Relationship Between Geological Structure and Landslides Triggered by the 2007 Mid-Niigata Offshore Earthquake

Abstract

Compared with the case of the 2004 Mid-Niigata Earthquake, by which so many large and middle-scale landslides were triggered, only a few middle-scale ones occurred with many small-scale ones by the 2007 Mid-Niigata Offshore Earthquake in spite of their similar geology and topography [landslide-scale with apparent volume (the Japan Landslide Society, 2004); small-scale: 10²–10⁴ m³, middle-scale: 10⁴–10⁶ m³, large-scale: 10⁶–10⁸m³]. Some of them, however, caused serious damage to the infrastructures. Around the Hijirigahana cape in Yoneyama town located at about 30 km south of the epicenter, some middle to small-scale landslides occurred as a group. Geology of the study area is composed of sedimentary rocks simply dipping to the sea. An open anticline gently plunges to north, and the study area is just located on and around its axial part. Due to the topography ‘cuesta’, both of translational and rotational slides occurred on the cataclinal and orthoclinal slope, respectively. It had been thought that mechanism of those landslides was not so complicated because of the simple structure. During the excavation for the countermeasure of those landslides, however, a vertical fault, which looked like a tectonic one, with some others including bedding faults were found behind the affected area. Although the most hazardous slide occurred on the cataclinal slope was a simple translational one, rotational slide occurred on the orothoclinal slope was a little complicated, and non-tectonic structure due to the pre-historical event was found behind the new slide.

Activities in WG 8 of JLS-ELRP.