Gentle-slope movements induced by the 2003 Tokachi-Oki Earthquake, in the Kyowa area of Tanno Town, Hokkaido, Japan

摘要

The 2003 Tokachi-Oki Earthquake caused extensive ground collapses accompanied by slightly lateral displacement . subsidences, and lateral spreads of gentle slopes in the Kyowa area of Tanno Town, situated about 230km away from the epicenter. The direct inducement of such slope movements was the liquefaction of the volcanic ash that had artificially covered the valley. Especially, at Locality No.1 in the Kyowa area, liquefied volcanic ash of about 10, 000m3spouted from the slope to the sides, and a wide farmland collapsed. Such large ground collapses caused by lateral jetting of the liquefied volcanic ash are new phenomena and new landslide disasters induced by the earthquake.The old landforms of the liquefied and moved area were narrow waste-filled valleys and swamps formerly utilized as paddy but these were converted into the gently-sloping farmland by the land fill of the volcanic ash. Though the field of similar landform transition and ground condition is large in number, it is characteristic that the slope movements caused by the liquefaction of volcanic ash were confined to the fields where the reclaimed planes tilted to the south or south-southwest. In addition to, the dip direction of the reclaimed planes coincided with the direction of the main lateral displacement of the surface in each liquefied area. The dip direction of the reclaimed planes might have brought about the geomorphic effect.