The Relationship between the Yasukuni Slide and a Hydrothermal Interstratified Illite/Smectite Minerals Zone

Abstract

The geology of the Yasukuni landslide area consists mainly of tuffaceous clastic rocks and acidic volcaniclastic rocks of the Upper Miocene Ikutawara Formation, in which an interstratified illite/smectite minerals zone exists, formed by hydrothermal alteration related to the 8.1-7.7 Ma Ryuo hydrothermal system. The slide clays and debris contain large amounts of interstratified illite/smectite minerals. The occurrence of the Yasukuni slide is strongly related to the unstable nature of the interstratified illite/smectite minerals zone, the bulk of which is swelling clay mineral. This fact is useful in evaluating landslide risk and creating a hazard map of such slides in ancient and active hydrothermal fields.