Time-lapse changes of P- and S-wave velocities and shear wave splitting in the first year after the 2011 Tohoku earthquake, Japan: shallow subsurface

We detect time-lapse changes in P- and S-wave velocities (hereafter, V-P and V-S, respectively) and shear wave splitting parameters associated with the 2011 Tohoku earthquake, Japan, at depths between 0 and 504 m. We estimate not only medium parameters but also the 95 per cent confidence interval of the estimated velocity change by applying a new least squares inversion scheme to the deconvolution analysis of KiK-net vertical array records. Up to 6 per cent V-S reduction is observed at more than half of the analysed KiK-net stations in northeastern Japan with over 95 per cent confidence in the first month after the main shock. There is a considerable correlation between the S-wave traveltime delay and the maximum horizontal dynamic strain (MDS) by the main shock motion when the strain exceeds 5 x 10(-4) on the ground surface. This correlation is not clearly observed for MDS at the borehole bottom. On the contrary, V-P and shear wave splitting parameters do not show systematic changes after the Tohoku earthquake. These results indicate that the time-lapse change is concentrated near the ground surface, especially in loosely packed soil layers. We conclude that the behaviour of V-P, V-S and shear wave splitting parameters are explained by the generation of omnidirectional cracks near the ground surface and by the diffusion of water in the porous subsurface. Recovery of V-S should be related to healing of the crack which is proportional to the logarithm of the lapse time after the main shock and/or to decompaction after shaking.