While another Fuji-san eruption could cause a lot of damage, the one that everyone fears is a major eruption of the very active Asama-san, which is not that far northwest of the small city of Karuizawa. If we have a major eruption of Asama-san, the major ashfall would affect the Kanto Plain itself (e.g., Tokyo/Yokohama/Chiba) and would effectively cut off Shinkansen service on the Nagano, Jōetsu, and possibly Tōhoku lines. That's why Asama-san is so heavily instrumented to give a lot of warning of a major eruption.