South facing Karasawa Daishamen bowl and the nearby Goryu ski area, showing boundaries of January 28 avalanche. This area is not a part of the Goryu ski area, and riding there is actively discouraged by the ski area. Base photo courtesy of Hakuba Goryu Ski Patrol.
South facing Karasawa Daishamen bowl and the nearby Goryu ski area, showing boundaries of January 28 avalanche. This area is not a part of the Goryu ski area, and riding there is actively discouraged by the ski area. Base photo courtesy of Hakuba Goryu Ski Patrol.
Skiers' right flank, looking downslope on our approach.
Dave Enright points to the weak layer sandwich of 10 cm of weak, sugary faceted grains over a 0.5-0.6 cm faceted melt form thaw crust with 3 cm of faceted grains below it. The slab was all the storm snow from the previous five days of heavy dry snowfall. Our profile site was on a ridge at the skiers' right edge, near the probable trigger point but in a relatively thinner and less-southerly facing area than most of the starting zone.
A team from the Japan Avalanche Network works on a profile on the central portion of the crown face.
A compression test column fractures on the weak layer that caused the slide. Test values were moderate, and fracture character ranged from sudden planar and sudden collapse to resistant planar. Notably though, they were mostly clean and fast, Quality 1 shears. Those and the sudden collapse and sudden planar fractures are associated with human-triggered avalanches even when test values are otherwise relatively high. This block did not slide out, probably because of the lower slope angle as we dug back from the crown, which was right on 40°. Interestingly, fracture propagation tests did not indicate propagation, but increases in strength and decreases in propagation 24 hours after a slide occurs are not unusual.