The Lake Ohara-West Yauneyama fault system, which is composed of multiple active faults, extends in a NE-SW to ENE-WSW trend, from central Yamaguchi Prefecture to central-western Shimane Prefecture. This fault system occurs along the Yamaguchi-Izumo seismic zone characterized by an alignment of microseismicity, suggesting a causal relation to the seismic zone. In this article, i) active-fault outcrops and the prevention facility along the fault system, ii) the causal relationship between discharge of the Yuda hot spring, and tectonics of an active fault and the 1707 M=8.6 Hoei earthquake generated probably along the Nankai trough, iii) an outcrop of a fault process zone at Chomonkyo Gorge, Ato, Yamaguchi City, iv) the 1872 M=7.1 Hamada earthquake and a fault damage zone at Tatamigaura coastal plateau in Hamada City, v) episodes of forerunners who engaged in their geology, and vi) legends and topics of damaging earthquakes and tsunamis, are described as a la carte.

The very large slip on the shallow portion of the subduction interface during the 2011 Tohoku-oki earthquake (Mw 9.0) caused a huge tsunami along the northeast coast of Honshu, Japan. In order to elucidate the mechanics of such tsunamigenic slip, the Integrated Ocean Drilling Program Expedition 343 (Japan Trench Fast Drilling Project, JFAST), was carried out one year after the earthquake and succeeded in recovering rocks constituting the active plate boundary fault. Mineralogical analyses using X-ray diffraction revealed that the shallow portion of the megathrust is significantly enriched in smectite (60-80wt.%) compared to the surrounding sediments. This mineralogical feature is a fundamental reason for realizing the weak fault zone under various slip conditions as demonstrated by laboratory friction experiments. The smectite-rich deposits are broadly distributed in the northwestern Pacific Ocean, and may therefore potentially enhance conditions for large shallow slip during earthquakes, which would result in large tsunamis for this region.

Clay minerals are considered to play important roles in the generation and nature of earthquakes occurred at subduction zone because of their unique frictional characteristics. Friction coefficients of clay minerals are strongly influenced by the presence of water due to incorporating interlayer water into the structure and absorption onto the crystal surfaces. Velocity dependence of friction is generally characterized by velocity-strengthening (stable sliding) at relatively low temperatures, however, as elevating temperature, clay minerals show velocity-weakening behavior (unstable sliding). Frictional healing of clay minerals is significantly smaller than other silicates such as quartz under wet environments, suggesting weak fault strength on the clay-rich surfaces. These distinctive frictional characteristics of clay minerals may explain regional variation and complex behavior of earthquakes on the subducting plate interface.

Frictional strength of mica and clay minerals is lower than that of other fault-forming minerals and rocks. These mica and clay minerals have layered structure and high affinity with water. The low frictional behavior has been interpreted by the relationship with the interlayer bonding energy (ILBE) and the presence of adsorbed and interlayer water. In this review, we discuss whether the low frictional behavior of mica and clay minerals can be interpreted by the ILBE and water based on recent experimental and theoretical results.

Geographical and geological features as the primary cause and intense precipitation as the trigger are related to landslides. The most important factor is the occurrence of clay minerals. Clay minerals are observed in clay vein and micro- crack, are generally developed in granite. Specifically, smectite expands or swells in wet conditions, and is important as the material sharing the responsibility of landslides. To elucidate mechanisms by which landslides occur, an understandings of heterogeneity of sediments and/or rocks is needed.

The frictional force and surface physical properties of high-purity clay minerals at the nanometer level were measured with the atomic force microscope (AFM). The results revealed that the frictional force (micro strength) of the clay unit layer is larger than its shear force (macro strength) determined by soil testing. Frictional forces remained constant regardless of increasing vertical load, and it was accordingly concluded that micro strength is the result of frictional phenomena occurring in the total contact state. In contrast, shear stress increased with increasing vertical load, accordingly indicating that macro strength is the result of frictional phenomena occurring in the partial contact state. The relationship between these two types of strength indicates that the value obtained by simply integrating micro strength does not correspond to the macro strength. In frictional force measurement, adsorption of water molecules by the clay unit layer surface affects the velocity dependency of frictional stress. This phenomenon indicates that water adsorbed on the clay unit layer surface exhibits viscosity. The results of viscosity measurement revealed that the viscosity of the adsorptive water is approximately 25,000 times larger than that of water under normal conditions. It is presumed that this large viscosity is caused by the adsorption power produced on clay particle surfaces. This adsorption power is gradually lost as adsorption progresses to form water molecule layers on the clay particle surface. Adsorption power for clay having formation of the first, second and third water molecule layer is reduced by 82%, 86% and 91%, respectively, from adsorption power for clay having no water layer.