GEOMORPHOLOGICAL ASPECT OF LANDSLIDE AND TIME CHANGE OF RAINFALL CHARACTER IN THE SOUTH-WEST PART OF ROKKO-SAN MOUNTAINS

Severe landslides at the south-west part of Rokko-san mountains in 1938, 1961 and 1967are discussed and a time series of rain fall character which concerned the landslides is analyzed.From statistical analysis of the landslides and topography in 1967, it is shown that the numberof landslides per unit area increases till the critical angel (31) as the average slope angle in thearea increases. It is also shown that the number of landslides increases till the number of 1st orderstreams and the drainage density become the critical points, 105/km2 and 15 km/km2, respectivelyand it decreases above the values.The pattern of distribution in the number or the total area of landslides does not change in theseveral decades from the data of landslides in 1938, 1961 and 1967.Annual landslide rainfall is introduced as one of the rainfall characters which is the annualsummation of excess daily rainfall which is the excess quantity over 100 mm/day. The result ofharmonic analysis for the rainfall shows that the big extrema of the harmonic function are corre-sponding to the ages when severe landslides happened and the values also correspond to the scalesof the severe landslides.