Two-stage nonlinear site amplification modeling for Japan with VS30 and fundamental frequency dependency

We describe a two-stage nonlinear site amplification model that considers both the sediment depth and the fundamental frequency of the soil. Adopting a non-reference site approach, empirical site amplification ratios for 1591 Kyoshin and Kiban Kyoshin network sites are developed. This process involves analyzing 92,025 surface ground motion recordings from 305 earthquakes between 1997 and 2012. The model is developed in two stages: first, we regress the total empirical site effects in terms of VS30; second, we fit a functional form to the residuals from the first stage using the fundamental period of the site. We validate the model through residual analyses and statistical methods. This study shows that the peak soil period increases with the equivalent shear strain ( Iγ), while the peak amplitude decreases with Iγ. The first-stage site term reduces the error at mid-to-long period ranges (>0.1 s) by 0.11 from the error of the initial site term without any constraints. The second-stage site term reduces the error further as 0.06. Finally, we discuss the model limitations and the applicability range of the predictor variables.

The synthetic spectra of potential earthquakes for Taipei basin

<p>Modeling the amplitude spectra based on the source term, the path one and site ones for 54 sites located in and around the Taipei basin is the aim of this study. The site term includes the amplification function varied with frequency and the site-specific parameter (k0). The amplification functions for Class-C, -D, and -E site are from Huang et al. (2007) for the central Taiwan. Meanwhile, the amplification function for Class-B site can be referred to Boore and Joyner (1997). The root-mean-squared spectral amplitudes of two horizontal shear waves after three-point smoothing from the observed seismograms are compared to the synthetic amplitude spectra. The goodness of fit coefficient (GFC) and the residual errors (ERR) are calculated for concluding the fitness of the modeling amplitude spectra. Results show both the GFC and ERR of stations are varied with the earthquake magnitude and hypo-central distance. The averaged GFC are larger than 0.8 for 42 stations. Meanwhile, there are 12 station with averaged GFC smaller than 0.8. Besides, the ERRs of 28 stations are less than 0.5. Meanwhile, there are 18 stations with ERRs in the range of 0.5-0.6. The obtained results may be used for modeling the amplitude spectra for the Taipei area. The more accurate amplitude spectra can be improved by updating the parameters utilized in the source-, the path- and the site terms.</p>

Ground motion prediction equation for Taiwan subduction zone earthquakes

A ground motion prediction equation (GMPE) is presented for computing the median and standard deviation of peak ground acceleration (PGA) and 5% damped pseudo-spectral acceleration (PSA) for periods between 0.01 s and 5.0 s for probabilistic seismic hazard analysis (PSHA) and engineering applications in Taiwan. An integrated strong motion dataset consisting of two subduction earthquake regions was selected from 3314 recordings from Taiwan with M4.5 to M7.1 and 3376 recordings from Japan with M6.5 to M9.1. This dataset was then used to validate, and refit where necessary, the function form provided by Abrahamson et al. for application to Taiwan subduction earthquakes. The proposed model accounts for the extrapolation behaviors associated with the large-magnitude scaling and the near-source scaling terms, both of which were developed empirically by using the combined Taiwan–Japan dataset. The distance attenuation and site term were developed specifically for the Taiwan region. The site term is based on two parameters; the time-averaged shear wave velocity of the top 30 m depth ( VS30) and the depth-to-the-shear wave velocity horizon of 1.0 km/s ( Z1.0).

Scenario‐Dependent Site Effects for the Determination of Unbiased Local Magnitude

We explore the role of scenario‐dependent site amplification on local magnitude (ML) and possible bias it may introduce. ML is strongly influenced by local site response, which is conditioned by unique local geological factors. To isolate the effect of the near‐surface amplification on ML, relative differences between station‐specific ML at the surface and borehole (ΔML,STN) are studied for 34 sites from the KiK‐net network, Japan. We find strong moment magnitude (M) dependent scenario‐specific ΔML,STN trends over the range 3.0<M<6.5. To model these trends, we employ the stochastic method, initially using empirical surface‐to‐borehole (S/B) Fourier spectral ratios for the site term. Simulated data, ΔML,STN(M), based on the available site‐response information are shown to closely match the empirical ΔML,STN trends. Subsequently, the site term is replaced with (a) linear 1D shear‐wave (horizontal) transfer function (1D‐SHTF) amplification, (b) horizontal‐to‐vertical ratios, and (c) quarter wavelength amplification to calculate ΔML,STN(M) in the absence of S/B. We find that ΔML,STN(M) trends are best estimated with S/B as the site term, but in many cases using a linear 1D‐SHTF model is adequate. Furthermore, we discuss how this phenomenon may be related to the observed inequality between M and ML at low magnitudes and how ΔML,STN(M) may be used in the future to compute unbiased ML with greater confidence.

Testing the Depths to 1.0 and 2.5 km/s Velocity Isosurfaces in a Velocity Model for Japan and Implications for Ground‐Motion Modeling

In the Next Generation Attenuation West2 (NGA‐West2) project, a 3D subsurface structure model (Japan Seismic Hazard Information Station [J‐SHIS]) was queried to establish depths to 1.0 and 2.5 km/s velocity isosurfaces for sites without depth measurement in Japan. In this article, we evaluate the depth parameters in the J‐SHIS velocity model by comparing them with their corresponding site‐specific depth measurements derived from selected KiK‐net velocity profiles. The comparison indicates that the J‐SHIS model underestimates site depths at shallow sites and overestimates depths at deep sites. Similar issues were also identified in the southern California basin model. Our results also show that these underestimations and overestimations have a potentially significant impact on ground‐motion prediction using NGA‐West2 ground‐motion models (GMMs). Site resonant period may be considered as an alternative to depth parameter in the site term of a GMM.

Non-Ergodic Site Response in Seismic Hazard Analysis

Probabilistic seismic hazard analyses are usually performed with semi-empirical ground motion models (GMMs) following the ergodic assumption whereby average source, path, and site effects from global databases apply for a specific site of interest. Site-specific site response is likely to differ from the global average conditional on site parameters used in GMMs (typically V S30 and basin depth). Non-ergodic site response can be evaluated using on-site ground motion recordings and/or one-dimensional wave propagation analyses, and allows site-to-site variability to be removed from the within-event standard deviation. Relative to ergodic, non-ergodic hazard analyses often reduce ground motions at long return periods. We describe procedures for replacing the site term in GMMs with a non-ergodic nonlinear mean over its appropriate range of periods (returning to the ergodic mean outside that range). We also present procedures for computing non-ergodic standard deviation by removing site-to-site variability while considering effects of soil nonlinearity. We illustrate application of these procedures, and their effect on hazard curves and uniform hazard spectra, as implemented in OpenSHA.

An alternative approach to site factors for NBCC 2015

The site amplification factors in the 2015 edition of the National Building Code of Canada (NBCC 2015) for South Western British Columbia (SWBC) were developed using only the single site term associated with the ground motion prediction equation (GMPE) for crustal sources. The seismic hazard in SWBC is derived from three source types; crustal, subduction intraplate, and subduction interface sources. Therefore site factors based only on a crustal site term are approximate. This study was conducted to assess the degree of approximation. Alternative site amplification factors for Vancouver and Victoria, British Columbia were calculated using site specific look-up tables incorporating all the GMPEs associated with each seismic source type and their associated site terms. The alternative site factors are significantly greater than those given in NBCC 2015 for periods 0.3 s, 0.5 s, and 1.0 s. These results suggest that the process for calculating site amplification factors should be reassessed by the committee currently reviewing seismic hazard for NBCC 2020.