Historical record of lahar related to phreatic or phreatomagmatic eruption

Abstract

Once the volcano erupted, fallen ash accumulated on the ground flew easily even by slight rainfall and generated lahar, which sometimes caused disaster. Therefore, certain criteria are required to evaluate the possibility of lahar after the eruption. It is, however, not well understood under which process or conditions lahar were occurred in the past. For the purpose of future volcanic disaster prevention, we reviewed the historical records of the lahar in japan related to phreatic or phreatomagmatic eruption. Totally 60 records were investigated, and the process of the lahar could be classified into 12 patterns. The most frequent type was secondary lahar （triggered by rain） and the next was primary lahar （induced by hydrothermal water）. According to the research of rainfall records on four volcanoes, initial secondary lahar may be produced particularly when the rainfall rate was approximately 11-35 mm/hour which was maximum experienced hourly rainfall after the eruption.