Repeated rock avalanche runout from the upper Okumatashirodani Basin, Kamikochi Valley, northern Japanese Alps

Abstract

In and around the Shinmurabashi Bridge in Kamikochi Valley of northern Japanese Alps, debris hummocks and a minor ridge features both 10 m high and with angular gravels are developed on the alluvial fan along Benten-sawa River and Okumatashiro-dani River, respectively.  These landforms are not moraines but rock avalanche origin supplied from the east face of Maehotakadake North Ridge about 2500-3000 m ASL.  Terrestrial cosmogenic nuclide chronology reveals that these features were formed in the period during 6.0-7.9 ka (hummocks) and 0.8-1.1 ka (minor ridge).