Journal of the Japanese Society of Snow and Ice

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Sixty-two avalanches were recorded in Soya District, Hokkaido from 1953 to 1974, which resulted 29 casualties and 49 fallen houses. The maximum yearly frequency of 18 avalanches occurred in 1975. Places of most frequent occurrence were Rishiri Island and Wakkanai City, subsequently Omisaki Coast and Rebun Island, where the houses were located on the sea shores with steep slopes at the back. Most of the avalanches occurred when a developing cyclone approached the west side of Hokkaido and stayed there. From late January to late March, many surface avalanches are caused by snow storms or heavy snowfalls. In the first 10 days of April, ground avalanches are caused by temperature rise and snow melting. The avalanches occurred frequently at 0900-1100 hrs and 1500-1700 hrs and very few at night. When the avalanche occurred either the daily maximum wind speed was more than 10 m/s and the daily maximum temperature was above 0°C, or more than 15 m/s when the temperature was below -4°C.

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An avalanche occurred in Wakkanai City Hokkaido on April 4, 1974, brought loss of two lives and one injured and one house was broken. The actual place was on a steep slope about 80 m high, 37° in average inclination and the upper part higher than 60 m was about 42°, which was facing east in the western part of Wakkanai City. The size of avalanche was 20 to 25 m in width and about 120 m in length. Before the avalanche, the slope was covered with snow of about 1 m deep, and some snow cornices formed by strong westerly monsoon existed near the top of the slope. A parting of snow cornice which triggered the avalanche was caused by continuous warming of 2 days before the accident. The surface weather map at 0900 JST on the day indicated that Soya district was situated in the west of developed migrating anticyclone and a weak cyclone was located in the northern Japan Sea, exhibiting a so-called “south-high-north-low” pressure pattern.

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