Seismic site response analysis allows an engineer to assess the effect of local soil conditions on the ground motions expected during an earthquake. In seismic site response analysis, an input ground motion on rock is propagated through a site specific soil column. The computed response at the surface is dependent on both the input ground motion and the soil properties that characterize the site. However, there is uncertainty in both the input ground motion and the soil properties, as well as natural variability in the soil properties across a site. To account for the uncertainty in the input ground motions, engineers use a suite of motions that are selected and scaled to fit a scenario input motion. This study introduces a semi-automated method to select and scale the input motions to fit a target input motion and its variability. The proposed method is intended to replace tedious trials of combinations by hand with combinations performed by a computer. However, as in the traditional selection methods, the final selection of the combination is done by the engineer.The effect of the selected ground motion combination on the computed surface response spectrum from the site response analysis, and its variability, was investigated in this study. The results show by using a combination with as few as five motions, the median surface response spectrum can be predicted with an error of 10%. Additionally, the manner used to scale the input motions does not impact the accuracy of the median surface response spectrum, as long as the median response spectrum of the input combination agrees with the target input response spectrum. However, if the standard deviation of the surface response spectrum is to be considered (e.g., to develop median plus one standard deviation spectra), a input combination of at least 20 motions is recommended and the combination must be scaled such that the standard deviation of the input combination matches the standard deviation of the input target spectrum. Monte Carlo simulations were used to assess the impact of soil property variability on surface spectra computed by seismic site response. The results from this study indicate that by accounting for the variability of the shear-wave velocity profile of a site can cause a significant decrease in the median surface response spectrum, as well as a slight increase in the standard deviation of the surface response spectrum at periods less than the site period. By considering the variability of the nonlinear properties (shear modulus reduction and damping ratio) the median response spectrum decreased only slightly, but the standard deviation increased in a manner similar to the increase observed when considering the variability of the shear-wave velocity profile. Simultaneously considering the variability of the shear-wave velocity profile and nonlinear properties resulted in a median surface response spectrumsimilar to the median surface response spectrumcomputed with considering the variability of the shear-wave velocity alone. However, the standard deviation of the surface response spectrum was larger than the standard deviation computed by independent consideration of the variability of the shear-wave velocity or nonlinear properties.

While site effects are accounted for in most modern U.S. seismic design codes for building structures, there exist no standardized procedures for the computationally efficient integration of nonlinear ground response analyses in broadband ground motion simulations. In turn, the lack of a unified methodology affects the prediction accuracy of site-specific ground motion intensity measures, the evaluation of site amplification factors when broadband simulations are used for the development of hybrid attenuation relations and the estimation of inelastic structural performance when strong motion records are used as input in aseismic structural design procedures. In this study, a set of criteria is established, which quantifies how strong nonlinear effects are anticipated to manifest at a site by investigating the empirical relation between nonlinear soil response, soil properties, and ground motion characteristics. More specifically, the modeling variability and parametric uncertainty of nonlinear soil response predictions are studied, along with the uncertainty propagation of site response analyses to the estimation of inelastic structural performance. Due to the scarcity of design level ground motion recording, the geotechnical information at 24 downhole arrays is used and the profiles are subjected to broadband ground motion synthetics. For the modeling variability study, the site response models are validated against available downhole array observations. The site and ground motion parameters that govern the intensity of nonlinear effects are next identified, and an empirical relationship is established, which may be used to estimate to a first approximation the error introduced in ground motion predictions if nonlinear effects are not accounted for. The soil parameter uncertainty in site response predictions is next evaluated as a function of the same measures of soil properties and ground motion characteristics. It is shown that the effects of nonlinear soil property uncertainties on the ground-motion variability strongly depend on the seismic motion intensity, and this dependency is more pronounced for soft soil profiles. By contrast, the effects of velocity profile uncertainties are less intensity dependent and more sensitive to the velocity impedance in the near surface that governs the maximum site amplification. Finally, a series of bilinear single degree of freedom oscillators are subjected to the synthetic ground motions computed using the alternative soil models, and evaluate the consequent variability in structural response. Results show high bias and uncertainty of the inelastic structural displacement ratio predicted using the linear site response model for periods close to the fundamental period of the soil profile. The amount of bias and the period range where the structural performance uncertainty manifests are shown to be a function of both input motion and site parameters.

Local soil conditions influence the characteristics of earthquake ground shaking and these effects must be taken into account when specifying ground shaking levels for seismic design. These effects are quantified via site response analysis, which involves the propagation of earthquake motions from the base rock through the overlying soil layers to the ground surface. Site response analysis provides surface acceleration-time series, surface acceleration response spectra, and/or spectral amplification factors based on the dynamic response of the local soil conditions. This dissertation investigates and compares the results from different site response methods. Specifically, equivalent-linear time series analysis, equivalent-linear random vibration theory analysis, and nonlinear time series analysis are considered. In the first portion of this study, hypothetical sites and events are used to compare the various site response methods. The use of hypothetical events at hypothetical sites allowed for the seismic evaluation process used in engineering practice to be mimicked. The hypothetical sites were modeled after sites with characteristics that are representative of sites in the Eastern and Western United States. The input motions selected to represent the hypothetical events were developed using the following methods: stochastically-simulated time series, linearly-scaled recorded time series, and spectrally-matched time series. The random vibration theory input motions were defined using: seismological source theory, averaging of the Fourier amplitude spectra computed from scaled time series, and a response spectrum compatible motion. All of the different input motions were then scaled to varying intensity levels and propagated through the sites to evaluate the relative differences between the methods and explain the differences. Data recorded from borehole arrays, which consist of instrumentation at surface and at depth within the soil deposit, are used to evaluate the absolute bias of the site response methods in the second portion of this study. Borehole array data is extremely useful as it captures both the input motion and the surface motion, and can be used to study solely the wave propagation process within the soil deposit. However, comparisons using the borehole data are complicated by the assumed wavefield at the base of the array. In this study, sites are selected based on site conditions and the availability of high intensity input motions. The site characteristics are then developed based on site specific information and data from laboratory soil testing. Comparisons between the observed and computed response are used to first assess the wavefield at the base of the array, and then to evaluate the accuracy of the site response methods. / text

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21950号 / 理博第4528号 / 新制||理||1650(附属図書館) / 京都大学大学院理学研究科地球惑星科学専攻 / (主査)教授 釜井 俊孝, 教授 千木良 雅弘 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Local site conditions are known to influence ground motion during earthquake events and increase the severity of damage. Data from earthquakes are useful to study the response but they are available only from active regions. Ubiquitous ambient vibrations on the other hand offer a more practical approach to quantify site responses. This thesis explores the use of various methods for obtaining site responses. The primary area of study is the Kachchh rift basin, NW India, a Mesozoic rift that features significant lateral variations in surface geology and has experienced ground responses during 1819 and 2001 earthquakes. The Mw 7.6, 2001 event was followed by hundreds of aftershocks, which were recorded by temporary networks. In this study we have used earthquake signals as well as ambient vibrations to understand site response in various parts of the basin. In addition we have collected data from a few sites from the Indo-Gangetic plains and Kathmandu valley, both affected by large earthquakes, 1934 the M ~ 8 (Bihar) and 2015, Mw 7.8 (Nepal). Velocity and acceleration records from a network of eight stations in the Kachchh Rift were used to evaluate site responses using Standard Spectral Ratio (SSR) and Horizontal to Vertical spectral ratio (HVSR-E) methods. Ambient vibrations were analyzed following Nakamura’s H/V method (HVSR-AV), for data collected from 110 sites that represent different field conditions within the Kachchh Rift. Fundamental resonance frequency (f0) varied between 0.12 – 2.30 Hz, while the amplification factor (A0) was in the range of 2.0 – 9.1. We found that higher A0 and liquefaction index (Kg) values were mostly associated with higher liquefaction potential. Using a close network of stations, we studied the role of site response in damage to the Bhuj city that suffered maximum damage in 2001; our results suggest that site response was not a significant factor. Studies based on passive data were complemented by Multi-channel Analysis of Surface Waves (MASW) to map shear wave velocities of the various subsurface units up to depths of 10m (Vs10) and 30m (Vs30). Our results imply average Vs could be a good proxy to characterize site amplifications where sediment thicknesses are shallow. Power law relationship between f0 and thickness (h) suggest a strong positive correlation (r = 0.89) adding credence to HVSR-AV method, making it a cost-effective alternative to MASW to infer site conditions. Further, to understand the influence of topography on site effects, we analyzed data from hills, valleys and their edges, both from the Kachchh rift and Kathmandu valley. Sites on the edges of valleys showed multiple, fuzzy peaks in the low frequency range (< 1 Hz) and broad peaks attributable to sites prone to higher damage. Spectrograms generated through Huang-Hilbert Transforms (HHT) suggested focusing of energy in narrow frequency bands on the edges, while valleys tend to scatter energy over wide frequencies. Although our current results are based on limited observations, we recognize spectral analysis as a powerful tool to quantify site effects in regions with significant topography. It is known that coseismic liquefaction could lead to nonlinear behavior wherein the near-surface soil layer loses its shear strength, causing a reduction of its fundamental resonance frequency. We used data from selected sites of coseismic liquefaction to highlight the significance of nonlinear effects in site response. Earthquake signals and ambient vibrations from Umedpur, a region that experienced intense liquefaction during 2001 were used in this analysis. Here we followed an empirical decomposition method based on HHT and signals were decomposed as many intrinsic mode functions (IMFs) that showed characteristic peaks for events of various values of PGAs. Thus, the first IMF for events with relatively higher PGAs (0.03g) showed distinct peaks for the S wave coda part, which were not noted for those with lower PGA (0.01g). These observations in a region of coseismic liquefaction are useful in developing models for quantifying nonlinear behavior. In conclusion, site response studies using different types of data and processing techniques in regions affected by recent earthquakes brings out the scope and limitations of each of these sets of data and techniques. This study suggests that ambient vibrations provide reasonable estimates of site response and can be reliably used in regions where earthquake data are not available.