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1D velocity profiles at a strong motion station in the northern part of the Kumamoto plain, Japan, were submitted in Step-1 of the blind prediction exercise of strong ground motions in the sixth international symposium on effects of surface geology on seismic motion (ESG6). Individual participants were requested to estimate a 1D S-wave velocity profile of sedimentary layers from the given data obtained by microtremor array explorations and surface-wave explorations at the site. This paper reports the target site, methods used by the individual participants, and the submitted results. More than half of the participants estimated the phase velocities of the Rayleigh-wave in the frequency range from 0.53 to 29.8 Hz. The statistical analysis of the phase velocity dispersion curves indicates that the standard deviation was below 40 m/s at the frequencies above 3.4 Hz, and it was below 20 m/s above 20 Hz. The S-wave velocity profiles are also similar to a depth of 20 m. The standard deviation was below 45 m/s. The average S-wave velocity in the top 30 m from the surface is 207.3 ± 60.7 m/s for the submitted profiles. The large variation is related to the introduction of the near-surface low velocity layers. The large variation of the S-wave velocities was found in the deep part. The average S-wave velocity at a depth of 1500 m was 2674 m/s with the standard deviation of 786 m/s. We compared 1D amplifications for the submitted profiles. Common peaks can be identified at 0.3–0.4 Hz and 1–2 Hz, excluding two teams. However, the amplifications vary much in the frequency range higher than 4 Hz. Through the experiment, it was found that the dispersion curves and the shallow S-wave velocity structures are estimated with a low standard deviation among the participants. Further development of the techniques for deep S-wave velocity profiling was found to be required. [ABSTRACT FROM AUTHOR]

To evaluate local site effects in Kumamoto City, we installed six temporary seismic stations along a 6-km north-south survey line in the city immediately after the 2016 Kumamoto earthquake foreshock (Mj 6.4), which occurred on April 14, 2016. Seismic data from the 2016 Kumamoto earthquake (Mj 7.3), which occurred on April 16, 2016, were successfully recorded at two sites and indicated large amplitudes in the frequency range of 0.5-3 Hz. Site amplifications estimated from weak ground motion data, with a station at Mt. Kinbo used as a reference, are relatively variable along this survey line; however, site amplification factors in the frequency range of 0.5-3 Hz are not large enough to explain the amplitudes produced by the main shock. Nevertheless, site amplifications estimated from strong ground motion data recorded at the two sites during the main shock are large in the frequency range of 1-3 Hz. These findings reveal that the strong ground motions in the frequency range of 1-3 Hz were enhanced by nonlinear behavior of the subsurface soil in Kumamoto City. Moreover, it is observed that the frequency contents of the main shock data in the frequency range of 0.7-3 Hz differ significantly between the two sites, despite the proximity of these sites (600-m interval). Therefore, we also performed single-station microtremor measurements with an interval distance of approximately 100 m between these two sites. We confirmed that the peak frequencies of the horizontal-to-vertical spectral ratios of microtremors have trends that are similar to those of the site amplification factors between the two sites. However, these results could not explain the differences in strong ground motions observed at the two sites during the 2016 Kumamoto earthquake. [ABSTRACT FROM AUTHOR]

We have conducted observation of earthquake ground motion due to aftershocks of the 2016 Kumamoto earthquake at 26 temporary stations in damaged areas of Kumamoto city, Mashiki town, Nishihara village and Minami-Aso village (partly in Aso city) in Kumamoto prefecture, Japan. Continuous recordings of ground acceleration were acquired in a period of about 1 month after the occurrence of the main shock on April 16, 2016. This preliminary analysis of the observed records clearly indicates strong effects of local geological condition in the heavily damaged districts in Mashiki town and Nishihara village. Spectral ratios of the ground motions at the stations in the severely damaged districts to those at the reference sites are characterized by large amplitudes at periods of 0.5-1 s. Peak ground velocities and seismic intensities are also large at the sites. Seismic intensities at the stations in the damaged districts are larger by an intensity of one at the maximum than those at the stations with the minor damage. The ground motions at the stations in Kumamoto city are rich in later phases with long duration suggesting basin effects. However, site amplification effects could not clearly be identified at the stations in the Minami-Aso area from the results in the conventional spectral ratio approach. [ABSTRACT FROM AUTHOR]