Fig. 5.

Surface/borehole spectral ratios stacked in different time period before and after the Tohoku main shock for the six stations. The time periods are: every month before the Tohoku main shock in linear time scale, and every 0.25 in the logarithmic time scale after the Tohoku main shock. The stacked spectral ratio values are color-coded with red being high and blue being low. The vertical solid and dashed lines indicate the occurrence time of the Tohoku main shock, and the ending of Tohoku main shock record (~300 s). The horizontal solid and dotted grey lines show the reference value and uncertainties of the resonance frequency before the Tohoku main shock. The tilted black solid line is the least-squares fitting of the peak resonance frequencies after 300 s, which is projected back to the time when the highest PGA is recorded. The two green tilted dashed lines in panel (f) are the least-squares fittings of two stages of recovery within and after ~5 hours of the Tohoku main shock, respectively. The residual standard deviation (RSD) value and the slope of the regression line are shown in the bottom and top right corner of each panel, respectively.