北海道常呂郡留辺蕊町金華地すべり地域における地すべりハザードマッピング

Abstract

Seven ancient slide topographies in the Kanehana landslide area have been identified as concave polyhill- or concave monohill-type.The geology of this area consists primarily of the Cretaceous-Paleocene Yubetsu Group, the Upper Miocene Ikutawara and Yahagi Formations, Late Miocene-Early Pliocene rhyolite, andesite and basalt dikes, Lower Pliocene Komatsuzawa and Hakugindai Lavas, and Quaternary System. The Quaternary System consists of the Rubeshibe Formation and pumice flow, river terrace, talus, slide, alluvial fan and recent fluvial deposits. The Ikutawara and Yahagi Formations consist primarily of clastic rocks, acidic volcaniclastic rocks and rhyolite lavas. The rocks of the Ikutawara and Yahagi Formations and rhyolite and andesite dikes have been affected to various extents by hydrothermal alteration related to Late Miocene-Early Pliocene epithermal goldsilver mineralization.The hydrothermal alteration zones are divided into smectite, zeolite, propylitic, interstratified illite/smectite minerals, illite, K-feldspar, steam-heated alteration and advanced argillic alteration zones, based on the mineral assemblage of hydrothermally altered rocks. The seven ancient slides and the Kanehana-Toge slide occurred in the hydrothermal alteration zones, and were mainly related to the smectite zone.The slope instability evaluation system for slides used here is based on topography, slope geology, geologi-cal structure and the type of hydrothermal alteration according to 1: 5, 000-scale topographic and geologic mapping of the area. Using this evaluation system slope areas can be classified into four ranks ; stable hard-rock slope (class I), stable soft-rock slope (class II), potential slide area (class III) and ancient slide area (class IV). Based on the evaluation, a slide hazard map of the area was created.