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After the 1995 Hyogo-ken Nanbu (Kobe) earthquake (M7.2), the Research Group for Explosion Seismology (RGES) conducted a refraction seismic experiment in the northern part of the Awaji Island in order to investigate a detailed structure around the source region. In the surveyed area, 145 temporal observation stations were deployed two-dimensionally at 200–250 meters intervals, and 7 shots of 150 kg charges were detonated. Using the first arrival times, we derived a three-dimensional P wave velocity structure of the shallow crust in this region. A dominant feature was the low velocity in the northern part of the region as compared with the southern part. The averages of velocities in the northern area were 4.1 km/s (at a depth of 0.09 km) and 4.5 km/s (at 0.49 km). In the southern region, these were 4.9 and 5.2 km/s, respectively. The study region was mostly within the Cretaceous granitic rocks, and we cannot find a difference in geology that corresponds to the velocity variation. On the other hand, the Bouguer anomaly and the resistivity distributions are consistent with the lateral velocity difference.

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range - a strain concentration zone in the interseismic period - shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

We propose a source fault model for the 2011, April 7, earthquake (M 7.1) deduced from a dense GPS network. The coseismic displacements obtained by GPS data analysis clearly show the spatial pattern specific to intraslab earthquakes, not only in the horizontal components but also the vertical ones. A rectangular fault with uniform slip was estimated by a non-linear inversion approach. The results indicate that the simple rectangular fault model can explain the overall features of the observations. The amount of moment released is equivalent to Mw 7.17. The hypocenter depth of the main shock estimated by the Japan Meteorological Agency is slightly deeper than the neutral plane between down-dip compression (DC) and down-dip extension (DE) stress zones of the double-planed seismic zone. This suggests that the depth of the neutral plane was deepened by the huge slip of the 2011, M 9.0, Tohoku Earthquake, and the rupture of the thrust M 7.1 earthquake was initiated at that depth, although more investigations are required to confirm this idea. The estimated fault plane has an angle of ∼60 degrees from the surface of the subducting Pacific plate. It is consistent with the hypothesis that intraslab earthquakes are thought to be a reactivation of the pre-existing hydrated weak zones produced in the bending process of oceanic plates around outer-rise regions.