Distribution of Landslides and its Genetic Machanism in the Kano Valley

Abstract

Japan forming a part of the circum-pacific orogenic zone is rich in various kinds of volcanoes in their shapes and structures. This study has been done with the view of clarifying some characters of lanslides occurred in such volcanic areas, in relation to the topographical and geological feartures. Landslides and debris flows in the Kano valley caused by the Kanogawa typhoon are characteristics of landslide arising from the volcanic area. The characters of these landslides and outflow of the debris are summarized as follows: (A) Landslides occurred on and around the contact zone of the two different rocks. This is due to the fact that the Izu Peninsula is composed of weak formation and covered with various kinds of volcanic ejecta. Therefore, the distribution and structure of geological formation, by which Izu Peninsula is composed of, are considerably complicated. Consequently these landslides occurred by the divergence of physical properties to water between the upper formation and the under one, by which the mountain slope is composed of. (B) Landslides caused by the exfoliation of the weathering materials. The outbreak of these landslides is due to the fact that fair part of the geological formation, by which Izu Peninsula is composed of, had been mollified by severe metamorphism or by weathering. (C) Landslides and debris flows occurred in ancient times. The outbreak of these landslides is closely related not only to the condition of (C) but also to the conditions of (A) and (B). In this region, large scale of volcanic activities and earthquakes have occurred so frequently that the destruction of volcanic mass, the formation of earthquake fault and landslides or landcreep caused by them have repeatedly occurred. It is due mainly to that reason that the Izu Peninsula is apt to cause the outbreak of landslide. Therefore, a portion of the deposit of landslides and debris flows occurred in ancient times (perhaps in the prehistorical and historial ages) gave rise to landslides much like to the landcreep and landslip recurred on the deposit of the old ones. And owing to the outbreak of these collapses, great quantities of debris and gravels were carried to the river floor. (D) Lateral erosion by the flood-streams, and the outflow of debris and gravels. As a result of lateral erosion by the flood-stream and the outbreak of debris flows, the river floor was supplied with great quantities of debris and gravels. And also in the lower part of the Kanovalley, the fine grained materials ejected by volcanoes originating in various landslides as mentioned above were deposited in rich amount. Judging from the characters of landslides and the outflow of debris and gravels as mentioned above, it can be said that these natures are characteristics of landslides arising from volcanic area, especially in the green tuff region where severe volcanic activities have continued on and after the Miocene epoch.