Difference of horizontal-to-vertical spectral ratios of observed earthquakes and microtremors and its application to S-wave velocity inversion based on the diffuse field concept

Abstract

We have been discussing the validity of using the horizontal-to-vertical spectral ratios (HVRs) as a substitute for S-wave amplifications after Nakamura first proposed the idea in 1989. So far a formula for HVRs had not been derived that fully utilized their physical characteristics until a recent proposal based on the diffuse field concept. There is another source of confusion that comes from the mixed use of HVRs from earthquake and microtremors, although their wave fields are hardly the same. In this study, we compared HVRs from observed microtremors (MHVR) and those from observed earthquake motions (EHVR) at one hundred K-NET and KiK-net stations. We found that MHVR and EHVR share similarities, especially until their first peak frequency, but have significant differences in the higher frequency range. This is because microtremors mainly consist of surface waves so that peaks associated with higher modes would not be prominent, while seismic motions mainly consist of upwardly propagating plain body waves so that higher mode resonances can be seen in high frequency. We defined here the spectral amplitude ratio between them as EMR and calculated their average. We categorize all the sites into five bins by their fundamental peak frequencies in MHVR. Once we obtained EMRs for five categories, we back-calculated EHVRs from MHVRs, which we call pseudo-EHVRs (pEHVR). We found that pEHVR is much closer to EHVR than MHVR. Then we use our inversion code to invert the one-dimensional S-wave velocity structures from EHVRs based on the diffuse field concept. We also applied the same code to pEHVRs and MHVRs for comparison. We found that pEHVRs yield velocity structures much closer to those by EHVRs than those by MHVRs. This is natural since what we have done up to here is circular except for the average operation in EMRs. Finally, we showed independent examples of data not used in the EMR calculation, where better ground structures were successfully identified from pEHVRs again. Thus we proposed here a simple empirical method to estimate S-wave velocity structures using single-station microtremor records, which is the most cost-effective method to characterize the site effects.Keywords: Site effect, Subsurface structure, Seismological bedrock, Horizontal-to-vertical ratio, Hybrid heuristic search, S-wave, P-wave © The Author(s) 2017. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.

Implementing horizontal-to-vertical Fourier spectral ratios and spatial correlation in a ground-motion prediction equation to predict site effects

“…Numerous studies have demonstrated that both the EHVRs and the MHVRs can be used for site classification; however, for the same station, EHVRs and MHVRs can differ. Kawase et al (2018) determined that MHVRs and EHVRs share similarities, especially from low frequency until their first peak frequency but exhibit significant differences in the higher-frequency range. They argued that this is because microtremors mainly consist of surface waves, so that peaks associated with higher modes would not be prominent, whereas seismic motions mainly consist of upwardly propagating body waves, so that higher mode resonances can be observed in high-frequency domain (Kawase et al, 2018).…”

Ground motion prediction model for southeastern México removing site effects using the Earthquake horizontal-to-vertical ratio (EHVSR)

“…In order to evaluate seismic hazard, site effects have to be incorporated back correcting the GMPE using HVSR with the appropriate corrections as proposed by Kawase et al (2018). Note that HVSR is a proxy of empirical transfer functions in low frequencies with obvious underestimations in higher frequencies.…”

“…The source effect is approximately removed. In a recent paper, Kawase et al (2018) suggested to consider the amplification due to vertical motion to avoid overreduction of FAS. However, this requires recordings both in soil and rock sites.…”

Site Effect Assessment in Ulaanbaatar, Mongolia through Inversion Analysis of Microtremor H/V Spectral Ratios

“…In this theory, MHVR can be numerically computed as the imaginary parts of Green's functions, and the theoretical H/V ratio depends on underground layer thickness and elastic properties of soil. Hence the theory allowed for inverting V S profiles from MHVR [13][14][15][16]. One of the authors also examined the applicability of the DFA for the MHVR-based V S profiling and presented the validity of the technique for the site characterizations [17].…”