Slope Failures caused by the Tokachi-oki Earthquake in the Southeastern Part of Aomori Prefecture

Abstract

On May 16th, 1968, many slope failures occurred in the southeastern part of Aomori prefecture by the severe shock of the Tokachi-oki earthquake after a heavy rainfall of 160-210mm in preceding three days. Most of slope failures were concentrated in the small tributaries of the Asamizu River which had suffered the heaviest rainfall.
Each failure site is composed of erosional part, runway of collapsed materials and depositional part of them. According to the form of its erosional part, each failure is classified into “seauper” or “plane” type. The former is situated in the small shallow depression excavated in the upper gentle slope of valley side dissecting the uplands and the latter on steeper straight slope. Runways are varied in length, but at this time, slope failures are characterized by long-distance transportation of detrital mass in proportion to its volume. The runways on slope surfaces and narrow valley floors are not eroded and covered with a little debris.
Rapid stream of mud with large blocks soaked by water as the result of the shock of the earthquake flew down through the runways and spread over the paddy fields in the valley plain like a sheet or deposited in the lower part of V-shaped valley. Materials connected with these slope failures are, from the upper to the lower, black surface soil, brown subsoil, yellowish brown volcanic ash with yellow pumice grains and greyish white clayey pumice, but underlying coase pumice was not disturbed. The permeability and liquidity of these tephras, as well as their distribution, are the basic factors of the event.