Numerous numbers of debris flows were induced by the heavy rain in Hiroshima Prefecture southwest Japan in July 2018, causing damage characteristically in different ways in granite and rhyolite areas. In granite areas, where tors are distributed on ridgetops, debris flows contained a large number of big rock masses of rock columns and corestones, which struck and heavily destroyed residential houses in the downstream of steep valleys. In some granite area, when debris flows traveled several kilometers far from the initiating point of the debris flows, sediment flood flows reached and buried residential houses deeply. In some rhyolite areas on the other hand, the debris flows were dominated by water with less amount of fine-grained materials; the bedrock was impervious because bedrock cracks were filled with clayey materials made by weathering and the surface water could not infiltrate downward. Those debris flows were highly mobile and went down on straight slopes as well as valley bottoms, and they floated many building structures up and washed away them downhill. Thus, detailed investigation of geology and geomorphology in the field could be used to predict the behaviors of debris flows and to evaluate the degrees of possible damage. Such a prediction is very useful for the planning of disaster prevention.

There are cases that subsidiary faults are distributed around main faults. Many surveys and studies have been conducted on main and subsidiary faults in the past, but none have focused on microstructural analysis of active subsidiary faults that slipped together with main faults during the Quaternary and have a high possibility of slipping in the future.

The 2014 Northern Nagano Prefecture earthquake （Mw 6.2） ruptured the Kamishiro Fault （main fault） and three subsidiary faults. We conducted a paleoseismic trench survey and microstructural analysis in order to reveal activity of one （latest slip plane） of the subsidiary faults and microstructures in a fault gouge along the slip plane. The trench survey identified that the latest slip plane had slipped at least twice during the Holocene. The microstructural analysis showed that a layered structure and gouge fragments were found in the fault gouge by using a helical X-ray CT scanner, polarized light microscopy, X-ray diffractometer, and transmission electron microscopy.