Mechanism of Rapid Long Runout Motion in the May 1997 Sumikawa Reactivated Landslide in Akita Prefecture and the July 1997 Harihara Landslide-Debris Flow, Kagoshima Prefecture, Japan

Abstract

Two disastrous long runout rapid landslides caused big disasters in Japan, 1997. One is a reactivated type landslide which traveled over 1.5 km in Akita Prefecture. Another is a rapid landslide-debris flow which killed 21 persons living on a debris fan in Kagoshima Prefecture. The authors took samples in both landslides and simulated the stress condition at the initiation of landslide motion by the stress control ring shear apparatus. The test results suggested that the sliding surface liquefaction of the saturated andesitic debris in both landslides played a key role for the rapid long runout landslide motion. One of the remarkable test results is that the andesitic debris showed a low apparent friction angle of about 5 degrees after failure even in the drained state of shear box and during gradual increase of pore pressure. Excess pore pressure generation in the shear zone after failure was much greater than the pore pressure dissipation, and the permeability in the shear zone was much decreased due to grain crushing by shearing.