The microtremor measurement was intermittently conducted at TMP03 (see Fig. 1) after the foreshock including just after the main shock. At GS-MSK-1 nearby TMP03 (see Fig. 1), a PS logging test was performed on July 28, 2016, by Yoshimi et al. (2016, 2017) and revealed depth distribution for the P-wave and S-wave velocities with 50-cm interval to engineering bedrock as shown in center and right panels of Fig. 2. In left panel of Fig. 2, we also obtained the subsurface consisting mainly of volcanic soil. Figure 1 shows the observed JMA seismic intensities during the foreshock and the main shock at the earthquake observation stations including TMP03. In Fig. 1, the largest JMA seismic intensity (6.9) in central Mashiki Town during the main shock was observed at TMP03 (Hata et al. 2016a, b). The initial measurement of microtremor at TMP03 was conducted 15.2 h after the foreshock, that is, 12.8 h before the main shock (Hata et al. 2016c). After the main shock, the earliest measurement was taken 32.0 h after the foreshock, that is, 4.0 h after the main shock. Then, the measurement was taken intermittently 5657 h after the foreshock (i.e., 5629 h after the main shock).
In particular, the measurements were taken 15.2, 32.0, 43.6, 67.5, 206, 449, 494, 710, 831, 902, 1219, 2513, 3062, 3640, 4280, 4933 and 5657 h after the foreshock. The measurement was also taken for three components (NS, EW and UD). The arithmetic mean of the two horizontal components was adopted in the calculation of the H/V spectral ratio. The measurement was taken for one hour (≒163.84 s × 22 sections), and the sampling frequency was 100 Hz. The specifications of the instrument for the microtremor measurement were the same as those reported in Senna et al. (2006).
The comparison of the microtremor H/V spectra at TMP03 is shown in Fig. 3. The procedure to calculate the H/V spectra can be summarized as follows: First, a high-pass filter of 0.1 Hz was adopted, and seven time sections of 163.84 s each were extracted from the original data considering recorded noise. Next, Fourier amplitude spectra for these seven time sections were calculated with a Parzen window of a band width of 0.05 Hz. Then, a microtremor H/V spectral ratio was calculated for each of the seven time sections, where the mean of the horizontal two components was adopted as the numerator. Finally, the spectral ratio was averaged for the seven time sections. Here, the frequency range to evaluate the microtremor H/V spectral ratio was from 0.2 to 10 Hz considering the performance of the instrument for the microtremor measurements (Senna et al. 2006). More details about the procedure to calculate the H/V spectra can be found in Hata et al. (2014b).