Geologic and Petrologic Characteristics of the Lahar Deposits at the Western Foot of Zao Volcano

The lahars are one of the most hazardous volcanic phenomena causing the third greatest causalities among the volcanic hazards since the 16th century worldwide. Lahars can flow down a long distance and cause tremendous disaster at the foot of volcanoes often beyond the areas of primary volcanic impacts of pyroclastic fall and pyroclastic density currents. Therefore, the research on lahar history of active volcanoes approaching from an analysis of a geological record in distal volcanic regions is significant for lahar hazard risk evaluation. Zao volcano has high risks of future eruptions, because volcanic tremors have been detected since 2013. The presence of a crater lake at the summit area, and steep slopes at the high altitude of Zao indicates high potential energy for future lahars, if triggered by an eruption starting underneath the crater lake. This study firstly reports the existence of lahar deposits at the western foot of Zao and discusses the depositional features and the origin of these as well as the lahar hazard risk at this volcano. The lahar deposits were exposed during the archaeological excavation of the Fujiki ruin, western foot of the Zao volcano. Two major lahar units, L1 and L2, were recognized. Based on the 14 C dating and stratigraphic relationships, the ages of units L1 and L2 were estimated to be