Abstract
The Izu-Oshima 2013 lahar occurred on the western slope of Izu-Oshima volcano in response to heavy rain brought by the 26th Typhoon (WIPHA) on 16 October 2013. The lahar damaged Motomachi village, resulting in many fatalities. We immediately carried out field surveys and compared the grain-size distributions of this lahar deposit with those of various reference lahars, including the Unzen 1993, Pinatubo 1991-1995, Bandai 1888, Izu-Oshima S2-c, Adatara-Sukawa L1, and Numazawa-Numazawako lahars. Based on the grain-size analysis, we divided these deposits into three types: cohesive mud flow, non-cohesive debris flow, and hyperconcentrated flood flow. The Izu-Oshima 2013 lahar deposit consists of moderately sorted, coarse- to medium-grained sand with pebbles and well-sorted fine- to very-fine-grained sand. The bi- and uni-modal grain-size distributions of this deposit suggest that the lahar was emplaced as a hyperconcentrated flood flow. This lahar was generated by a shallowseated slope failure within ash fall deposits covering the Y5-stage collapse wall, and it cascaded turbulently down to Motomachi village at high speed.
