Large-Scale Landslides in Rapidly Uplifted and Extremely Snowy Mountains in the Northern Japanese Alps

Mount Shirouma-dake (2932 m ASL, 36.75°N, 137.75°E) and its surrounding mountains have experienced rapid uplift and snowy climate over the late Quaternary period. The geology of this area, which comprises Paleozoic to Mesozoic ultramafic/sedimentary/metamorphic rocks and Neogene to Quaternary volcanic rocks, is very complicated. In addition, glaciers existed during MIS4 and 2 (no glaciers currently exist). Moreover, the active faults in the piedmont area are significant. This is because according to trenching studies, they have produced large earthquakes with a 1100-2400 y interval in the Holocene epoch. Therefore, it is recognized that the environment of this area is conducive to the occurrence of a landslide. Although the landforms in this area have been well described from glacial/periglacial geomorphological viewpoints, few details are available on landslides. The local sustainable tourism with geohazard mitigation requires more information on landslides because this area provides a spectacular landscape and attracts many visitors. Since the 1970s, many lodging and tourism facilities have been developed for such visitors. This area has also hosted the 1998 Winter Olympics because of its snowy climate. We performed geomorphological/geological analysis of the landslides and found that landslides and their precursors (uphill-facing scarps etc.) are common in most altitudinal zones. Many different types of landslides are observed in the bedrock slopes. Slumping is a typical mode. Mass rock creep (sagging), shallow failure, toppling, and landslide complexes are also observed. However, debris avalanche is restricted to the area in which volcanic rocks are distributed. Sometimes, the collapse of unconsolidated debris such as tills, colluvium, and alluvium occurs. In most cases, it is difficult to determine the depths of deformed zones without drilling results; however, it is assumed that a few landslides have a slip surface with a depth of 100 m. In some cases, the volumes of the collapsed material appear to exceed the order of 107-108 m3. The estimated average relative vertical mass transfer driven by the landslides for the past 10 ky (106 t m km-2 a-1) is one to three orders of magnitude larger than the measured values of the present-day periglacial processes. Chronology with tephras and 14C dates is helpful. In some cases, it is estimated that the primary movements of a landslide occur after the lateglacial period, mainly in the Holocene epoch. A landslide older than 28 cal ka has also been observed. The latest large landslide (V=1.5×108 m3), probably with debris avalanche behavior, occurred in 1911. The information available on basic factors for landslide occurrence is still inadequate. However, the stress release in the bedrock after deglaciation, increase in rain/snowfall since the early Holocene epoch, and degradation of permafrost appear to have been significant. Furthermore, earthquake shaking by the active faults should be considered. Further dating of the timing of the landslides is required in this regard. This study is significant in further considering the regional tectonic and climatic implications for formation of the basic factors of landslides occurrence.