Geomorphological characteristics of mountain slopes with landslides induced by rainfalls after the Iwate-Miyagi nairiku earthquake in 2008

Abstract

  Here, we investigated the topographic characteristics of slopes where a rainfall-induced landslide had occurred after the Iwate-Miyagi Nairiku Earthquake on June 14, 2008. We compared shaded relief maps, contour maps, and cross sections from LiDAR data measured immediately and 3 months after the earthquake. The landslides were caused by rainfall less than those observed in usual rainfall-induced landslide occurrences. From a comparison of the maps, we observed that prior to landslide occurrence, oval or horseshoe-shaped gentle slopes had formed on the slope after the earthquake and cracks had appeared on these slopes. Similar micro-topographies were observed and checked by field survey on neighborhood slopes without landslide occurrences. From the results of a dynamic cone penetrometer and soil cross section, we determined that the weathering layer of the base rocks (Nc value of 10-30) was relatively thick on the slopes where the cracks had appeared. Further, it became clear that a weak layer (Nc value below 5) had formed in the weathering layer or near the border with the lower hard layer (Nc value over 40). Therefore, we consider that the weak layer became a slip plane and subsequently, landslide occurred despite lesser rainfall than that normally observed after an earthquake.