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The Current State of the Osawa Failure

The Osawa failurecollapse, which is one of the largest scale collapse areas in Japan. It exists on the west slope of Mt. Fuji between the summit and 2,200 meters, 2.1km in length, 500m maximum width, 150m maximum depth, 1km2 collapsed area, and about 75,000,000km3 of collapsed soil (as large as 60 Tokyo Domes). It has been producing a large amount of debris and brought debris flow. Debris flows were often hit to the downstream area of the Osawa River.
It is not clear when the Osawa Failure had been formed, but the results of the analysis on an old piece of wood buried in debris, was said to be about 1000 years ago.
Figure 1 shows the expansion of the collapsed area in the past 34 years. The collapse of the Osawa failure collapsing is still in progress and 15,000,000 debris in an annual average are flowing out.

Photo 1 (right photo) is a picture near the Osawa failure at an altitude of 3,500m. The steep cliff is more than 100m in height with some overhangs.
Photo 2 (below) is the dried riverbed in the vicinity at an altitude of 2,200m. The scoria layer is eroded, the lava layer is already collapsed partially, and the lava also has a 2m or more overhang in height.
Photo 3 (bottom right) is a riverbed in the vicinity at an altitude of 1,900m (waterfall No 6).


The mechanism collapse

The geological feature in the Osawa failure is the alternate layer structure like sandwiches. The soft scoria layer (ejecta such as volcanic ash) and the hard lava layer are overlapping alternately.  
Wind, rain, and change in temperature affects the soft scoria layer and the weathered layer to collapse. After a while, the hard and heavy lava layer would collapse.  
The phenomenon like this is repeated without limit, causing the collapsed areas to spread. These collapsed rocks and sands accumulate in the valley, bottom of the Osawa River, and flow out to the downstream at the time of sediment flow generation.