Unique geological features behind huge typhoon damage on Oshima: experts
Geological features unique to Izu Oshima Island were key factors behind the massive damage caused by powerful Typhoon Wipha, which triggered heavy flooding and landslides, leaving at least 18 people dead and dozens missing on the tiny island south of Tokyo, experts point out.
Hiroshi Ikeya, specially-appointed professor of erosion control engineering at the National Graduate Institute for Policy Studies, believes that the collapsed sediment of a mountain split into two groups, slid down the slope and directly hit two neighborhoods at the foot of the mountain. On the fact that landslides had caused massive damage in the Motomachi district on the island, Ikeya said, "The fracture energy increases when the earth and sand that collapses on a wide scale flow into a valley and converge there. It is especially conceivable that the speed of movement of the sediment quickly increases in a narrow creek."
Mt. Mihara, a 758-meter-high volcano which has periodically erupted in the past, constitutes the main composition of Izu Oshima Island. Massive amounts of volcanic pebbles and ash that spewed from Mt. Mihara have built up on the island.
Yo Uesugi, professor emeritus at Tsuru University who is knowledgeable about the geological formation of Izu Oshima Island, said, "Generally speaking, even if trees are knocked down by pyroclastic flows caused by a volcanic eruption, new plants grow over several decades to prevent the earth and sand from collapsing. But Mt. Mihara has repeatedly erupted and plants have presumably not grown enough to withstand such a large-scale typhoon as the latest one." Because there is a layer of lava and volcanic ash that does not easily soak up water beneath the layer of pumice-like volcanic stones with lots of holes, "The water-holding ability remains high with a little rain. But excessive rains make it easier for the upper layer such as that of pebbles to slide. There are many slopes on the island whereby the risk of earth and sand being washed out increases further."
Furthermore, because the island, stretching nine kilometers from east to west and 15 kilometers from north to south, is so small there are no obstacles around to block clouds. Ikeya said, "Because there is little distance between the ocean and the mountain top, it creates an environment in which rain clouds maintain their momentum and it can rain heavily."
October 17, 2013(Mainichi Japan)