Microstructural comparison in fault gouges and slip surfaces of active and inactive faults: A case study at Watarigawa area in northeastern Yamaguchi Prefecture, SW Japan

Abstract

In recent years, surveys and studies have been conducted on active faults that have been active since the late Pleistocene and inactive faults that have not been active since at least the late Pleistocene, respectively, in order to develop fault activity evaluation methods for fault zones by using fault rocks. This study analyzed and compared microstructures in fault gouges and slip surfaces of an active fault and an inactive fault at Watarigawa, Ato-Ikumo-Higashibun, Yamaguchi City, Yamaguchi Prefecture. We also examined the constituent minerals in a hydrothermal clay vein by using polarized light microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning transmission electron microscopy (STEM). The active fault analyzed in this study is called the Chomonkyo Fault by previous studies, by the way. The polarized light microscopy analysis revealed gouge fragments (i.e., reworked fault gouge) are found in the fault gouge of the active fault, but not in that of the inactive fault. The SEM and STEM analyses revealed that the latest slip surface of the inactive fault is covered with randomly oriented illite crystals and crossed by clusters of barite crystals. This indicates that the inactive fault has not moved since the illite and barite were formed. In contrast, the latest slip surface of the active fault does not exhibit these features.