Depth-dependence of seismic velocity change associated with the 2011 Tohoku earthquake, Japan, revealed from repeating earthquake analysis and finite-difference wave propagation simulation

We detect time-lapse changes in seismic velocity associated with the 2011 Tohoku earthquake, Japan (M-w 9.0), by applying the moving time window cross-correlation analysis to repeating earthquake records registered by Hi-net borehole seismograms. The phase delay curve of the repeating earthquake records demonstrates up to 0.2 per cent apparent velocity reduction for S-wave time windows, while the reduction is only 0.1 per cent at the maximum for P-wave time windows. The apparent velocity reductions for S-wave time windows are especially large for offshore region near the large slip area of the Tohoku earthquake. To investigate the sensitivity of the phase delay curve to partial velocity change, we perform a finite difference (FD) wave propagation simulation using short-wavelength random inhomogeneous media. We found that a 1 per cent velocity reduction at the top 150 m depth (shallow zone) is the most possible model to explain the observed phase delay curve. The evaluated velocity reduction at the shallow zone is consistent with previous studies that detected velocity changes due to strong ground motion. Through the FD simulation, we also found that displacement of the source location after the Tohoku earthquake is not likely the primary cause of the observed apparent velocity change. The average velocity reduction at depths from 150 m to 80 km (deep zone) is evaluated to be much smaller than 0.1 per cent. This small velocity reduction at the deep zone can be caused by a static strain change due to the Tohoku earthquake and its post-seismic deformation.