Recovery process of shear wave velocities of volcanic soil in central Mashiki Town after the 2016 Kumamoto earthquake revealed by intermittent measurements of microtremor
© The Author(s) 2017
Received: 19 February 2017
Accepted: 16 May 2017
Published: 22 May 2017
During the main shock, Hata et al. (2016b) were given an unexpected opportunity to observe strong motions at TMP01, TMP02 and TMP03 as shown in Fig. 1. The observed strong motions at TMP03 exceeded the largest observed ground motions during the 1995 Kobe earthquake in terms of spectral acceleration and JMA seismic intensity (Nishimae 2004) and have a significant importance to the engineering as well as seismological communities (Hata et al. 2016a). With installation of the seismograph at TMP03 on April 15, 2016, between the foreshock and the main shock, Hata et al. (2016c) have previously conducted microtremor measurement in order to confirm the validity as a candidate site for the temporary earthquake observation (see photograph in Fig. 1). After the main shock, intermittent measurements of microtremor at TMP03 were also carried out within December 6, 2016, in order to evaluate recovery process of properties of subsurface soil for this study.
Previous studies have reported a phenomenon in which shear modulus of soil is decreased during a large earthquake and then gradually recovers over a time interval of several months (e.g., Arai 2006; Houlsby and Wroth 1991; Nagao et al. 2016; Nishimura et al. 2005; Sugito et al. 2000; Rubinstein et al. 2007; Tokimatsu and Hosaka 1986; Vlastos et al. 2006). It is important to fully understand such a phenomenon, because it could be related to the dissipation of excess pore water pressure and hence to a potential long-term deformation of soil after a large earthquake. However, in spite of its importance, there have been relatively few field data with respect to the recovery process of shear modulus of soil after a large earthquake.
Since we confirmed the similar phenomenon at TMP03 based on the intermittent measurements of microtremor, the obtained results were reported in this express letter. In particular, the recovery process based on the time history of peak frequencies of the microtremor H/V spectra at TMP03 after the main shock was confirmed. We also confirmed the similarity between the monitored H/V spectrum at TMP03 and the theoretical ones based on original PS logging tests at proximity site of TMP03 (Yoshimi et al. 2016, 2017), indicating applicability for the observed shear wave velocities to TMP03.
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).
Monitored microtremor H/V spectra
List of the observed ground motion indices at permanent and temporary stations in central Mashiki Town during the 2016 Kumamoto earthquake sequence (e.g., Hata et al. 2016b)
Mashiki Town Office
PGAs (m/s2) [N–S]
PGAs (m/s2) [E–W]
PGAs (m/s2) [U–D]
PGVs (m/s) [N–S]
PGVs (m/s) [E–W]
PGVs (m/s) [U–D]
JMA seismic intensities
PGAs (m/s2) [N–S]
PGAs (m/s2) [E–W]
PGAs (m/s2) [U–D]
PGVs (m/s) [N–S]
PGVs (m/s) [E–W]
PGVs (m/s) [U–D]
JMA seismic intensities
On the other hand, the peak frequency just after the main shock (see Fig. 3b) was significantly lower than the final peak frequency 5657 h after the foreshock (i.e., 5629 h after the main shock), indicating the reduction of the shear wave velocities of the volcanic soil. Note, at KiK-net Mashiki (see Fig. 1), we have already confirmed that the same significant change of H/V spectral ratio featuring spectral shape and peak frequency has not occurred just before/after the main shock. Then the peak frequency gradually increased and approached to the final value (nearly equal to the initial value), indicating the recovery process of the shear wave velocities. The H/V spectrum about 2500 h after the foreshock and the main shock was almost identical to that before the main shock (see Fig. 3l). Thus, the recovery process of the shear wave velocities of the volcanic soil at TMP03 was clearly documented in the monitored microtremor H/V spectra.
Recovery process of shear wave velocities
Figure 3l also shows the comparison of the theoretical with monitored H/V spectral ratios at TMP03. In Fig. 3l, the monitored H/V spectral ratio was based on the measurement results of microtremor at TMP03 during the PS logging test at GS-MSK-1 on July 28, 2016. On the other hand, the theoretical H/V spectral ratio (e.g., Haskell 1953) based on the fundamental mode Rayleigh wave was calculated from the soil profiles at GS-MSK-1 nearby TMP03 (see Fig. 2) with the conventional soil densities (Goto et al. 2017). We can find a good agreement between the monitored and theoretical H/V spectral ratios as shown in Fig. 3l, indicating applicability for detection of the shear wave velocities of the volcanic soil at TMP03 with good accuracy by the monitoring results.
Summary and conclusions
The recovery process of shear wave velocities of the volcanic soil was clearly documented in the time-dependent peak frequency of the measured microtremor H/V spectra.
The measured microtremor H/V spectrum at the site of interest during the PS logging test after the main shock nearby the site of interest was reproduced good accurately in the theoretical H/V spectrum based on the fundamental mode Rayleigh wave, which indicates that the P-wave and S-wave velocities distribution after the main shock was detectable with good accuracy by the monitoring results.
As a future study, we would like to simulate the obtained recovery process of the shear wave velocities based on dynamic FEM analyses considering pore water pressure with observed strong motions for the main shock and major large aftershocks.
YH conducted the measurement and the simulation. YH, MY and HG drafted the manuscript. MY and TH acquired data of the ground investigation. HG, HM and TK participated in reconnaissance survey of the seismic damage around TMP03. MY, HG, HM and TK participated in the discussion and the interpretation. All authors read and approved the final manuscript.
The authors thank the residents of Mashiki Town and a staff of Mashiki Town Office for generously cooperating in conducting the microtremor measurements. The authors also appreciate the assistance of Mr. Fumihiro Minato, a graduate student of Osaka University, during the measurement. This study was partially supported by JSPS KAKENHI (Grant No. JP15H05532), and the special research found for collaborative research in Disaster Prevention Research Institute (DPRI), Kyoto University (Grant Number: 28U-05 and 28U-07). This study was carried out as an activity of the Subcommittee on Aggregation and Application of Seismic Trace Data in Geographical Feature [Head: Prof. Kazuo Konagai (Yokohama National University)], the Subcommittee on Mechanism of Earthquake Motion around Active Fault [Head: Prof. Takao Kagawa (Tottori University)] and the Subcommittee on Survey Analysis of Damage for the 2016 Kumamoto Earthquake Sequence [Head: Prof. Takaaki Ikeda (Nagaoka University of Technology)], organized by the Earthquake Engineering Committee, Japan Society of Civil Engineers [Chairperson: Prof. Sumio Sawada (DPRI, Kyoto University)].
The authors declare that they have no competing interests.
Data and resources
Strong motion data at TMP01, TMP02 and TMP03 can be obtained from Division of Dynamics of Foundation Structures, Disaster Prevention Research Institute (DPRI), Kyoto University, at http://wwwcatfish.dpri.kyoto-u.ac.jp/~kumaq/ (last accessed February 2017).
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