Evaluating The Site Specific Applicability Of One Dimensional Seismic Ground Response Analysis

One-Dimensional (1D) seismic ground response analysis is the most commonly performed analysis in geotechnical earthquake engineering. However, previous studies have shown a troubling fact that only a small fraction of sites are modeled well by 1D analysis. The objectives of this research are to assess the site-specific suitability of 1D analysis by identifying the issues that hinder the performance 1D analysis and to develop approaches to better match the observed sites response. The downhole array technique is used in this work to evaluate 1D analysis because it provides the most direct observations of how seismic waves are modified by the subsurface soil and rock. An important phenomenon in downhole array analysis is the potential presence of pseudo-resonances, which has not been effectively taken into account in previous studies and which affects the assessment of the accuracy of 1D analysis. The first part of this research provides insights into the cause and effect of pseudo-resonances and an approach is outlined to distinguish true-resonances from pseudo-resonances. The small-strain damping (D [subscript min] ) is a key parameter in linear ground response analysis and using laboratory-measured values tend to over-predict the response because it does not account for wave scattering present in the field. The second part of this research focuses on methods of increasing the D [subscript min] values in the profiles to better match observed site response, with the site response evaluated in terms of different ground motion characteristics. Alternatively, the randomization of shear wave velocity profiles is also assessed to provide more insights into the variable seismic properties at a site. A hypothesis that links the level of increased damping to the level of spatial variability in materials implied by the geologic conditions is proposed. To broaden the application of the 1D analysis, it is crucial to be able to identify sites that can be modeled accurately by 1D analysis. A taxonomy scheme is developed that classifies sites into different groups based on the similarity in their responses in terms of being modeled well by 1D analysis. This classification system is based on downhole array data but can be applied to non-downhole array sites. The taxonomy results presented in this study show that an increased portion of sites are suitable for 1D analysis.

TRB’s National Cooperative Highway Research Program (NCHRP) Synthesis 428: Practices and Procedures for Site-Specific Evaluations of Earthquake Ground Motions identifies and describes current practice and available methods for evaluating the influence of local ground conditions on earthquake design ground motions on a site-specific basis.

In this dissertation, we study the effects of site response on earthquake ground motions, the uncertainty in site response, and incorporating site response in probabilistic seismic hazard analysis. We introduced a guideline for evaluation of non-ergodic (site-specific) site response using (a) observations from available recorded data at the site, (b) simulations from one-dimensional ground response analysis, or (c) a combination of both. Using non-ergodic site response is expected to be an improvement in comparison to using an ergodic model which is based on the average of a global dataset conditional on site parameters used in ground motion models. The improvement in prediction when using non-ergodic analysis results in the removal of site-to-site variability which is a part of the uncertainty in ground motion prediction. The site-to-site variability is evaluated by partitioning the residuals to different sources of variability. We illustrate application of these procedures for evaluating non-ergodic site response, and use examples to show how the reduction in site response uncertainty results in less hazard for long return periods. We utilize a dataset of recordings from vertical array sites in California in order to study the effectiveness of one-dimensional ground response analysis in predicting site response. We use the California dataset for comparing the performance of linear ground response analysis to similar studies on a dataset from vertical arrays in Japan. We use surface/downhole transfer functions and amplification of pseudo-spectral acceleration to study the site response in vertical arrays. For performing linear site response analysis for the sites, we use three alternatives for small-strain soil damping namely (a) empirical models for laboratory-based soil damping; (b) an empirical model based on shear wave velocity for estimating rock quality factor; and (c) estimating damping using the difference between the spectral decay ( ) at the surface and downhole. The site response transfer functions show a better fit for California sites in comparison to the similar results on Japan. The better fit is due to different geological conditions at California and Japan vertical array sites, as well as the difference in the quality of data for the two regions. We use pseudo-spectral acceleration residuals to study the bias and dispersion of ground response analysis predictions. The results of our study shows geotechnical models for lab-based damping provide unbiased estimates of site response for most spectral periods. In addition, the between- and within-site variability of the residuals do not show a considerable regional between California and Japan vertical arrays. In another part of this dissertation, we develop ground motion models for median and standard deviation of the significant duration of earthquake ground motions from shallow crustal earthquakes in active tectonic regions. The model predicts significant durations for 5-75%, 5-95%, and 20-80% of the normalized Arias intensity, and is developed using NGA-West2 database with M3.0-7.9 events. We select recordings based on the criteria used for developing ground motion models for amplitude parameters as well as a new methodology for excluding recordings affected by noise. The model includes an M-dependent source duration term that also depends on focal mechanism. At small M, the data suggest approximately M-independent source durations that are close to 1 sec. The increase of source durations with M is slower over the range M5 to 7.2-7.4 than for larger magnitudes. We adopt an additive path term with breaks in distance scaling at 10 and 50 km. We include site terms that increase duration for decreasing VS30 and increasing basin depth. Our aleatory variability model captures decreasing between- and within-event standard deviation terms with increasing M. We use the model for validating the duration of ground motion time series produced by simulation routines implemented on the SCEC Broadband Platform. This validation is based on comparisons of median and standard deviation of simulated durations for five California events, and their trends with magnitude and distance, with our model for duration. Some misfits are observed in the median and dispersion of durations from simulated motions and their trend with magnitude and distance. Understanding the source of these misfits can help guide future improvements in the simulation routines.

This book presents state-of-the-art information on seismic ground response analysis, and is not only very valuable and useful for practitioners but also for researchers. The topics covered are related to the stages of analysis: 1. Input parameter selection, by reviewing the in-situ and laboratory tests used to determine dynamic soil properties as well as the methods to compile and model the dynamic soil properties from literature;2. Input ground motion; 3. Theoretical background on the equations of motion and methods for solving them; 4. The mechanism of damping and how this is modeled in the equations of motions; 5. Detailed analysis and discussion of results of selected case studies which provide valuable information on the problem of seismic ground response analysis from both a theoretical and practical point of view.

One dimensional (1D) site response analysis using total stress approach is a popular framework for evaluating seismic hazard at a site where no significant excess pore water pressure generation is expected. Several site response analysis codes are available, but their variabilities to predict site response at shear stress levels approaching the shear strength of the soil have not been demonstrated. This study evaluates the performance of different soil models employed in each code to predict the nonlinearity behavior of soil over a wide range of strain levels. This research performed a set of 1D site response analyses utilizing input motions, scaled to various intensity levels, against sites that were underlain by cohesive deposits with determined shear wave velocity profiles. The analyses utilized several available nonlinear soil models while the model-to-model variability was characterized. These codes were then validated against free-field downhole data from a vertical array at a relatively well characterized site. The evaluations of the variabilities of ground motion amplitude, duration, response spectra and cyclic hysteresis loop at various strain levels were performed by comparing all predictions to data from a vertical array. The results showed that all codes give consistent predictions with reasonable accuracy at small to moderate shear strain levels. At larger shear strain levels, only some of current nonlinear soil models were capable of predicting reasonable cyclic behavior in terms of being able to approach the peak shear strength with reasonable damping behavior. The analyses show that the variability of predicted peak shear strain parameters are higher than peak acceleration and shear stress parameters. It also shows that the coefficient of variation of the ground motion parameters predicted by all codes tended to increase at greater shear strain levels.

This volume brings together contributions from world renowned researchers and practitioners in the field of geotechnical engineering. The chapters of this book are based on the keynote and invited lectures delivered at the 7th International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. The book presents advances in the field of soil dynamics and geotechnical earthquake engineering. A strong emphasis is placed on proving connections between academic research and field practice, with many examples, case studies, best practices, and discussions on performance-based design. This volume will be of interest to research scholars, academicians and industry professionals alike.

This volume presents select papers presented at the 7th International Conference on Recent Advances in Geotechnical Earthquake Engineering and Soil Dynamics. The papers discuss advances in the fields of soil dynamics and geotechnical earthquake engineering. Some of the themes include ground response analysis & local site effect, seismic slope stability & landslides, application of AI in geotechnical earthquake engineering, etc. A strong emphasis is placed on connecting academic research and field practice, with many examples, case studies, best practices, and discussions on performance based design. This volume will be of interest to researchers and practicing engineers alike.

As recognized universally by both seismology and earthquake engineering communities, the amplitude and frequency content of ground motions are influenced by local site effects, including the effects of near-surface geologic materials, surface topographic and basin effects, and so on. Strong linkage between seismic site effect and earthquake damage has been commonly demonstrated from many past earthquakes. Therefore, quantitative and reliable evaluation of the seismic site effect is one of the crucial aspects in seismic hazard assessment and risk mitigation. With the significant advancement of modern seismic monitoring networks and arrays, huge amounts of high-quality seismic records are now being accumulated. This encourages us to measure the site responses and its associated uncertainty for selected seismic stations by some record-dependent approaches, such as horizontal-to-vertical spectral ratio (HVSR) measurements, generalized spectral inversion (GIT) methods, etc. Machine learning techniques also show significant promise in characterization of the near-surface geologic properties and prediction of site response. These data-driven approaches help us to better understand the physics of spatial and temporal variabilities of ground motions. Due to more and more site-specific data being captured, invoking non-ergodic assumptions in seismic response analysis has recently been a topic of great interest in the community. For specific site response analysis, numerical simulations are carried out to model the dynamic process of seismic waves propagating and scattering in the subsurface strata. With development of modeling capacity, great efforts have been taken to evaluate quantitatively the complex 2D and 3D effects on seismic site response.

Ground motion models (GMMs) are used to predict ground motion intensity measures given parameters descriptive of source, path, and site conditions. These GMMs incorporate source, path, and site response models that represent approximately the average conditions in the database from which the GMMs were derived. In the case of NGA-type models, global data is used, potentially with path and site adjustments for large regions with ample data (e.g., California), so the predictions represent either global or regional averages. In contrast, when GMMs are applied for a specific engineering project, the source, path, and site response attributes of interest are those local to the site, which may depart from the global or regional averages represented by the GMM. In this context, I refer to the source, path, and site models in the GMM as ergodic. Alternative models that consider local, or site-specific features, are considered non-ergodic, and have the potential to significantly reduce the ground motion variability that is considered in probabilistic seismic hazard analysis. My thesis work is concerned principally with the site response component of GMMs, and in particular, with evaluating the effectiveness of predictive models available for non-ergodic site response analysis. The ergodic site amplification within a GMM represents the global or regional average for the site's value of time-averaged upper 30 meters shear wave velocity and basin depth. Many local effects may introduce departures in site response from the ergodic model, including strong impedance contrasts within the shear wave velocity profile, an unfavorable location relative to a basin edge, complexity of local terrain, and perhaps other factors. Therefore, the ergodic site response model has two drawbacks: (1) potential for a biased estimate of mean site response and (2) because the ergodic model averages over a diverse array of conditions having many different site responses, the model carries a relatively large standard deviation. The alternative of non-ergodic site response takes into account the particular geologic conditions at a site that control site response. If applied properly, non-ergodic site response can produce unbiased estimates of site response and remove site-to-site variability from the total standard deviation, which is a significant contributor. One method of evaluating non- ergodic site response in practice is to utilize recordings at the site to evaluate misfits from a GMM, and then use this information to construct a median site response model. However, when on-site recordings are not available, site-specific analysis requires the application of various predictive models. The questions addressed in this research relate to the effectiveness of different predictive models for estimation of site response. The general approach followed in this research was to develop a database of available recordings for sites in a study region, analyze the data to develop non-ergodic site responses, and then either 1) apply existing predictive models to the sites with "measured" (i.e., non- ergodic) site responses and then evaluate their effectiveness over the population of sites or; 2) develop a new predictive model where existing models cannot be reasonably applied. The first approach of evaluating existing tools is applied to a population of 159 sites in California. The second approach of developing a new model is applied to 7 sites in Obihiro (Japan), where soft soil conditions (VS30 = 102 to 211 m/s) require the development of a novel modeling framework. For the California sites, the predictive models considered are ground response analysis (GRA; one-dimensional shear wave propagation through a soil column), square-root impedance method (SRI), and models conditioned on horizontal-to-vertical spectral ratio (HVSR) vs frequency plots. The GRA and SRI methods require a shear wave velocity (VS) profile for the site and models for material damping for each soil horizon in the profile. Among the 159 sites, the profile depth range is 30 to 255 m (profile period range is 0.06 to 1.02 sec). The HVSR model requires HVSR data, which can be derived from microtremors or earthquake recordings. A challenge that was encountered in the application of GRA and SRI methods was the lack of soil profiles to accompany VS profiles. I developed protocols for estimation of soil type parameters that allow geotechnical damping models to be applied. Additional damping models were also considered, including one that is informed by high-frequency spectral decay of site ground motions ( 0). Despite the depth of the profiles considered in this work being relatively modest, ground response analyses (or square-root-impedance analyses) are able to improve site response predictions relative to ergodic models for approximately 36% of sites (for periods less than or equal to the site period). The inability of site-specific methods to improve prediction accuracy for the 64% sites could stem from three potential sources: (1) simulations of one-dimension wave propagation do not accurately characterize the physics of site response; (2) the measured VS profile from the site does not accurately represent site conditions, either because of strong site heterogeneity or inaccurate measurements; (3) portions of the site profile beneath the profile depth significantly impact the site response in the frequency range of the measured profile. These problems are common to some extent in virtually all site response simulations, so understanding their collective impact is of practical importance. The unknown influence of these factors introduces epistemic uncertainties, which we quantify. Lacking any knowledge of whether a given site is well represented with one-dimensional simulations, this epistemic uncertainty is only slightly reduced from that of the site-to-site variability in ergodic models within soil column period range. For the subset of sites where this modeling is effective, the epistemic uncertainty is more substantially reduced by amounts ranging from 0.05-0.10 in natural log units. The HVSR model considered in this work (adapted from a model in literature) uses the frequency and amplitude of peaks in HVSR spectra. I identify three populations of sites based on microtremor data - those for which a clear HVSR peak is evident (40%), those for which no peak occurs (40%), and intermediate/ambiguous cases (20%). When the ergodic model is used, sites with a peak are observed to have higher bias and site-to-site variability than sites without peaks; as a result, commonly used models for site-to-site variability represent a blending of these condition because the occurrence of peaks is not accounted for. Use of the HVSR model for sites with peaks does not appreciably change the bias but reduces dispersion at long periods (> 1 sec) relative to what is obtained with an ergodic model. The lack of improvement at short period could be caused by false positives (peaks in HVSR that do not appear in site response) and not well-aligned peak positions between HVSR and site response, and may also be influenced by the model used in our analyses having been derived for conditions in Japan. I recommend a California-specific bias correction for sites without a peak. For the Obihiro (Japan) sites, I developed a region-specific site amplification model applicable to the peaty organic soils in this region. The analysis of site response from regional data required removal of source-specific biases and careful consideration of source-to-site path effects. These considerations were essential to avoid mapping source- or path-related model misfits into estimates of site response. I considered two subduction ground motion models as reference models. By paying special attention to the conditions for which the path models are effective, and making adjustments for between-island path misfits (Hokkaido to Honshu and vice-versa), I found the proposed approach effectively identifies site effects, and that the results are insensitive to the selected ground motion model. Observed site responses are characterized by strong resonances at first-mode site frequencies as derived from HVSR measurements.

Earthquake Geotechnical Engineering for Protection and Development of Environment and Constructions contains invited, keynote and theme lectures and regular papers presented at the 7th International Conference on Earthquake Geotechnical Engineering (Rome, Italy, 17-20 June 2019. The contributions deal with recent developments and advancements as well as case histories, field monitoring, experimental characterization, physical and analytical modelling, and applications related to the variety of environmental phenomena induced by earthquakes in soils and their effects on engineered systems interacting with them. The book is divided in the sections below: Invited papers Keynote papers Theme lectures Special Session on Large Scale Testing Special Session on Liquefact Projects Special Session on Lessons learned from recent earthquakes Special Session on the Central Italy earthquake Regular papers Earthquake Geotechnical Engineering for Protection and Development of Environment and Constructions provides a significant up-to-date collection of recent experiences and developments, and aims at engineers, geologists and seismologists, consultants, public and private contractors, local national and international authorities, and to all those involved in research and practice related to Earthquake Geotechnical Engineering.