Comparisons of Landslide Densities between Rock Types in Weathered Granitoid in Obara Village, Aichi Prefecture

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  • TOBE Hayato
    Department of Geophysics, Graduate School of Science Kyoto University
  • CHIGIRA Masahiro
    Disaster Prevention Research Institute, Kyoto University
  • DOSHIDA Shoji
    Department of Geophysics, Graduate School of Science Kyoto University

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Other Title
  • 愛知県旧小原村の風化花崗岩類における崩壊発生密度の岩相関での比較
  • アイチケン キュウ オバラムラ ノ フウカ カコウガンルイ ニ オケル ホウカイ ハッセイ ミツド ノ ガンソウカン デノ ヒカク

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Abstract

In 1972, numerous numbers of shallow landslides were induced in granite and granodiorite areas by a heavy rainstorm in Obara village, Aichi prefecture, central Japan. We examined quantitatively the effects of petrologic types and precipitation amounts on the landslide distribution and densities in the disaster area. The landslide density in granite area was 293/km2 and was larger than ten times of that in granodiorite area (13/km2), even though both the areas had experienced a strong precipitation of about 200mm per 5 hours. Landslide densities increased with the amounts of precipitation in the granite area, but did not in the granodiorite area. Investigation with airborne laser scanner and aero photographs was successively applied to detect landslides by the 1972 rainstorm and also preceding landslides in an area of 3km2, which indicated that the above contrast of landslide densities had already existed before the 1972 disaster. The differences in landslide densities and in increase of landslide density according to the amount of precipitation between granite and granodiorite areas could result from the difference in weathering profiles of these two rocks. The most widely distributed granite in the surveyed area is weathered granite with grade DH, (decomposed granitoid) and/or DM (moderately weathered gruss), which likely is loosened with a well defined front and provides preferable conditions for landslide. In addition, a certain type of granite is micro-sheeted in the surveyed area, also providing a preferable condition for landslide occurrence. On the other hand, the most widely distributed granodiorite is of grade DL (strongly weathered saprolite), of which granodiorite is not likely loosened with a well-defined front. This may be the reason why fewer landslides occurred in the granodiorite area.

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