Multiple slump landslides in Onikobe Basin, Miyagi Prefecture, Japan

Many landslides have developed at the end of low relief surface consisting of soft clay deposit (Late Pleistocene) in Onikobe basin, northwestern area of Miyagi Prefecture. In this paper the authors report microlandforms, landform process, occurrence condition and age, and indicate one of the landslide types, such as multiple slump.The results are summarized as follows.1) Landslide in northwest of Kitataki consists of microlandform units such as main scarp, some blocks (Kimata and Miyagi 1985) and secondary scarps. It is clarified that blocks have been formed by slumping and this landslide is multiple rotational slips (Hutchinson 1968) or multiple slump (Hatano et. al. 1974) type.2) The 14C age of wood which was obtained from landslide deposit of Kitataki is 3, 130±240y. B. P.. And some lower terrace surfaces in this area which have emerged in 1, 400-500y. B. P. (Omoto 1992) are overlain by landslide deposits. It appears from the above that landslides in and around Kitataki have been mainly active since middle Holocene, 4, 000-3, 000y. B. P. and have continued.3) Soft clay deposits in this area are high moisture content and slope of this clay was expected to be unstable once lateral support had been lost resulting from downward erosion.