秋田県谷地地すべり周辺の地質構造と初生地すべりの発生機構

The Yachi landslide located in the south-eastern part of Akita Prefecture is an active and large-scale one which covers approximately 100ha or more. However, it occupies only a part of the extensive old landslide area in which many scarps, indentations and other irregularities in the topography remain. The study inquestion is underlain by Miocene strata which are mainly composed of hard shale. The strata are complexly folded and the geologic structure of the area has been interpreted until now as a synclinorium. However, it has now been clarified that some parts of the structure were more or less formed by the primary landslide. In this paper, on the basis of the field investigation, the issue of the relationship between the geologic structure and the primary landslide around the Yachi landslide are discussed. The view that primary landslides mainly formed the complicated structures of this area has not necessarily been proven. However, there is evidence that some geologic phenomena and topography which are observed in this area were formed by old rock glides. One example: the strata observed in Nishikozawa which is the type-locality of the Nishikozawa Formation are composed of detached segments of bedrock. Another example: although it was believed that the Narusegawa Fault is a big one and passes through the point at which two adjacent outcrops of strata show completely different dip and strike each other, this abrupt change between them is clearly due to the displacement of an old rock glide. On the basis of this field evidence, the interpretation of the mechanisms of geologic structure and primary landslide in this area can be summarized as follows; 1. Composite and/or other complicated deformations are concentrated on the anticlinal axis zone, and short extended faults which are parallel to the axial trace of fold are recognized. These structures are fundamentally due to gravitation, that is, they are due to the primary landslide, and stress release resulting from river erosion might have been the main cause. 2. The Narusegawa Fault is intercepted by some younger faults (oblique or transverse faults). Therefore, it is not directly related to the formation of geologic structure in the final stage. 3. Some oblique or transverse faults control the slide direction of primary landslides, and each block surrounded by these faults shows respectively different inner structure. 4. It is inferred that the sliding surface of the primary landslide originates from bedding fault and its maximum depth reaches 200m or more. 5. The primary landslide presumably occurred during the period between the formation of Middle Terrace and Lower Terrace. However, there was no over-all reactivation excepting a few subsequent secondary landslides.