Catastrophic Landslides and Their Precursory Deep-Seated Gravitational Slope Deformation Induced by the River Rejuvenation in the Kii Mountains, Central Japan

Abstract

Slope movements are one of the major processes of denudation as well as erosion and transportation by rivers or glaciers, and many slope movements themselves are induced by river or glacier erosion. We investigated the linkage of river erosion, deep-seated gravitational slope deformation, and catastrophic landslides in the Kii Mountains Japan, where tens of catastrophic large landslides during 1889 Typhoon Totsukawa and 2011 Typhoon Talas occurred. The consequent fatalities were 168 and 56, respectively. The Kii Mountains is underlain by the Cretaceous to Neogene Shimanto accretional complex and has paleosurface remnants at higher elevations. The paleosurfaces have been newly incised by rivers, of which the Kumano River catchment occupies the central part of the Kii Mountains. The new incision of the Kumano River proceeded with the upstream propagation of knickpoints, which developed well-defined convex slope breaks on interfluve slopes. High-resolution DEMs clearly delineated deep-seated gravitational slope deformations, which are characterized by scarps, linear depressions, and bulges, and are aligned along the convex slope breaks, suggesting that they were induced by gravitational instability induced by the erosion undercutting. The catastrophic landslides during the 1889 Typhoon Totsukawa and 2011 Typhoon Talas had been preceded by deep-seated gravitational slope deformation on newly incised inner gorge slopes. Most of the landslides had sliding surfaces along undulating minor faults, probably thrusts, which might have made rock bridges when shearing along preferably oriented parts of the faults occurred. Catastrophic slope failure may occur when those rock bridges are finally fractured.