Relations between landslide disasters and geological and geomorphological conditions in the central part of Kochi Prefecture (1)

Abstract

The central part of Kochi Prefecture is geologically situated in the Outer zone of South west Japan. Several largescale belt-shaped geological tectonic lines are developed from east-north-east to west-south-west. The distribution of sliding areas and collapsed places reflects this belt-shaped structure of the tectonic lines. Along the tectonic lines such as Kiyomizu tectonic zone and Mikabu tectonic line, a number of sliding areas exist. Since the erosion of the ground proceeds rapidly there, the height of summit level becomes low and the gradient of slopes gentle. The Shikoku mountain ranges have been upheaved by from 500 to 1000 meters since the beginning of the Quaternary, affected by the eustatic movements and climatic changes. Thus, several stages of gentle slopes, in which the secondary deposits are accumulated and the rocks are weathered, are developed. These marginal areas are referred to as “an erosion front”. A number of landslides occurred in these marginal areas a: the attack of the typhoon No.5, 1975 and the typhoon No.17, 1976, and, subsequently, landslide disasters were caused. At the typhoon No.5, 1975 and No.17, 1976, the rainfall zone was almost the same, but great differences were seen in the rainfall intensity and the duration time of rainfall. Thus, the occurrence of landslide disasters at the typhoon No.5, 1975 was greatly different from that at the typhoon No.17, 1976. That is, the typhoon No.5, 1975 was of the medium scale, but it was a summer-type typhoon with heavy rainfall. Landslide disasters were caused by large-scale collapses and mudflows occurred along the tectonic lines. The typhoon No.17, 1976 was of the very large scale. Since the typhoon No.17, 1976 stayed for about one week with a front, the rainfall time was very long and the precipitation reached 50% of the annual precipitation. After the latter half of the typhoon No.17, large-scale landslides occurred in the gentle slopes along the tectonic lines.