Landslides have become one of the most important matters of concern because of the physical characteristics of our country and the frequent occurrence of landslide in recent years. The studies of landslides have not been fully made because of their momentary nature of occurrence. Kobe and Kure Cities suffered from great disasters of this kind not only once but twice during the last 15 years. On 28 of June, 1953, in the northernmost peninsular part of Kyusyu including Moji City, there took place 619 landslides and countless slides of a small scale, which killed 137 persons, injured 477 persons and damaged 2533 houses. The writer made an investigation of these landslides. The writer subdivided the areas affected into three zones: the zone of surface breaking, the. intermediate zone of motion, and the zone of deposition. However, since the zone of surface breaking was the basic zone from the standpoint of prevention of disasters, be has concentrated his study on that zone. The principal facts revealed by the the writer are as follows:
(1) The gradient of slope where the landslides occurred most numerously is 30° to 35°, although the genesis of a landslide depends upon not only the gradient but the amount of rainfall, the angle of repose of the surface soil, and the change of gradient in the slope, etc. The gradient for maximum occurrence of landslides in the area studied around Moji is lower than that observed in Kobe area. (Fig. 1)
(2) The dimension of the zone of surface breaking, is not so large as in the cases preveously reported. It is liable to be overestimated owing to the fact that the out-wash debris often covers an extensive area. The type and scale of the landslides in the Schalstein regions of the area (i. e. the Kazashi Mountainland) are similar to those in the Granite region along the River Watarase in Northern Kanto reported by Mr. Ichikawa. They are of a wide and deep type (not of a long and shallow type), but are more or less different from those in the Palaeozoic region (i. e. the Tonoue Mountainland) of the area studied. (Fig. 2).
(3) As it is of a wide and deep, type, the quantity of the debris is commonly large, being 1500-2000m
3 in cases of landslides from slope of 30°-35°. In cases where the gradient is over 35°, its quantity is less than those above-described. (Fig. 4).
(4) If the vegetation is taken into consideration, it can be said that landslides occurred most numerously in the area covered with Japanese reed-grass. Dwarf deciduous tree cover seems to have little resistance against slides. That there is an extensive area under Japanese reed-grass is regarded as an important factor in the multiple occurrence of landslides. On the other hand, the quantity of the debris which slides decreases as the age of the forest trees and their density increases.
5) The density of surface breaking is in proportion to the altitude in the respective regions. The density is especially high in the north-eastern hills chiefly composed of shale, schalstein, sandstone, and in the Kazashi Mountainland chiefly made up of schalstein, diorite, clayslate. It is lower in the Tonoue Mountainland composed of Palaoezoic rocks.
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