Geology of the area where debris flows occurred by a heavy rainfall in Hiroshima at August 20, 2014

Abstract

A new geological map has been prepared of an area in Hiroshima where numerous debris flows occurred on account of heavy rainfall on 20 August 2014. Rocks in the area are mainly the Late Cretaceous Hiroshima Granites and its roof pendant, which consists of the Late Cretaceous Takada Rhyolites, a Jurassic accretionary complex correlative of the Kuga Group, and schists correlative of the Triassic-Jurassic Suo metamorphic rocks. All these rocks have been intruded by porphyritic rhyolite dikes. The most disastrous debris flows were due to the collapse of steep slopes composed of the porphyritic rhyolite dikes. Many other disastrous debris flows occurred because of small collapses of steep slopes along the ridge lines composed of schist that had experienced contact metamorphism, and also of porphyritic granite.