*Teruki Oikawa1, Mitsuhiro Yoshimoto2, Yoshihiro Takeshita3, Fukashi Maeno4, Jiro Komori5, Setsuya Nakada9, Taketo Shimano10, Hisashi Sasaki6, Kishimoto Hiroshi6, Tatsuro Chiba6, Yasuhiro Ishimine7, Kae Tsunematsu8, Yoshihiro Ishizuka1
(1.GSJ, National Institute of Advanced Industrial Science and Technology , 2.Mt. Fuji Reserch Institute, Yamanashi Prifectural Goverment, 3.Faculty of Education, Shinshu University, 4.Earthquake Reserch Institute, University of Tokyo, 5.Teikyo Heisei University, 6.Asia Air Survey Co., Ltd., 7.Reserch and Education Center for Natural Hazards, Kagoshima University, 8.Yamagata University, 9.National Reserch Insitute for Earth Science and Disaster Resilience, 10.Graduate School of Environment and Disaearch, Tokoha University)
Keywords:Mt. Ontake, eruption, ejected material, distribution density, damaege, hazard mitigation
The 2014 eruption of Ontake Volcano, central Japan killed 56 people for ballistic blocks. We entered near the crater after this eruption, investigated the size and distribution density of ballistic blocks. This eruption formed the new craters extending about 0.8 km from the northwest to southeast. From the distribution of ballistic rocks, it became clear that they were released from the new craters located in the Jigokudani. It is within 0.9 km radius from its new craters that these ballistic blocks are distributed high densely (> one/m^2). The area where fatalities occurred from this eruption was consistent with the area where such ballistic blocks fell at high density. The size of the largest ballistic rock is 96 × 70 × 64 cm. It is commonly observed in other phreatic eruptions in the Japanese Islands that ballistic rocks fall at high density within about 1 km from the crater. Therefore, in order to prevent severe damage of ballistic rocks due to phreatic eruption, it is necessary to leave 1 km from the crater.