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In this study, we compared observations and predictions submitted by participants for blind prediction experiments for ground motions using aftershocks, foreshock, and mainshock of the 2016 Kumamoto earthquake sequence, Japan, to improve our understanding of the quality of state-of-the-art methods on the reproducibility of the effects of surface geology on seismic motions. In the blind predictions, 1D, 2D, and 3D methods, Green's function methods, spectral ratio approaches, and other approaches were applied. As for PGA/PGV, acceleration/velocity duration, Fourier spectrum, pseudo-velocity response spectrum, and site amplification factors, the observed values are mostly within the range of average ± σ of all the predictions in the case of weak and strong ground motions. The results of the mean absolute percentage errors for these indices show that the applied methods can predict weak and strong ground motions for the three components in the range of one-half to twice the observations. The average goodness-of-fit (GOF) scores for weak and strong ground motions indicate either a very good fit (6.5–8) or a good fit (4.5–6.5) for the three components. Finally, examples of the categorized methods are quite limited; however, results indicate that the predictions by all the categorized methods can adequately reproduce weak and strong ground motions within either a very good or good fit. Although we could not find a significant difference in the results from the categorized methods, scores by the 2D and 3D methods in the frequency range of 0.5–1 and 1–2 Hz for all the blind predictions are higher than the scores by the other methods. The GOF score for the part after the S-wave by the 2D and 3D methods is higher than that by the 1D method. This supports that the predictions by the 2D and 3D methods due to the accounting of the proper geometry could reproduce the basin-induced and/or basin-transduced surface waves excited by the basin-edge effect more than the 1D method using the earthquake record observed at the reference site. [ABSTRACT FROM AUTHOR]

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]

In the Kanto Basin, Japan, it has been reported that the dominant periods of long-period ground motions vary depending on the areas where earthquakes occurred. This suggests that the characteristics of long-period ground motions are closely related to their propagation paths in the sedimentary layers. We, firstly, investigated the azimuthal dependence of amplifications for long-period ground motions based on the spectral ratios of ground motions on the surface to those on the basement, using both the observation data and the simulation data in the Kanto Basin. In both the data for periods of more than 3 s, the seismic stations where the dominant periods vary for individual earthquakes are concentrated in the central part of the Tokyo Metropolitan Area. The azimuthal dependence on amplifications for long-period ground motions suggests that the wave propagation features of long-period ground motions are related to the laterally complex three-dimensional subsurface structures along the propagation paths between stations and epicenters. We, secondly, investigated the amplification characteristics for body and surface waves, to evaluate their contributions to earthquake ground motions for periods of more than 1 s observed in the basin. It is found that body and surface waves tend to dominate at periods of less than 2 s and more than 3 s, respectively. We, finally, understood the amplification characteristics of body and surface waves including the dominant periods and those amplification factors, to appropriately predict broadband earthquake ground motions. Highlights: The characteristics of long-period ground motions including the dominant periods and those amplification factors were quantitatively investigated based on the spectral ratio by seismic data observed on the surface and the bottoms of the deep boreholes. The azimuthal dependence of amplifications for long-period ground motions propagating in the Kanto Basin was demonstrated by analyzing both the observation data and the simulation data. The dominant periods of the spectral ratios vary for individual earthquakes in the central part of the Tokyo Metropolitan Area. This suggests that the wave propagation features of the long-period ground motions depend on the laterally complex subsurface structures in the Kanto Basin along the propagation paths between stations and epicenters. The amplification characteristics for body and surface waves for periods of more than 1 s observed were investigated to evaluate their contributions to earthquake ground motions. Surface and body waves significantly influence earthquake ground motions at periods of more than 3 s and less than 2 s in the Kanto Basin, respectively. [ABSTRACT FROM AUTHOR]