Common Geographic and Geological Features of Earthquake-Induced Landslides in Northern Ibaraki Prefecture

Abstract

As a result of the 2011 Great Earthquake off the Pacific coast of Tohoku, four large landslides occurred in northern Ibaraki Prefecture and caused damage that closed roads. These landslides were characterized by the following geological and geographic features: 1, each slide occurred near the tip of a ridge; 2, each ridgeline was almost perpendicular to a line drawn from the landslide to the epicenter; and 3, geologically, each slope was a dip slope of soft Neogene sandstones and/or siltstones. In an earthquake, the ground motion of the upper part of the slope is bigger than that of the lower part of the slope. We can infer that if the tip of the ridge is equivalent to the upper part of the slope, then a large amount of ground motion likely occurred there in the 2011 Great Earthquake; this might have placed fracture stress along the bedding planes of the dip slope, thus triggering a landslide. In future disaster management, we need to be aware of slopes that have the geographic and geological features mentioned above.