Estimation of shallow S-wave velocity structure using microtremor array exploration at temporary strong motion observation stations for aftershocks of the 2016 Kumamoto earthquake

Abstract

Shallow S-wave velocity V S profiles were estimated for 26 temporary strong motion observation sites surrounding the epicenters of a sequence of the 2016 Kumamoto earthquake. The microtremor array method was used to gather the dispersion characteristics of Rayleigh waves. V S profiles were obtained by inverting the dispersion curves for each site and those of three permanent strong motion stations that recorded the sequence of seismic events. The shallow V S profiles near two of the permanent strong motion stations in the town of Mashiki were almost identical. However, the V S profiles at other stations varied. The V S profiles were found to have the common feature of the uppermost low-velocity layer being widely distributed from Mashiki to the village of Minami-Aso, and it was especially thick in the areas that suffered heavy damage. This low-velocity layer was a major contributor to the site amplification. The horizontal-to-vertical spectral ratios of the microtremors indicate that both the shallow soil and deep sedimentary layers may control the site response characteristics over a broad frequency range.

Simple assessment of shallow velocity structures with small-scale microtremor arrays: interval-averaged S-wave velocities

“…An empirical law, before everything else, has limitations of accuracy. At site KMMH16, array measurements of microtremors were taken and Rayleigh-wave phase velocity dispersion curves were shown by Arai and Kashiwa (2017) and Chimoto et al (2016) as well. We have read their dispersion curves, applied our present method to each of them and compared the results with the interval-averaged S-wave speeds obtained in this study (Figure 6b).…”

“…For example, the distribution of layers of a subsurface structure strongly influences the analysis results, and therefore, requires time for validation; an interested reader can compare between the inversion results in the publications of Arai and Kashiwa (2017) and Chimoto et al (2016). Our present method has a different application.…”

Development and numerical tests of a Bayesian approach to inferring shallow velocity structures using microtremor arrays

“…Chimoto et al, 2016). In some studies, V S (S-wave velocity) is fixed and only the layer thickness is estimated (e.g.…”

“…This assumption seems quite common, in particular, for the spatial autocorrelation (SPAC) method (Aki, 1957), which has been widely adopted in recent years (e.g. Chimoto et al, 2016;Goto et al, 2017). However, higher modes are indeed easily excitable in shallow structures (e.g.…”

Special issue “2016 Kumamoto earthquake sequence and its impact on earthquake science and hazard assessment”

“…Yamanaka et al (2016) conduct dense observation of aftershocks in heavily damaged areas and find local amplification effects in Mashiki, Nishihara and Kumamoto, but not in Minami-Aso. Chimoto et al (2016) conduct array analysis in Mashiki and Nishihara and obtain the S-wave velocity structure, which they consider controls site amplification. On the other hand, Kawase et al (2017) make a detailed survey of damages and reanalyze strong-motion records in the Mashiki town, attributing severe structural damages to a strong westward velocity pulse, rather than to site effects.…”