A large number of volcanoes are distributed in Japanese Island. Many types of debris disasters occurred on volcanic regions. Volcanic region have particular geological and geomorphlogical condition. Various types of landslide disaster occurred in volcanic areas. Various researches for such disaster was conducted.
This paper is a review on research on landslide disaster in volcanic region in Japan. This paper describes characteristics of landslide disaster on volcanic regions, historical review of the disasters and researches, recent research findings and task for the future.

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The 1997 Sumikawa landslide suggested that the pre-Yakeyama caldera covered by the younger Yakeyama volcano had played an essential role for the landslide to take place. Recently, about eighty calderas of late Miocene to late Pleistocene in age have been found along the Ohu Backborn Mountains in Tohoku district, northern Japan.
Our studies have revealed that those calderas are responsible for landslides of extremely large-scale in this district. The dissection stage of caldera volcanoes can be divided into three substages, 1) the post-caldera volcano edifice and exotic volcanics erosion substage, 2) caldera lacustrine-deposit erosion substage and 3) caldera-fill pyroclastic (ash-flow) deposit erosion substage. At the first substage, landslides medium to small in scale often observed on post-caldera volcanoes but sagging can be found in some of them. At the second substage, extremely large-scale landslides (-5km wide and-6km long) develop in caller deposits, because of their cap-rock structure composed of volcanic rocks and underlying lacustrine deposits. Combination of grabens and radial landslides of large-scale can be identified in some volcanoes. So many landslides of medium to small scale can be usually found on the surface of the lacustrine deposits that the topography of the lacustrine deposits sometimes becomes extremely irregular. At the third subtage, rockfalls, rockslide-avalanches and debris flows occur, sometimes causing severe disasters. In co-operation with hydrothermal activity play the lacustrine deposits an important role for occurrence of large-scale landslides especially.

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Rockmagnetic studies were conducted on three boring cores drilled at Bandai Volcano of Fukushima Prefecture. The cores include the debris avalanche deposit of 1888 collapse. Debris avalanche deposits in BD 1 and BD 2 cores, with the thickness of 20m and 80m respectively, show a stable remanent magnetization with fairly constant inclination. It suggests that the 1888 debris avalanche deposit was transported without significant deformation, i.e., slid a distance of 1 to 2km as a block to rest on the area that is BD 1 and BD 2 at present. In the BD 3 core at the center of collapse area, the magnetizations of the samples show appreciable deflected and scattered directions from the geomagnetic inclination, suggesting that both the debris and the underlying volcanic sequence have been severely fractured.
Rockmagnetism was also studied on the hummocky hill of Okinajima debris avalanche, 5km from the crater. Eight samples collected from the outcrop (20m high and 80m wide) showed the concentrated direction of remanent magnetization. It indicates that hummocky hill of the Okinajima debris avalanche (collapse at 80-90ka; Suzuki, 1987) was transported 5km as a sliding block.
Thus, the rockmagnetic method is proved to be useful in the study of transportation and depositional process of mega-block debris avalanches as a huge landslide event.

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The 2-D electrical resistivity prospecting (1, 000m in survey depth) was conducted in order to investigate the subsurface structure of the volcanic body and the large-scale collapse site at the Shirataka volcano, Yamagata Prefecture, northeast Japan. As a result of the prospecting, the resistivity structure around the Shirataka volcano shows two layer systems. According to a surrounding geological structure and the geology of borehole cores, the upper layer of high resistivity corresponds to the volcanic product and debris avalanche deposit, and the lower layer of low resistivity corresponds to the Tertiary which is the basement of the Shirataka volcano. The hydrothermal alteration part, recognized as a low resistivity part which is distributed widely from surface to deep in the Bandai volcano, is not found in the Shirataka volcano. This result supports the opinion that an immediate cause to collapse the volcanic body is not volcanic activity but landslides.

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This paper presents that moving directions of volcanic activity induced large-scale slope failures of volcanic edifices have a close relation to regional stress field. Many of the large-scale slope failures were strongly influenced by the regional stress field, and then their moving directions trended sub-vertical to axes of horizontal maximum principal stress of the regional stress field. Under the condition of compression tectonics, volcanic edifices built up on the ridge of highly uplifted mountains were controlled by local extensional stress field, and then the large-scale slope failures took place toward the direction sub-parallel to the axes of horizontal maximum principal stress. In volcanic edifices built up under the condition of extension tectonics, when medium principal stress parallel to the axes of horizontal maximum principal stress changed to minimum principal stress due to stress release caused by volcanic activity, the large-scale slope failures took place toward the direction sub-parallel to the axes of horizontal maximum principal stress.

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The offshore-bottom of Toya Caldera Lakc at Toyako-onscncho, Hokkaido, was studied using detailed bathymetry. Lake-bottom images and subbuttom profiles show that landslide deposits are distributed in the lake bottom 0.8-1km width and over 2.2km length. The furrow surface of the large landslide deposits, similar to that of the Blackhawk landslide in California, can be interpreted as imbricate structure of sliding sediments. The formation age of the landslide deposits is unknown, but the slide was probably induced by crustal deformation or volcanic earthquake at Usu Volcano.

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