Abstract: The R/V (Research Vessel) Yokosuka YK16-01 Cruise was performed during 19 days from Friday, April 8, 2016 (Harumi, Tokyo) to Tuesday, April 26, 2016 (Harumi, Tokyo) in the southeastern part of the Japanese Exclusive Economic Zone (EEZ) around Minamitorishima Island., to investigate on widespread distribution of ferromanganese nodules: toward basic studies for the origin of marine mineral resources, conducted by the chief scientist: Dr. Shiki MACHIDA (JAMSTEC). Shinkai 6500 dives of 8 times were performed during the cruise (6K#1459-6K#1466). A dense field of ferromanganese (Fe-Mn) nodules was discovered by the Shinkai 6500 submersible dive (6Kdive#1207: observer, ISHII) during R/V Yokosuka YK10-05 Cruise in 2010 in the skirts of a small seamount approximately 300 km east of Minamitorishima Island in the Japanese EEZ. Textural and geochemical features indicate that the nodules are morphologically and compositionally similar to Fe-Mn crusts recovered from the near field, suggesting that they owe their origin the same geochemical sources. The REY (=rare earth elements and yttrium)-rich mud and ferro- manganese nodules also suggest genetic relation by mean of geological and geochemical features. Ocean oxide resources (=manganese nodules, manganese crust and REY-rich mud) have genetic deep relation, each other. They are assumed as the geological Trinity (Machida et al., 2016 in Japanese). On the basis of this working hypothesis, we attend the cruise. Onboard as well as on- & sub-bottom geological and geophysical several observations (ADCP, SBP, MBES, 4Kvideo, etc.) were also successfully done during the Cruise.
Keywords: ferromanganese nodule, ferromanganese crust, REY-rich mud, Shinkai 6500, R/V Yokosuka, Minamitorishima EEZ, ocean oxide resources, Trinity working hypothesis
p. 1-28

Abstract: We studied in detail the six landslide source areas which developed into large debris flows devastating severely on housing areas situated at alluvial cones in Hiroshima city on August 20, 2014. The source areas are deeper (~5.5m) and larger (~3200m3) than superficial slides (1m ± in depth). Those larger source areas have a characteristic slope formation consisting of bedrock (weakly weathered granite and rhyolitic dyke rock), reddish brown soil−debris bed (~5m), yellowish brown soil−debris bed (~3m), brown soil bed (~0.6m), and rhizome-humus soil (~0.2m). The main part of the surface of rapture can be found near the basal part of the reddish brown soil−debris beds in those source areas. The topographical locations of the source areas are situated on lineaments where bedrocks are severely sheared and show concave sub-surfaces, filled thickly with the reddish brown soil−debris beds, and saddles at the rear ridges. This suggests for the possibility of the site prediction for somewhat large landslide-debris flow source areas. We have a hypothesis on the formation of the reddish brown soil−debris bed, which played the key role on the 2014 disaster, have been formed after MIS 5e (0.125Ma).
Keywords : granitic rocks, source area, debris flow, slope body formation, reddish brown soil−debris bed, yellowish brown soil−debris bed
p. 29-56

Abstract: The Tohoku Earthquake which occurred at March 11, 2011 with a moment magnitude Mw9.1 brought a lot of problems to us including the organization of earthquake prediction of Japan. In this short note, I will try to summarize the history of earthquake prediction project of Japan, and make a personal reflection about these problems as a researcher who has at least committed earthquake prediction and made many experimental studies concerning the forecasting earthquakes. Some people have argued that the earthquake prediction is intrinsically impossible and others have claimed that the earthquake prediction should be a scientific aim to be realized. The history of this argument will be reviewed and I will show my personal consideration.
Keywords: earthquake prediction, earthquake foreseeing, Tohoku earthquake
p. 57-72

Abstract:A group of earthquakes began to occur in the region near Kumamoto City, central Kyushu along the Futagawa Fault system on the Japan Median Techtonic Line (MTL) from 14th, April 2016. The first earthquake (M6.5) occurred at 21:26 in Mashiki town , and 32 hours after it, at 01:25 26th, April the main shock (M7.3) broke out at several kilometers NW of the first event. Just about two hours after the main shock, two eminent earthquakes (M5.9 and M5.8) were induced in the Caldera region of Volcano Aso, and six hours after the main shock another earthquake (M5.4) was induced in Yufuin Basin in Ooita prefecture. On 19th April, another eminent earthquake (M5.0) occurred at Yatsushiro city, which is located on the SW prolongation of the Futagawa fault system. In 1889, the Meiji Kumanoto earthquake (M6.3) occurred near at the same place as the present events. After the occurrence of the Meiji Kumamoto earthquake a series of induced earthquakes took place on the MTL on Kyushu and in the Seto Inland Sea region in the period up to the end of 1898. In addition that, two earthquakes (M5.3 and M6.3) occurred at Chiran town in Kagoshima prefecture, and an earthquake (M6.0) was induced at a point west of Fukuoka city in the same period. In the present events, 47 people were killed in the narrow zone north of the Futagawa fault which has 3 kilometers in width.
Keywords: The 2016 Heisei Kumamoto earthquake, the 1889 Meiji Kumamoto earthquake, Futagawa fault, the Japan Median Tectonic Line, relationship between the distribution of deaths and the location of the active fault
p. 73-87

Abstract: We investigated three-year post-seismic surface displacement of the 2011 Tohoku earthquake around the Kanto Plain, using a global positioning system network and persistent scatterer interferometry of TerraSAR-X data. Uniform uplift owing to viscoelastic relaxation and afterslip on the plain has been reported previously. In addition to the general trend, we identified areas where surface displacement velocity was faster than surrounding areas, with as much as ~5 mm/year decay over time. The local uplift areas were about 30×50 km2, and showed complex a spatial distribution with irregular shape. Based on the observed groundwater level increase, we deduced that the local ground uplift was induced by permeability enhancement and pore pressure increase of the aquifer system, attributable to the mainshock vibration. The study is the first observation to reveal ground displacement caused by permeability enhancement from an earthquake.
Keywords: the 2011 Tohoku earthquake, GNSS, PSInSAR, surface displacement, groundwater
p. 89-96

Abstract: Rock mass classification has been widely used for designing and constructing engineering structures such as tunnels and dams. There are several classification methods such as RMR and Q-value, and the methods proposed by related organizations in Japan. As these methods, however, include some qualitative evaluation criteria, it has been long recognized that the rock mass classification strongly depends on those who make classification. We have, therefore, studied a classification method which can more quantitatively and objectively classify the rock grade with multiple geophysical data based on rock physics. This paper demonstrates an application of the proposed model to P-wave velocity and resistivity data acquired with seismic refraction and electric surveys on a planned tunnel route to estimate rock grades before and after excavation. The rock grade after excavation is determined based on P-wave velocity reduced due to loosening of the tunnel wall which is predicted by the model. Comparison with rock grades determined with the conventional way proves that the model-based method can be used for more quantitative rock mass classification.
Keywords: rock mass classification, tunnel rock classification, seismic velocity, resistivity, rock physics model, tunnel
p. 97-104

Abstract:The confined tri-axial extension tests are conducted using cylindrical specimens of Kimachi sandstone. The test is a simulation of stress release, such as construction of underground structure and rock materials surfaced on the ground. In addition, the tests are conducted as two stress paths. In the case of stress path I, the confining pressure (σ1=σ2=σ3) has been increased to maximum value, then σ3 has been decreased at a rate of 0.125 kN/sec, as σ1＝σ2>σ3. Another case of stress path II, the confining pressure (σ1=σ2=σ3) has been increased to minimum value, then σ1(=σ2) has been increased as σ1＝σ2>σ3. In spite of the two stress paths, all test specimens are failed with shear and tensile fractures. The shear fractures are traced and the dipping angles are measured on the lateral surface images of the specimens. It is shown that the fracture angles are increased with confining pressure when the specimens are yield. This phenomenon can be explained by Mohr-Coulomb yield criterion, and the fracture envelope can be calculated by yield stress and fracture angles. In addition, surface roughness of tensile fractures are measured by photogrammetry. The surface roughness is planer with increasing confining pressure. Therefore, the tensile fractures have been created in the confining pressure, after the formation of shear fractures.
Keywords: shear fracture, tensile fracture, Mohr-circle, fracture envelope, roughness
p. 105-114

Abstract:Flow localization is one of the most fundamental and common issues for characterizing flow in fractured rocks. In this paper, based on the results from the laboratory tests and the numerical study, the formation mechanism of the preferential flow paths and the contribution of those paths to the macroscopic hydraulic conductivity are investigated. The results of the hydraulic tomography and X-ray CT using a 30 cm cube of fractured chert showed that the localized flow is occurring along the interconnected horizontal and smaller scale semi-vertical fractures. From numerical study, it was shown that the flow localization starts to occur when, σ, the standard deviation of the logarithm of the local hydraulic conductivity, is larger than one. The preferential flow pathways are created within the contour surface of top 25 % of the hydraulic conductivity that is equivalent to the “critical path” defined by the percolation theory. The location of the preferential pathways predicted by the numerical simulation was successfully reproduced by the 3D printed fracture network model. It was also pointed out that the macroscopic conductivity becomes larger, more than one order of magnitude comparing to the logarithmic mean of the local conductivity, when σ becomes larger than 1.5 which was also predicted by the percolation theory and the numerical simulation.
keyword: fractured rock, flow localization, critical density, hydraulic tomography, X-ray CT, 3D printer
p. 115-124

Abstract:DCDA is a new method for evaluating the in-situ stress of rocks based on the elastic and elliptical deformation of boring cores with differential stress relief. However, rocks are seldom isotropic. The elliptical deformation of cores must be caused by not only differential stress relief but also anisotropic elasticity of rocks. In order to evaluate the in-situ rock stress precisely, we must consider anisotropic elasticity of rocks in DCDA. Laboratory drilling verifications were conducted using 300 mm cube Ohshima granite samples under bi-axial stress condition using two pairs of flat jacks. Cross sectional shapes of a drilled core were measured precisely by the DCDA measuring device and its anisotropic elasticity was determined by Brazilian test. Then we developed DCDA for rock stress evaluation considering anisotropic elasticity.
Keywords: stress relief, elliptical, rock core, anisotropic elasticity, Brazilian test
p. 125-134

Abstract:Mountain tunnels constructed in the deep rock mass have been vaguely believed to be safe for earthquakes up to now. Actually, the structural stability of mountain tunnel has been examined by considering the static loads such as excavation released stress and load due to excavation disturbed zone. Stability of tunnels against earthquakes has been examined for the large-scale underground caverns for underground power plant and oil or LPG storage facilities, and also for the tunnel with special conditions where the damage by earthquake is clearly supposed. However in the recent destructive earthquakes, significant damages of mountain tunnels caused by seismic vibration and earthquake fault displacement have been reported. Once the mountain tunnel which plays an important role as a social infrastructure structure causes earthquake damages, it will exert a large influence on the social activity as well as it requires a lot of time and effort in its own restoration. In this paper, the present situation of aseismicity of mountain tunnels is summarized and the problem to be solved is discussed.
Keywords: mountain tunnel, earthquake damage, aseismicity evaluation
p. 135- 152