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Following the devastating damage caused by the heavy rains in western Japan in 2018, the Japan Geotechnical Society compiled and published recommendations for future disasters. This report introduces an outline of the recommendation on slope disasters. In particular, the recommendation emphasises the need for measures to evacuate residents from dangerous places before a disaster occurs. For this purpose, it is noted that it is important to identify potentially dangerous places and their characteristics in the long term based on past disaster information and to inform the residents of the identified areas. As a pioneering effort to achieve this recommendation, this paper describes the results of research conducted in Yamaguchi, Hiroshima, Tokyo, and Kumamoto in Japan, to clarify the frequency of the occurrence of debris flows and its utilisation measures. The main result is that debris flows occur once every few hundred years in weathered granite areas, whereas volcanic ash areas experience even more frequent debris flows than granite areas.

We describe the detailed sedimentary characteristics of a tsunami deposit associated with the 2011 Tohoku-oki tsunami in Hasunuma, a site on the Kujukuri coastal plain, Japan. The thick tsunami deposit was limited to within 350 m from the coastline whereas the inundation area extended about 1 km from the coastline. The tsunami deposit was sampled by excavation at 29 locations along three transects and studied using peels, soft-X imaging and grain-size analysis. The deposit covers the pre-existing soil and reached a maximum measured thickness of 35 cm. It consists mainly of well-sorted medium to fine sand. On the basis of sedimentary structures and changes in grain size, we divided the tsunami deposit into several sedimentary units, which may correspond to multiple inundation flows. The numbers of units and their sedimentary features vary among the three transects, despite the similar topography. This variation implies a considerable influence of local effects such as elevation, vegetation, microtopography, and distance from footpaths, on the tsunami-related sedimentation.

The 2016 Kumamoto earthquake sequence was a rare event worldwide in that the surface ruptures associated with the largest foreshock (Mj 6.5) of 21:26 (JST), 14 April ruptured again during the mainshock (Mj 7.3) of 01:25 (JST), 16 April. The 14 April Mj 6.5 earthquake produced 6-km-long surface ruptures along the central portion of the Futagawa–Hinagu fault zone (FHFZ). The mainshock produced 31-km-long surface ruptures along the central to northeastern part of the FHFZ. Field observations and eyewitness accounts documented that the offsets of the ruptures associated with the 14 April foreshock became larger after the 16 April mainshock, suggesting that the same portion of the fault ruptured to the surface twice in the Kumamoto earthquake sequence. The 6-km-long surface ruptures associated with the largest foreshock are located near a geometric bend of the FHFZ characterized by ~50° change in strike. The epicenter of the mainshock is also located near the bend. These observations imply that the Kumamoto earthquake sequence was initiated due to a stress concentration on the bend of the FHFZ, and the mainshock was initiated approximately at the same place about 28 h after the largest foreshock. This foreshock/mainshock sequence of the Kumamoto earthquake is not successive events on the adjacent different fault zones, because the 6-km-long surface ruptures of the largest foreshock are part of the 31-km-long surface ruptures of the mainshock.

The Kitakami Lowland fault zone (KLFZ) is an active reverse fault zone that extends about 70 km eastward along the Quaternary volcanic front of the northeast Japan arc. The activity of this fault zone in the Quaternary is mostly recognized as a fault reactivation of Miocene normal faults in the area. We conducted a high-resolution seismic reflection profile survey, Nanshozan 09, to define the geological structure of the Nanshozan fault group, northern portion of the KLFZ. The Nanshozan fault group is composed by three reverse faults, F1, F2, and F3 faults, and deforms late Quaternary fluvial terraces and alluvial fans in the Kitakami Lowland. Several isolated andesite hills such as Jonaisan and Kitayachi-yama, line the eastern edge of the hanging-wall of the F3 fault. The high-resolution seismic reflection profile shows that the fault scarps and tilted fluvial terraces are traced down to a depth of 2 km along the Nanshozan fault group, formed by an west-dipping high-angle reverse fault (F1 fault) and along two of the thrusts that have a flat-ramp geometry (F2 and F3 faults). Furthermore, the results of K-Ar dating and field observations show that the Jonaisan hill is certainly composed of Pliocene andesitic rocks, not Quaternary intrusives. Cross-section restoring across the Nanshozan fault group implies that F2 fault activity occurred after the initiation of F3 fault activity. Hence, isolated hills near F3 fault, such as Jonaisan and Kitayachi-yama, are recognized as volcanic blocks that survived long periods of erosion while being deformed by F3 fault thrusting.

High-resolution seismic reflection profiles across coseismic fault scarps reactivated during the A.D. 1896 Rikuu earthquake along the eastern margin fault zone of the Yokote basin (EFZYB) in northeast Japan, correlated with borehole stratigraphy and geologic mapping, provide insights into its detailed kinematic history and structural evolution. In spite of along-strike variations of thrust geometries both at ground surface and at shallow depth, the EFZYB has commonly formed as forward breaking imbricate thrust systems. Near surface complexity of thrust geometries appears strongly affected by mechanical decoupling between layers within middle to late Miocene mudstone. Cross-section balancing across the Mahiru Mountains shows strong correlation of the millennial uplift rates with mountain topography but its weak correlation with the late Pleistocene uplift rates. Considering the thrust trajectories estimated by the balanced cross sections, the millennial dip-slip rate are consistent with the late Pleistocene dip-slip rate, suggesting that the EFZYB has accumulated strains at a constant rate since the onset of its fault activity.

The Eastern margin of the Yokote Basin Fault Zone (EYBFZ) is one of the seismogenic reverse faults that developed in Northeast Japan, generating the 1896 Rikuu Earthquake (M7.2). The EYBFZ is divided into range boundary fault (Kawaguchi Fault) and frontal fault. The major segment of the frontal fault consists of three faults : the Shiraiwa Fault, the Ota Fault, and the Senya Fault, at which coseismic surface ruptures with irregular and highly sinuous traces, gaps and echelon steps occurred during the 1896 Earthquake. In the study area, the irregular coseismic surface ruptures probably originate from differences in fault geometries and sedimentary conditions, which are associated with thrust-front migration in the Quaternary. We discuss the relationships among fault traces, geomorphic displacements, and fault geometries on the Senya Fault and the Kawaguchi Fault, based on data from high-resolution seismic reflection profiling, investigations into tectonic geomorphology and structural geology with the help of the balanced cross-section method.The seismic image obtained shows changes of geomorphic and geologic features around the Senya Hills expressed as differences in faulting geometries along the Senya Fault and the Kawaguchi Fault. In fact, the Senya Hills are situated as an unsymmetrical ordering.The disparity in the timing of thrust-front migration affects the development of fluvial surfaces, which are related to the appearance of the fault scarp on the surface. That is why, in the central portion of the Senya Hills, the Senya Fault is a typical emergent thrust with flat-ramp structures associated with the uplifting and the back-tilting of late Quaternary fluvial terraces. On the other hand, in the northern portion, the Senya Fault is a concealed thrust with step-like geometry and prominent folding and small faults in the hanging wall. The small faults infer the structure of the tectonic scarp and short wavelength deformation on the surface. This means that the Senya Fault at the central portion preceded the northern portion as an emergent fault. Moreover, fault traces show different dips between central and northern portions. The dip of the central portion is lower than that of the northern portion.By restoring the balanced cross-sections and assuming the uniform net-slip rate, the initial thrusting along the boundary faults is retroactive to 2.02.6 Ma at the central portion and 1.8 2.3 Ma at the northern portion. The initiation of thrust front migration from the boundary faults to the frontal faults is ca.1.6 Ma at the central portion and 0.6Ma at the northern portion.

The Northern Miyagi earthquake (Mj 6.4) on 26 July, 2003, was a shallow crustal earthquake produced by high-angle reverse faulting. To construct a realistic geologic model for this fault system from depth to the surface, seismic reflection profiling was carried out across the northern part of the source fault of this earthquake. The common mid-point seismic reflection data were acquired using a vibrator truck along a 12 km-long seismic line. The obtained seismic profile portrays a Miocene half-graben bounded by a west-dipping fault. Consistent with gravity anomaly data, the maximum thickness of the basin fill probably reaches 3 km. From the regional geology, this basin-bounding normal fault forms the eastern edge of the northern Honshu rift system and was produced by rapid extension during 17–15 Ma. The deeper extension of the fault revealed by seismic profiling coincides with the planar distribution of aftershocks. The hypocentral distribution of the aftershocks shows a concentration on a plane dipping 55 degrees to the west with listric geometry. Thus, the basin inversion has been performed using the same fault; the 2003 Northern Miyagi earthquake was generated by fault reactivation of a Miocene normal fault.