LAUSR

Abstract Typhoon Talas brought heavy rain to the Kii Peninsula, Japan, during 2–5 September 2011, causing hundreds of debris avalanches and debris flows in the southeastern part of the peninsula. We undertook field investigations to understand the geological and geomorphological context of the landslides and found that the most important geological factor is spheroidal weathering of granite porphyry. This granite porphyry is deeply weathered with corestones in higher elevations and forms low-relief surfaces, of which marginal areas were the major sources of debris avalanches. Corestones that failed were embedded in grus in-situ or on nearby slopes of mudstone in which the granite porphyry intruded. The corestones are made by spheroidal weathering from well-developed columnar joints and sheeting joints. Sheeting joints in the source granite porphyry acted as sliding surfaces both before and during the occurrence of the 2011 debris avalanches.