Investigation of volcanic deposits using a combined penetrometer-moisture probe: Application in Izu-Oshima Volcano, Japan

On 15-16 October 2013, Typhoon No. 26 (Wipha) struck Izu-Oshima Island, bringing record-breaking levels of rainfall (24-h precipitation>800 mm) and causing landslides that resulted in more than 40 casualties from debris downstream [Ishikawa et al ., 2014]. Shallow landslides in the large upstream area of the Okanazawa watershed were a major cause of the disaster. The Mount Miharayama volcano is located on Izu-Oshima Island and the collapsed slopes of the upper Okanazawa watershed consist of alternating layers of tephra (containing ash and scoria) and loess deposits, formed by a series of eruptions that occurred in historic times [Koyama and Hayakawa , 1996]. The tephra was deposited by eruptions and the loess is eolian dust that accumulated during the intervening periods. Some reports suggest that the landslides are attributable to the peculiar geology of volcanic regions as follows. The depth of slip surface is 0.5-2.0 m and generally less than 1.0 m, which corresponds to the upper surface of the loess layers. The loess layers act as a hydraulic aquiclude, leading to an increase in pore water pressure within the upper soil layers during heavy rainfall [Ministry of Land, Infrastructure, Transport and Tourism, Japan , 2015]. Hotta et al . [2016] studied the upper Okanazawa watershed and observed significant increases in pore water pressure at the outside edge of a landslide. These increases occurred during heavy rainfall and in the upper part of Original Article