Mechanism and future risk of slope instability induced by extreme rainfall event in Izu Oshima Island, Japan
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Mechanism and future risk of slope instability induced by extreme rainfall event in Izu Oshima Island, Japan Ikuo Towhata1 · Satoshi Goto2 · Shigeru Goto3 · Takeshi Akima4 · Junya Tanaka4 · Taro Uchimura5 · Gonghui Wang6 · Hiroshi Yamaguchi7 · Shogo Aoyama8 Received: 27 December 2019 / Accepted: 11 September 2020 © Springer Nature B.V. 2020
Abstract A volcanic slope in Izu Oshima Island in Japan experienced a profound rain-induced disaster in October 2013. Since this slope had been stable for centuries except for minor failures, a special investigation was carried out on the cause. Because of its volcanic origin, the failed slope consisted of layers of ash, sand and lava. While the investigation concerned many disciplines, the present paper addresses one part of the geotechnical studies. The particular emphasis was put on the reason why some part of the slope “did not” fail because the post-disaster construction of infrastructures in the affected area relied on the future stability of the affected mountain slope. In line with this, another focus of the study was on the geohydrological feature of the underlying lava layer that possibly controlled the instability of the volcanic slope. It was concluded finally that the lava layer is pervious and allows drainage of infiltrated rainwater and that, only during extremely heavy rain, the subsurface lava topography triggered slope failure only in its “valley” parts. For 7 years after the disaster, the remaining parts of the slope have been stable as judged and many local infrastructures have been reconstructed. Keywords Rainfall-induced disaster · Volcanic slope · Lava · Seepage · Restoration
* Ikuo Towhata [email protected] 1
Kanto Gakuin University, Yokohama, Japan
2
Yamanashi University, Kofu, Japan
3
Waseda University, Tokyo, Japan
4
Fudo Tetra Corporation, Tokyo, Japan
5
Saitama University, Saitama, Japan
6
DPRI, Kyoto University, Uji, Japan
7
Chuo Kaihatsu Corporation, Tokyo, Japan
8
University of Tokyo, Bunkyo City, Japan
13
Vol.:(0123456789)
Natural Hazards
1 Introduction The volcanic Izu Oshima Island is located in the Pacific Ocean, 120 km south of Tokyo (Fig. 1), and is 9 km in EW and 15 km in NS directions. The central part of this island is occupied by active Mihara Yama Volcano of 758 m in height (Fig. 2). The nature in this island is preserved as a national park. This island has been prone to two types of natural disasters caused by volcanic activities (Koyama and Hayakawa 1996) and typhoon-induced rainfalls. Figure 3 shows the history of eruption of Mihara Yama volcano. In the recent history, the events in 1684–1690 and 1777–1792 produced extensive lava flows that reached the sea and induced substantial damage. Another event in the fourteenth century (most probably in 1338) caused lava flow to the west of the mountain that reached the sea as well (Nakamura 1964; Disaster Prevention Board of Tokyo Metropolitan Government 1990). As a consequence of repeated volcanic activities, the mountain slopes in this island are com
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