Fig. 3: Recovery processes of seismic velocities detected in field observations and laboratory experiments.
a Recovery process of near-surface shear-wave velocity for the selected KiK-net stations. The gray dots represent raw data computed on time windows having peak acceleration lower than 0.2 m/s2. The horizontal dashed line and gray-shaded band show the mean and standard deviations of the velocities before the Tohoku-Oki Earthquake. The black dots and the error bars indicate the mean and standard deviations for velocity changes (Δc/c) measured over a time span of 0.5 in the logarithmic scale. The bi-linear red lines are the regression lines of the mean values. The fitting of the first recovery stage is obtained from the first five data points for each stations except for IBRH11, for which the first two points are ignored for fitting. The fitting of the second recovery stage is obtained from the rest of data points. The details regarding the regression are provided in Supplementary Table 1. In each subplot, the red arrow on the left shows the abscissa of the intersection of the two regression lines, whereas the red arrow on the right shows the abscissa of the intersection between the regression line of the second stage and the preseismic velocity level. b Normalized shear-wave velocity recovery process from field observations. The red bi-linear lines and arrows are the same as in a. The black dotted line and gray-shaded band represent the averaged normalized velocity recovery process and the corresponding 63% confidence limit, respectively. α1 and α2 are the log slopes in the first and the second stages, respectively; Δc1 and Δc2 are the recovery amounts of the shear-wave velocity during the first and the second stages, respectively. The relative velocity changes for different stations before normalization are shown in the bottom right corner. c Normalized compressional-wave velocity recovery process from laboratory experiments (modified from Shokouhi et al. 35). The red arrow on the left indicates the initiation of the first stage of the recovery process.
