Role of geological structure in the occurrence of earthquake-induced landslides, the case of the 2007 Mid-Niigata Offshore Earthquake, Japan

The landslides induced by the Mid-Niigata Offshore earthquake in 2007 (MJMA6.8) occurred mostly along the shore of the Japan Sea. This study focused on the landslides that occurred near the Hijirigahana Cape, Yoneyama Town in the Niigata Prefecture. The study area is located in the western region of the central Niigata Prefecture, along the Japan Sea. The geology of the landslide area is dominated by late Miocene sandstone-rich member of alternating beds of sandstone and siltstone. These strata gently dip to the north at approximately 25° to 30° and form cataclinal and orthoclinal slopes on the north- and west-facing slopes, respectively. The central Niigata region is one of the most landslide-prone areas in Japan. The study area is located approximately 30km from the epicenter of the earthquake with an estimated intensity of 6 lower in the JMA (Japan Meteorological Agency) scale. A group of landslides occurred in the study area. The area contains the most concentrated set of landslides from this earthquake, even though it is located far from the epicenter. Most of the landslides occurred on north-facing slopes, but two occurred on west-facing slopes. The landslides on the north-facing slopes are characterized as translational slides. The largest rock slide in the study area, Unit D (width 100m, length 230m), slipped along the bedding plane of the strata. Observations from slickenlines from the slip surface and outcrop in the upper portion of Unit D suggest that before this earthquake, landslides frequently occurred on the north-facing slopes and were controlled by the bedding plane. With the exception of the largest one, other landslides on north-facing slopes were mostly debris slides, indicating that sediments from older landslides exist. The second largest landslide occurred on a west-facing orthoclinal slope. A rotational rock slide, Unit A, was 80m wide and 100m long. During the excavation after the landslide, a high angle fault with a SW dip was identified behind the crushed zone. Based on field observation, the fault was considered to be a non-tectonic fault and created by gravitational movement. During the excavation, the crushed zone was identified on the cutting wall of the slope. These results revealed that the relatively larger rotational rock slides occurred because of the existence of the crushed zone due to older events that caused the slippage along a non-tectonic fault. •Field observations to Mid-Niigata Offshore earthquake triggered land