 subunit) by a phreatic eruption tends to be of small scale. These results indicate that the characteristics of a lahar depend largely on causal volcanic activity.