Dissertations / Theses on the topic 'Seismic site response'

A seismic hazard assessment was carried out for the Catania and Siracusa towns providing a comprehensive re-examination and re-processing of all the available seismic data. The site approach and the seismotectonic one were used and compared. The hazard assessment, using both methods, was performed following a logic-tree approach in order to consider and reduce the epistemic uncertainties. The combined use and comparison of these approaches is recommended since it allows to verify the robustness of the hazard estimates and allowed us to obtain useful elements to define the seismic hazard in Catania and Siracusa. Experimental data and numerical modeling were used to study the effect of local geology on the seismic response in the Catania area. Available boreholes data and elastic parameters were used to reconstruct a geotechnical model in order to perform 1D numerical modeling. Seismic urban scenarios were simulated considering destructive (Mw=7.0), strong (Mw=6.2) and moderate (Mw=5.7) earthquakes. PGA and spectral acceleration at different periods were obtained in the urban area through the equivalent linear numerical code EERA, and contour maps of different levels of shaking were drawn. Standard and horizontal-to-vertical spectral ratios were achieved making use of a dataset of 172 seismic events recorded at ten stations located on the main outcropping lithotypes. Spectral ratios inferred from earthquake data were compared with theoretical transfer functions. Both experimental and numerical results confirm the role of the geologic and morphologic setting of Catania. A study aimed to investigate on the dynamic properties of main lithotypes outcropping in the Siracusa area and their relationships with the local seismic response was performed. Non-invasive seismic prospecting techniques using the vertical component of surface waves (MASW and ReMi) were adopted, as well as ambient noise measurements, processed through the Nakamura technique. Moreover, a cluster analysis was performed to subdivide into homogeneous groups the experimentally obtained noise spectral ratios. Results pointed out that the use of combined different methods provides a more robust way to characterized the investigated soils and to reduce the problems linked to the non-uniqueness of solutions during the interpretation of geophysical data. The role of local geology and topography on the site response of a small hill, located in the northern part of Catania, was investigated. Ambient noise and earthquake data were processed through standard and horizontal-to-vertical spectral ratios. Directional effects were also investigated by computing the spectral ratios after rotating the horizontal components of motion and performing polarization analysis. Results of noise and earthquakes analysis, although show significant differences in amplitude, are comparable in frequency, especially in the sedimentary terrains. Pronounced directional effects are mostly observed on the slopes rather than at the hill top. Our findings appear linked to the complex wavefield generated by the lithologic heterogeneities existing in the area which seem to have a stronger influence with respect to the simple topographic effect. Seismic noise recorded by mobile stations in the Ortigia peninsula (downtown Siracusa) was analyzed through H/V spectral ratios, to investigate local site effects. Moreover, shear wave velocities were investigated through non-invasive techniques (MASW and ReMi) in order to assess the theoretical resonant frequency of the hill. Experimental results coming out from the spectral ratios show peaks in the frequency range 1.0-3.0 Hz which are consistent with the theoretical resonance frequency at Ortigia. The H/V azimuthal spectral analysis shows a clearly predominant E-W directional effect, transversal to the main axis of the peninsula, which is also confirmed by the polarization analysis in the time domain.

Thesis (M.S. in civil engineering)--Washington State University, 2004.
Title from PDF t.p. (viewed on Nov. 6, 2005). Pages 1-5 appear in duplicate. Includes bibliographical references (p. 158-168).

Topography effects, the modification of seismic motion by topographic features, have been long recognized to play a key role in elevating seismic risk. Site response, the modification of ground motion by near surface soft soils, has been also shown to strongly affect the amplitude, frequency and duration of seismic motion. Both topography effects and 1-D site response have been extensively studied through field observations, small-scale and field experiments, analytical models and numerical simulations, but each one has been studied independently of the other: studies on topography effects are based on the assumption of a homogeneous elastic halfspace, while 1-D site response studies are almost exclusively formulated for flat earth surface conditions. This thesis investigates the interaction between topographic and soil amplification, focusing on strong ground motions that frequently trigger nonlinear soil response. Recently, a series of centrifuge experiments tested the seismic response of single slopes of various inclination angles at the NEES@UCDavis facility, to investigate the effects of nonlinear soil response on topographic amplification. As part of this collaborative effort, we extended the search space of these experiments using finite element simulations. We first used simulations to determine whether the centrifuge experimental results were representative of free-field conditions. We specifically investigated whether wave reflections caused by the laminar box interfered with mode conversion and wave scattering that govern topographic amplification; and whether this interference was significant enough to qualitatively alter the observed amplification compared to free-field conditions. We found that the laminar box boundaries caused spurious reflections that affected the response near the boundaries; however its effect to the crest-to-free field spectral ratio was found to be insignificant. Most importantly though, we found that the baseplate was instrumental in trapping and amplifying waves scattered and diffracted by the slope, and that in absence of those reflections, topographic amplification would have been negligible. We then used box- and baseplate-free numerical models to study the coupling between topography effects and soil amplification in free-field conditions. Our results showed that the complex wavefield that characterizes the response of topographic features with non-homogeneous soil cannot be predicted by the superposition of topography effects and site response, as is the widespread assumption of engineering and seismological models. We also found that the coupling of soil and topographic amplification occurs both for weak and strong motions, and for pressure-dependent media (Nevada sand), nonlinear soil response further aggravates topographic amplification; we attributed this phenomenon to the reduction of apparent velocity that the low velocity layers suffer during strong ground motion, which intensifies the impedance contrast and accentuates the energy trapping and reverberations in the low strength surficial layers. We finally highlighted the catalytic effects that soil stratigraphy can have in topographic amplification through a case study from the 2010 Haiti Earthquake. Results presented in this thesis imply that topography effects vary significantly with soil stratigraphy, and the two phenomena should be accounted for as a coupled process in seismic code provisions and seismological ground motion predictive models.

Seismic hazard analyses require an estimate of earthquake ground motions from future events. These predictions are achieved through Ground Motion Prediction Equations, which include a prediction of the median and the standard deviation of ground motion parameters. The differences between observed and predicted ground motions, when normalized by the standard deviation, are referred to as epsilon (𝜖). For spectral accelerations, the correlation structure of normalized residuals across oscillator periods is important for guiding ground motion selection. Correlation structures for large global datasets have been studied extensively. These correlation structures reflect effects that are averaged over the entire dataset underlying the analyses. This paper considers the effects of site response, at given sites, on the correlation structure of normalized residuals. This is achieved by performing site response analyses for two hypothetical soil profiles using a set of 85 rock input motions. Results show that there is no significant difference between correlation coefficients for rock ground motions and correlation coefficients after considering the effects of site response for the chosen sites.
Master of Science

The understanding of the impact of site effects on ground motions is crucial for improving the assessment of seismic hazards. Site response analyses (SRA) can numerically accommodate the mechanics behind the wave propagation phenomena near the surface as well as the variability associated with the input motion and soil properties. As a result, SRA constitute a key component of the assessment of site-specific seismic hazards within the probabilistic seismic hazard analysis framework. This work focuses on limitations in SRA, namely, the definition of the elastic half-space (EHS) boundary condition, the selection of input ground motions so that they are compatible with the assumed EHS properties, and the proper consideration of near-surface attenuation effects. Input motions are commonly selected based on similarities between the shear wave velocity (Vs) at the recording station and the materials below the reference depth at the study site (among other aspects such as the intensity of the expected ground motion, distance to rupture, type of source, etc.). This traditional approach disregards the influence of the attenuation in the shallow crust and the degree to which it can alter the estimates of site response. A Vs-κ correction framework for input motions is proposed to render them compatible with the properties of the assumed EHS at the site. An ideal EHS must satisfy the conditions of linearity and homogeneity. It is usually defined at a horizon where no strong impedance contrast will be found below that depth (typically the top of bedrock). However, engineers face challenges when dealing with sites where this strong impedance contrast takes place far beyond the depth of typical Vs measurements. Case studies are presented to illustrate potential issues associated with the selection of the EHS boundary in SRA. Additionally, the relationship between damping values as considered in geotechnical laboratory-based models, and as implied by seismological attenuation parameters measured using ground motions recorded in the field is investigated to propose alternative damping models that can match more closely the attenuation of seismic waves in the field.
Ph. D.

My dissertation focuses primarily on the following three aspects associated with passing seismic waves in the field of earthquake seismology: temporal changes of fault zone properties, nonlinear site response, and dynamic triggering. Quantifying the temporal changes of material properties within and around active fault zones (FZ) is important for better understanding of rock rheology and estimating the strong ground motion that can be generated by large earthquakes. As high-amplitude seismic waves propagate through damaged FZ rocks and/or shallow surface layers, they may produce additional damage leading to nonlinear wave propagation effects and temporal changes of material properties (e.g., seismic velocity, attenuation). Previous studies have found several types of temporal changes in material properties with time scales of tens of seconds to several years. Here I systematically analyze temporal changes of fault zone (FZ) site response along the Karadere-Düzce branch of the North Anatolian fault that ruptured during the 1999 İzmit and Düzce earthquake sequences. The coseismic changes are on the order of 20-40%, and are followed by a logarithmic recovery over an apparent time scale of ~1 day. These results provide a bridge between the large-amplitude near-instantaneous changes and the lower-amplitude longer-duration variations observed in previous studies. The temporal changes measured from this high-resolution spectral ratio analysis also provide a refinement for the beginning of the longer more gradual process typically observed by analyzing repeating earthquakes. An improved knowledge on nonlinear site response is critical for better understanding strong ground motions and predicting shaking induced damages. I use the same sliding-window spectral ratio technique to analyze temporal changes in site response associated with the strong ground motion of the Mw6.6 2004 Mid-Niigata earthquake sequence recorded by the borehole stations in Japanese Digital Strong-Motion Seismograph Network (KiK-Net). The coseismic peak frequency drop, peak spectral ratio drop, and the postseismic recovery time roughly scale with the input ground motions when the peak ground velocity (PGV) is larger than ~5 cm/s, or the peak ground acceleration (PGA) is larger than ~100 Gal. The results suggest that at a given site the input ground motion plays an important role in controlling both the coseismic change and postseismic recovery in site response. In a follow-up study, I apply the same sliding-window spectral ratio technique to surface and borehole strong motion records at 6 KiK-Net sites, and stack results associated with different earthquakes that produce similar PGAs. In some cases I observe a weak coseismic drop in the peak frequency when the PGA is as small as ~20-30 Gal, and near instantaneous recovery after the passage of the direct S waves. The percentage of drop in the peak frequency starts to increase with increasing PGA values. A coseismic drop in the peak spectral ratio is also observed at 2 sites. When the PGA is larger than ~60 Gal to more than 100 Gal, considerably stronger coseismic drops of the peak frequencies are observed, followed by a logarithmic recovery with time. The observed weak reductions of peak frequencies with near instantaneous recovery likely reflect nonlinear response with essentially fixed level of damage, while the larger drops followed by logarithmic recovery reflect the generation (and then recovery) of additional rock damage. The results indicate clearly that nonlinear site response may occur during medium-size earthquakes, and that the PGA threshold for in situ nonlinear site response is lower than the previously thought value of ~100-200 Gal. The recent Mw9.0 off the Pacific coast of Tohoku earthquake and its aftershocks generated widespread strong shakings as large as ~3000 Gal along the east coast of Japan. I systematically analyze temporal changes of material properties and nonlinear site response in the shallow crust associated with the Tohoku main shock, using seismic data recorded by the Japanese Strong Motion Network KIK-Net. I compute the spectral ratios of windowed records from a pair of surface and borehole stations, and then use the sliding-window spectral ratios to track the temporal changes in the site response of various sites at different levels of PGA The preliminary results show clear drop of resonant frequency of up to 70% during the Tohoku main shock at 6 sites with PGA from 600 to 1300 Gal. In the site MYGH04 where two distinct groups of strong ground motions were recorded, the resonant frequency briefly recovers in between, and then followed by an apparent logarithmic recovery. I investigate the percentage drop of peak frequency and peak spectral ratio during the Tohoku main shock at different PGA levels, and find that at most sites they are correlated. The third part of my thesis mostly focuses on how seismic waves trigger additional earthquakes at long-range distance, also known as dynamic triggering. Previous studies have shown that dynamic triggering in intraplate regions is typically not as common as at plate-boundary regions. Here I perform a comprehensive analysis of dynamic triggering around the Babaoshan and Huangzhuang-Gaoliying faults southwest of Beijing, China. The triggered earthquakes are identified as impulsive seismic arrivals with clear P- and S-waves in 5 Hz high-pass-filtered three-component velocity seismograms during the passage of large amplitude body and surface waves of large teleseismic earthquakes. I find that this region was repeatedly triggered by at least four earthquakes in East Asia, including the 2001 Mw7.8 Kunlun, 2003 Mw8.3 Tokachi-oki, 2004 Mw9.2 Sumatra, and 2008 Mw7.9 Wenchuan earthquakes. In most instances, the microearthquakes coincide with the first few cycles of the Love waves, and more are triggered during the large-amplitude Rayleigh waves. Such an instantaneous triggering by both the Love and Rayleigh waves is similar to recent observations of remotely triggered 'non-volcanic' tremor along major plate-boundary faults, and can be explained by a simple Coulomb failure criterion. Five earthquakes triggered by the Kunlun and Tokachi-oki earthquakes were recorded by multiple stations and could be located. These events occurred at shallow depth (< 5 km) above the background seismicity near the boundary between NW-striking Babaoshan and Huangzhuang-Gaoliying faults and the Fangshan Pluton. These results suggest that triggered earthquakes in this region likely occur near the transition between the velocity strengthening and weakening zones in the top few kms of the crust, and are likely driven by relatively large dynamic stresses on the order of few tens of KPa.

3D seismic data are widely used for many different purposes. Despite different objectives, a common goal in almost all 3D seismic programs is to attain better understanding of the subsurface features. In gas injection projects, which are mainly for Enhanced Oil Recovery (EOR) and recently for environmental purposes, seismic data have an important role in the gas monitoring phase. This thesis deals with a 3D seismic investigation at the CO2 injection site at Ketzin, Germany. I focus on two critical aspects of the project: the internal architecture of the heterogeneous Stuttgart reservoir and the detectability of the CO2 response from surface seismic data. Conventional seismic methods are not able to conclusively map the internal reservoir architecture due to their limited seismic resolution. In order to overcome this limitation, I use the Continuous Wavelet Transform (CWT) decomposition technique, which provides frequency spectra with high temporal resolution without the disadvantages of the windowing process associated with the other techniques. Results from applying this technique reveal more of the details of sand bodies within the Stuttgart Formation. The CWT technique also helps to detect and map remnant gas on the top of the structure. In addition to this method, I also show that the pre-stack spectral blueing method, which is presented for the first time in this research, has an ability to enhance seismic resolution with fewer artifacts in comparison with the post-stack spectral blueing method. The second objective of this research is to evaluate the CO2 response on surface seismic data as a feasibility study for CO2 monitoring. I build a rock physics model to estimate changes in elastic properties and seismic velocities caused by injected CO2. Based on this model, I study the seismic responses for different CO2 injection geometries and saturations using one dimensional (1D) elastic modeling and two dimensional (2D) acoustic finite-difference modeling. Results show that, in spite of random and coherent noises and reservoir heterogeneity, the CO2 seismic response should be strong enough to be detectable on surface seismic data. I use a similarity-based image registration method to isolate amplitude changes due to the reservoir from amplitude changes caused by time shifts below the reservoir. In support of seismic monitoring using surface seismic data, I also show that acoustic impedance versus Poisson’s ratio cross-plot is a suitable attribute for distinguishing gas-bearing sands from brine-bearing sands.
CO2SINK Project

The central and eastern United States is subject to seismic hazards from both natural and induced earthquakes, as evidenced by the 1811-1812 New Madrid earthquake sequence, consisting of at least three magnitude 7 and greater earthquakes, and by four magnitude 5 and greater induced earthquakes in Oklahoma since 2011. To mitigate seismic hazards, both earthquake sources and their effects need to be characterized. Ground motion site response can cause additional damage to susceptible infrastructure and buildings. Recent studies indicate that Vs30, one of the primary site-response predictors used in current engineering practice, is not reliable. To investigate site response in the New Madrid Seismic Zone, ratios of surface-to-bedrock amplitude spectra, TFT, from S-wave recordings at the two deep vertical seismic arrays in the sediment-filled upper Mississippi Embayment (i.e., VSAP and CUSSO) were calculated. The mean TFT curves were compared with theoretical transfer functions; the results were comparable, indicating that TFT estimates of the empirical, linear SH-wave site responses at these sites. The suitability of surface S-wave horizontal-to-vertical spectral ratios, H/V, for estimating the empirical site transfer function was also evaluated. The results indicate that mean S-wave H/V curves are similar to TFT at low frequencies (less than the fifth natural frequencies) at both CUSSO and VSAP. SH-wave fundamental frequency, f0, and fundamental-mode amplification, A0, were evaluated as alternatives to the Vs30 proxy to estimate primary linear site-response characteristics at VSAP, CUSSO, and nine other seismic stations in the CEUS. In addition, calculated f0 and A0 were compared with the first peaks of S-wave H/V spectral ratios. The f0 and A0 were found to approximate the 1-D linear, viscoelastic, fundamental-mode responses at most stations. Also, S-wave H/V from weak-motion earthquakes can be used to measure f0. However, S-wave H/V does not reliably estimate A0 in the project area. S-wave H/V observations reveal site response within the frequency band of engineering interest from deeper, unmodeled geological structures. Because damaging or felt earthquakes induced by hydraulic fracturing and wastewater disposal have occurred in the CEUS, characterizing background seismicity prior to new large-scale subsurface fluid injection is important to identify cases of and the potential for induced seismicity. The Rogersville Shale in the Rome Trough of eastern Kentucky is being tested for unconventional oil and gas potential; production of this shale requires hydraulic fracturing, which has been linked to induced seismicity elsewhere in the CEUS. To characterize natural seismicity and to monitor induced seismicity during testing, a temporary seismic network was deployed in the Rome Trough near the locations of new, Rogersville Shale oil and gas test wells. Using the real-time recordings of this network and those of other regional seismic stations, three years of local seismicity were cataloged. Only three earthquakes occurred in the Rome Trough of eastern Kentucky, none of which was associated with the deep Rogersville Shale test wells that were stimulated during the time the network was in operation.

L’attività di monitoraggio sismico è basata sull’uso di sismografi ricettivi per la registrazione del moto del suolo causato dai terremoti. Gli strumenti sviluppati nell’ambito del monitoraggio sismico permettono di studiare la sismicità a livello sia regionale che globale e trovano un uso strategico nel contesto della stima della pericolosità sismica. Uno sforzo continuo è necessario per migliorare gli strumenti al servizio del monitoraggio sismico e le conseguenti applicazioni, sia nel campo della stima della pericolosità che in contesti di protezione civile ed ingegneria sismica, come ad esempio nelle mappe di scuotimento del suolo. Il primo fondamentale passo per migliorare gli strumenti e i modelli sviluppati nell’ambito del monitoraggio sismico è un corretto ed accurato trattamento dei dati. Un’attenta procedura di selezione ed elaborazione è stata seguita sulla base del tipo specifico di dati impiegati, differenziando tra dati strumentali, metadati e valori assegnati da esperti. Una nuova definizione di intensità strumentale, la quale fornisce una previsione dell’intensità macrosismica sulla base del livello di scuotimento del suolo, viene proposta per il caso dell’Italia. Lo scopo è sostituire le equazioni lineari comunemente impiegate a tale scopo (Ground Motion to Intensity Equations, GMICE), che per loro natura non riescono a trattare in modo completamente corretto il dato dell’intensità macrosismica e la relativa incertezza. Un modello basato sulla tecnica dei classificatori di Bayes Gaussiani (Gaussian Naïve Bayes, GNB) è stato sviluppato e calibrato per un set di diversi parametri di moto del suolo. Tale modello fornisce stime di intensità su classi intere, in accordo con la definizione originaria di classi di intensità, ed una relativa probabilità associata. I risultati sono stati testati rispetto ad una formulazione delle più classiche GMICE calibrata sullo stesso gruppo di dati. L’intensità strumentale basata sulla definizione da GNB è risultata fornire prestazioni migliori in termini di applicazione su dati indipendenti e di capacità di catturare l’incertezza associata al dato. Le stime di intensità basate sull’uso di parametri di massimo moto del suolo (in velocità e in accelerazione) sono risultate le più adatte all’applicazione diretta nei prodotti di monitoraggio sismico e convertite in una scala spezzata adatta all’uso. Un algoritmo per la modellazione degli spettri di ampiezza di Fourier è stato sviluppato per effettuare un’inversione parametrica da cui ottenere stime di comportamento specifico al sito. Il relativo software è stato sviluppato in maniera flessibile per permettere un facile adattamento nella selezione dei modelli, degli stimatori di incertezza, dei coefficienti di peso, del sito di riferimento, della metodologia e del numero di passi usati nell’inversione. Sulla base di osservazioni sismotettoniche è stata individuata un’area nella regione del Nord-Est Italia, da usare come caso di studio per la stima dei parametri spettrali associati ad un gruppo di eventi e stazioni sismiche. Le curve di amplificazione al sito sono state ricostruite dall’analisi congiunta dei prodotti e dei residuali dell’inversione, ed è stato di conseguenza suggerito un gruppo di stazioni adatte ad essere usate come riferimento per il livello regionale di amplificazione su roccia. Il raffronto con la letteratura ha confermato l’affidabilità dei risultati ottenuti, in particolar modo per i termini relativi alle sorgenti sismiche e ai siti. La validazione rispetto a diverse scelte di parametrizzazione ha confermato la stabilità dell’algoritmo di inversione e fornito suggerimenti per migliorare la stima dei parametri legati all’attenuazione lungo il percorso sismico. Le stime ottenute per le curve di amplificazione sono adatte all’uso nei modelli di stima della pericolosità sismica.
Seismic monitoring employs sensitive seismographs to record the ground motion generated by earthquakes. It provides tools to study regional and global seismicity that are fundamental when applied inside seismic hazard assessment. A continuous effort is necessary to improve monitoring for both hazard assessment and direct applications in civil protection and engineering contexts (e.g., shakemaps). The first step towards improving monitoring tools and models is to carefully select and handle data. A strict selection and processing procedure tailored to the specific kinds of employed data is followed to ensure the quality of the ensuing results. A novel definition of instrumental intensity for Italy is proposed, to provide a forecast of expected macroseismic intensity based on the ground motion shaking level. It is intended to substitute Ground Motion to Intensity Conversion Equations (GMICEs), which are linear relationships that do not correctly treat intensity and its associated uncertainty. A model based on Gaussian Naïve Bayes (GNB) classifiers is developed and calibrated for a set of ground shaking parameters, providing integer-valued intensity forecasts with a known, class-specific associated probability. The results are tested against a more classical GMICE formulation calibrated on the same dataset. Instrumental intensity based on GNB definition is proved to possess better performance on unseen data and better capability of capturing data uncertainty with respect to GMICEs. Forecasts based on peak ground parameters (velocity and acceleration) are selected as most suitable for direct application in seismic monitoring products and converted in a ready-to-use piecewise scale. An algorithm for Fourier amplitude spectra modelling is developed to perform parametric inversion and provide estimates of site-specific soil behaviour. A flexible software is developed that supports the customization of employed models, uncertainty estimators, weighting coefficients, reference settings, inversion techniques and number of inversion steps. A case study area in the North-Eastern Italy region is chosen based on seismo-tectonics considerations, and spectral parameters are estimated for a set of selected events and stations. Site-specific amplification curves are built from a combined analysis of inversion products and residuals, and a set of candidate stations to be used as regional rock amplification reference is suggested. Comparison with literature values reinforces the reliability of the results, especially in the case of source and site terms. Validation against different model parametrizations confirms the stability of the inversion algorithm and suggests additional steps to improve the estimate of path attenuation features. A case scenario is built to exemplify the possible use of the developed tools. Estimated amplification curves are found to be compatible with independent empirical observations and suitable for employment in hazard assessment models to better constrain site-specific response.

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.

One of the most difficult tasks towards designing earthquake resistant structures is the determination of critical earthquakes. Conceptually, these are the ground motions that would induce the critical response in the structures being designed. The quantification of this concept, however, is not easy. Unlike the linear response of a structure, which can often be obtained by using a single spectrally modified ground acceleration history, the nonlinear response is strongly dependent on the phasing of ground motion and the detailed shape of its spectrum. This necessitates the use of a suite (bin) of ground acceleration histories having phasing and spectral shapes appropriate for the characteristics of the earthquake source, wave propagation path, and site conditions that control the design spectrum. Further, these suites of records may have to be scaled to match the design spectrum over a period range of interest, rotated into strike-normal and strike-parallel directions for near-fault effects, and modified for local site conditions before they can be input into time-domain nonlinear analysis of structures. The generation of these acceleration histories is cumbersome and daunting. This is especially so due to the sheer magnitude of the data processing involved. The purpose of this thesis is the development and documentation of PC-based computational tools (hereinafter called EQTools) to provide a rapid and consistent means towards systematic assembly of representative strong ground motions and their characterization, evaluation, and modification within a performance-based seismic design framework. The application is graphics-intensive and every effort has been made to make it as user-friendly as possible. The application seeks to provide processed data which will help the user address the problem of determination of the critical earthquakes. The various computational tools developed in EQTools facilitate the identification of severity and damage potential of more than 700 components of recorded earthquake ground motions. The application also includes computational tools to estimate the ground motion parameters for different geographical and tectonic environments, and perform one-dimensional linear/nonlinear site response analysis as a means to predict ground surface motions at sites where soft soils overlay the bedrock. While EQTools may be used for professional practice or academic research, the fundamental purpose behind the development of the software is to make available a classroom/laboratory tool that provides a visual basis for learning the principles behind the selection of ground motion histories and their scaling/modification for input into time domain nonlinear (or linear) analysis of structures. EQTools, in association with NONLIN, a Microsoft Windows based application for the dynamic analysis of single- and multi-degree-of-freedom structural systems (Charney, 2003), may be used for learning the concepts of earthquake engineering, particularly as related to structural dynamics, damping, ductility, and energy dissipation.
Master of Science

Turkey is one of the most earthquake prone countries in the world. The study area, Erbaa, is located in a seismically active fault zone known as North Anatolian Fault Zone (NAFZ). Erbaa is one of the towns of Tokat located in the Middle Black Sea Region. According to the Earthquake zoning map of Turkey, the study area is in the First Degree Earthquake Zone. The city center of Erbaa (Tokat) was previously settled on the left embankment of Kelkit River. After the disastrous 1942 Niksar-Erbaa earthquake (Mw = 7.2), the settlement was moved southwards. From the period of 1900s, several earthquakes occurred in this region and around Erbaa. The 1942 earthquake is the most destructive earthquake in the center of Erbaa settlement. In this study, the geological and geotechnical properties of the study area were investigated by detailed site investigations. The Erbaa settlement is located on alluvial and Pliocene deposits. The Pliocene clay, silt, sand, and gravel layers exist in the southern part of Erbaa. Alluvium in Erbaa region consists of gravelly, sandy, silty, and clayey layers. The alluvial deposits are composed of stratified materials of heterogeneous grain sizes, derived from various geological units in the vicinity. The main objective of this study is to prepare a seismic microzonation map of the study area for urban planning purposes since it is getting more essential to plan new settlements considering safe development strategies after the disastrous earthquakes. In this respect, seismic hazard analyses were performed to deterministically assess the seismic hazard of the study area. Afterwards, the essential ground motions were predicted regarding near fault effects as the study area is settled on an active fault zone. 1-D equivalent linear site response analyses were carried out to evaluate the site effects in the study area. Amplification values obtained from site response analyses reveal that the soil layers in the study area is quite rigid. Furthermore, liquefaction potential and post liquefaction effects including lateral spreading and vertical settlement were also delineated for the study area. The above-mentioned parameters were taken into account in order to prepare a final seismic microzonation map of the study area. The layers were evaluated on the basis of overlay methodologies including Multi-Criteria Decision Analysis (MCDA). Two different MCDA techniques, Simple Additive Weighting (SAW) and Analytical Hierarchical Process (AHP), were carried out in GIS environment. The seismic microzonation maps prepared by SAW and AHP methods are compared to obtain a final seismic microzonation map. Finally, the map derived from the AHP method is proposed to be the final seismic microzonation map of Erbaa. As an overall conclusion, the northwestern part of the study area where the loose alluvial units exist is found to be vulnerable to earthquake-induced deformations. On the other hand, the Pliocene units in the southern and alluvial units in the northeastern part are quite resistant to earthquake effects. In addition, the proposed final seismic microzonation map should be considered by urban planners and policy makers during urban planning projects in Erbaa.

Une prédiction de la réponse sismique des structures de génie civil (telles que les centrales nucléaires ou les barrages), doit faire face à plusieurs difficultés majeures compte tenu de la complexité du problème traité. Pour cela, la simulation de la source, la propagation des ondes sismique et les effets de site, ont été étudiés par différentes approches dernièrement. Récemment, des méthodes combinées avec des ordinateurs massivement parallèles se sont avérées très efficaces pour modéliser la propagation des ondes sismiques depuis la source jusqu’au site, dans des environnements géologiques tridimensionnels complexes. Cependant, la précision reste limitée en raison du caractère multi-échelle du problème et des grandes incertitudes sur les données à introduire dans le modèle (i.e. la caractérisation géométrique et cinématique de la source sismique, le modèle géologique et numérique du chemin de propagation). Pour cela, l’utilisation d'un modèle numérique régional capable de simuler le phénomène sismique de la source au site permettrait de mieux comprendre et de classer l'origine de ces incertitudes.Ce travail vise à étudier numériquement l'effet de la géologie locale et régionale sur la réponse sismique d’un bassin et plus particulièrement le site d’Argostoli situé sur l'île de Céphalonie (Grèce). Dans un premier temps, le code numérique utilisé dans ce travail (SEM3D) est vérifié à l'aide de trois cas canoniques. Les résultats obtenus montrent un bon accord avec les solutions de référence. Dans les cas d’un modèle numérique avec diverses échelles ou avec des géologies complexes, l’un des points clefs est la conformité du maillage numérique avec les interfaces géologiques, ce qui se traduit par l’augmentation du coût numérique. Grâce aux caractéristiques de la méthode numérique utilisée, pour contourner cette difficulté, une approche possible est l’utilisation des maillages “non-conformes” ou “not-honouring”. Une étude paramétrique sur la faisabilité de cette approche est ainsi réalisée afin de mettre en évidence l'influence de certains paramètres numériques sur les résultats obtenus.Par la suite, des études paramétriques de plusieurs scénarii sismiques sur le site d'Argostoli ont été réalisées. Concernant le chargement sismique, deux types de sources ont été étudiés : des sources ponctuelles et des failles étendues. L’étude avec les sources ponctuelles révèle un phénomène d'amplification et de piégeage des ondes dans le bassin, conduisant à un signal complexe et allongé, avec une énergie importante par rapport à une étude avec une géologie simplifiée. Pour le second type de source, la faille modélisée est proche de la surface. Cela permet d'étudier l’effets du bassin et du champ-proche sur la réponse sismique du site. En effet, le mouvement du sol à proximité d'une faille peut être différent du mouvement du sol observé loin de la source sismique. D’après les résultats obtenus, l'effet du bassin est plus marqué mais avec une amplification plus forte et des fréquences de résonance différentes. De plus, l'effet du champ-proche a été mis en évidence, marqué par de fortes impulsions de vitesse à certains endroits du bassin. L’ordre de grandeur des spectres de réponses obtenues est comparable à ceux obtenus lors des séismes de magnitude semblable qui ont eu lieu en 2014 au même endroit.Dans la dernière partie, une étude paramétrique sur des aspects numériques liées à la précision du calcul a été réalisée. Cette étude a permis d’augmenter la résolution fréquentielle de 7 Hz à 10 Hz avec des caractéristiques mécaniques de sols mous avec la même taille de domaine. Ces simulations ouvrent plus de questions sur l’interdépendance de la finesse de la résolution des données physiques et des maillages pour les calculs numériques. En conclusion, cette thèse correspond à une première étape dans la caractérisation numérique de la réponse sismique du bassin d’Argostoli et les effets dus au bassin, au type de source et à leurs interactions
A prediction of the seismic response of civil engineering structures that requires a high level of safety (i.e. nuclear power plants or dams) faces several major difficulties given the complexity of the problem being treated. To this end, the source simulation, seismic wave propagation and site effects have been studied by different approaches over the last two decades. Recently, numerical methods, such as the spectral element, combined with massively parallel computers have proved a good efficiency in modelling the seismic wave propagation from source to site in complex three-dimensional geological environments. However, the accuracy of these predictions remains limited due to the multi-scale nature of the problem and the large uncertainties in the data to be introduced into the model (i.e. the geometric and kinematic characterization of the seismic source, the detailed geological and numerical model of the source-to-site propagation path). Therefore, the use of a regional numerical model able to simulate the seismic phenomenon from source to site would allow a better analysis and classification of the origin of the associated uncertainties.This work aims to study numerically the effect of local and regional geology on the seismic response of a basin and precisely the Argostoli site located on the island of Kefalonia (Greece). Firstly, the numerical code used in this work (SEM3D) is verified using three canonical cases. The simulated results showed a good agreement with the reference solutions. In the cases of a numerical model with different scales or with complex geologies, one of the important difficulties is the conformity of the numerical meshes with the geological interfaces, that will result an increase in the numerical cost. Because of the characteristics of the used numerical method, one possible approach was to use the “non-conforming” or “not-honoring” meshes to overcome this difficulty. A parametric study on the applicability of this approach was then carried out in order to highlight the influence of some numerical parameters on the obtained results.Thereafter, parametric studies on several seismic scenarios in the Argostoli site were conducted. Concerning seismic loading, two types of source have been studied: point sources and extended faults. The study with point sources clearly revealed a phenomenon of amplification and trapping of waves in the basin, leading to a complex and elongated signal, with significant energy compared to a study with a simplified geology. For the second type of source, the modeled fault is close to the surface. It allows to study, in addition to the influence of the basin, the effect of the near-field on the seismic response of the site. Indeed, near-fault ground motion can be significantly different from ground motion observed far from the seismic source. Based on the results obtained, the basin effect is even more pronounced but with higher amplification and different resonance frequencies. In addition, the near-field effect has been highlighted, marked by strong velocity pulses at some locations in the basin. The order of magnitude of the obtained response spectra is comparable to the ones obtained during the earthquake sequence of similar magnitude that took place in 2014.In the last part, a parametric study (allowed by the development of computing power) on the numerical aspects related to the computational accuracy was carried out. With this study, it is possible to increase the frequency resolution from 7 Hz to 10 Hz with soft soil mechanical characteristics while keeping the same domain size. These simulations open even more questions on the interdependence of the fineness of resolution of physical data and meshes for numerical simulations. In conclusion, this thesis corresponds to a first step in the numerical characterization of the seismic response of the Argostoli basin and the effects due to the basin, the type of source and their interactions

Occurrence of displacements of shallow mat foundations resting on saturated silt-clay mixtures were reported in Mexico City during 1985 Mexico Earthquake, and in Adapazari during 1999 Kocaeli (izmit) Earthquake. Soft surface soils, shallow ground water, limited foundation embedments and deep alluvial deposits were the common features pertaining to such foundation displacements in either case. Experience shows, while uniform foundation settlements, even when excessive, do not limit post earthquake serviceability of building structures, tilting is particularly problematic. In this study, a simplified methodology is developed to estimate the seismically induced irrecoverable tilting potential of shallow mats on fine saturated soils. The undrained shear and deformation behavior of silt-clay mixtures encountered at the Adapazari sites with significant foundation displacements are investigated through a series of standard and rapid monotonic, and stress-controlled cyclic triaxial tests conducted over anisotropically consolidated natural soil samples. Test results show that, while the shear strength of these soils do not significantly degrade under means of loading comparable to that of Kocaeli earthquake, their plastic strain accumulation characteristics critically depend on the mode of loading as well as the relative levels of applied load with regard to the monotonic strength. Based on the results of laboratory tests, the response of nonlinear soil-foundation-structure system is reduced to a single-degree-of-freedom oscillator with elastic-perfectly plastic behavior. The natural period of the system is expressed by simplified soil-structure-interaction equations. Pseudo-static yield acceleration, which is required to initiate the foundation bearing capacity failure when applied to the structural mass, is estimated by the finite-element method. Eventually, the tilting potential of the foundations is estimated utilizing inelastic response of the nonlinear oscillator. Response of the deep alluvium sites, which involves velocity pulses with periods consistent with the fundamental site period, is significant in determination of inelastic response of low bearing capacity systems. Predictive capability of the methodology developed is tested with actual case data. The methodology is observed to predict irrecoverable tilting potential of foundations consistent with the observations, except for the cases with low seismic bearing capacity. Deviations are explained considering the sensitivity of low-strength systems to asymmetrical behavior and uncertainties involved in seismic demand.

Predicting the ground response to the propagation of seismic waves is one of the most important aspects of geotechnical engineering. Advanced soil constitutive models provide significant opportunity to improve the understanding of nonlinear ground response during a seismic event, and offer the capability of simulating complex nonlinear soil behaviour which is not captured by means of traditional ground response analyses in geotechnical engineering. Moreover, observations of distinctive nonlinear soil behaviour during recent large earthquake events such as the 2011 Tohoku earthquake point towards the need to more reliably simulate realistic soil behaviour in order to understand the complex dynamic response of soils. The intent of this thesis is to utilize the SANISAND bounding surface plasticity model based on the work of Dafalias and Manzari (2004) to simulate the response of shallow sand deposits to a number of earthquake motions, with the aim of evaluating the ability of the model to simulate relatively complex nonlinear soil behaviour. Furthermore, both total and effective stress analysis techniques are carried out in order to highlight the importance of modeling the interaction between the pore fluid phase and the soil solid. For this purpose, two sites are analyzed, including a case history of a real downhole seismograph array and a generic site. The capability of the SANISAND model to simulate the phenomenon of high frequency dilation pulses is also explored. The SANISAND constitutive model is shown to adequately simulate the seismic ground response of a shallow sand soil column at a real downhole seismic array in Sendai, Japan by comparison to surface seismograph recordings for several earthquake events on the east coast of Japan. Soil permeability in the effective stress analyses is influential in the dynamic response of the soil to earthquake motions. Furthermore, modeling the pore fluid – soil solid interaction in an effective stress analysis is shown to be important for shallow medium dense sand sites subjected to cyclic mobility and strain stiffening. High frequency ground motion during the seismic response of a generic 10 m deep sand site is suggested to be caused by acceleration pulses as a result of soil dilation.
Applied Science, Faculty of
Civil Engineering, Department of
Graduate

Near surface soils can greatly influence the amplitude, duration, and frequency content of ground motions. The amount of their influence depends on many factors, such as the geometry and engineering properties of the soils and underlying bedrock, as well as the earthquake source mechanism and travel path. Building codes such as the 2012 International Building Code (IBC) define six site categories for seismic design of structures, which are based on the sites defined by the National Earthquake Hazards Reduction Program (NEHRP). Site categories A, B, C, D, and E are defined by the time averaged shear wave velocity over the top 30 meters of the soil deposit. Site category F is defined as sites that include liquefiable or sensitive soils, as well as sites with more than 3 meters (10 ft) of peat or highly organic clays, more than 7.5 meters (25 ft) of soil with PI > 75, and more than 37 meters (120 ft) of soft to medium stiff clays. The IBC specifies simplified procedures to calculate design spectra for NEHRP sites A through E, and requires a site specific investigation for NEHRP F sites. However, established procedures for performing the required site specific investigations for NEHRP F sites are limited.

The objective of this research is to develop a simplified procedure to estimate design spectra for non-liquefiable NEHRP F sites, specifically sites with organic soils, highly plastic soils, and deep soft soil deposits. The results from this research will directly affect US practice by developing much needed guidelines in this area.

There is little empirical data on the seismic response of non-liquefiable NEHRP F sites. As a result, this study focused on generating data from site response analyses. To capture the variability of ground motions, this study selected five base case scenarios according to tectonic environments and representative cases encountered in common US practice. Suites of ground motions for each scenario were created by collecting ground motions from online databases. Some of the ground motions were scaled and others were spectrally matched to their respective target response spectra. Fifteen different NEHRP E and F sites were created for the site response analyses. Seven of the sites are based on actual sites from the San Francisco Bay Area, New York City, Ottawa, Canada, Guayaquil, Ecuador, and Hokkaido, Japan. The other eight sites are variations of the seven base case sites. This study conducted a total of 14,541 site response analyses using a well documented site response analysis program.

This study then developed a simplified model to estimate response spectra for non-liquefiable NEHRP F sites. The simplified model was developed in two stages. In the first stage, the results for each site were regressed separately against the ground motion intensity to estimate the effect of the ground motion scenario. In the second stage, the site specific coefficients calculated from the first stage were regressed against site properties to determine their site dependence. These two parts were then combined to form the final model. The simplified model was validated against a separate database than the one used to develop it. This validation database consisted of 24 effective stress nonlinear site response analyses for three sites and eight ground motion scenarios.

The simplified model developed in this study does not replace a site response analysis, but rather augments it. It is hoped that the results of this dissertation will help practicing engineers gain a better understanding of their site before conducting site response analyses

Among all-natural hazards, earthquakes are the most damaging in terms of loss of lives and damage to infrastructure. Amplification and liquefaction are the major effects of earthquake that cause massive damages to infrastructures and loss of lives. Subsurface soil layers play a very important role in ground shaking modification. These ground shaking modifications when a seismic wave passes through soil are estimated by understanding of site effects. Site effects are the combination of soil and topographical effects, which can modify (amplify and deamplify) the characteristics (amplitude, frequency content and duration) of the incoming wave field. There are two stages of site effect evaluation. First is site characterization which is done by classifying the site based on soil properties. Next step is to estimate amplification of possible motions through these site classes. This is done by amplification factors or site coefficients in various provisions. Widely it has been agreed that site effects/amplification are different for deep and shallow soil deposits. Classifying the sites based on 30 m average shear wave velocity (Vs30) is useful for zonation studies because site amplification factor was defined as a function of Vs30 such that the effect of site conditions on the ground shaking can be taken into account. However, the definitions of site classes in different codes are not consistent. Seismic site classification and amplification for seismic micro zonation are obtained on basis of Vs30 irrespective of bedrock depth in Asia. As shear stiffness and the time period of soil column affect soil response most, parameters representing them are used for classification worldwide. Various codes use shear velocity (Vs) 30 and SPT-N as defining parameter for the purpose. Despite their wide use, the seismic site classification schemes considering top 30-m soil layers are being applied to dissimilar bedrock profile and are under significant research scrutiny. In this study, an attempt has been made to estimate amplification of shallow bedrock sites in Bangalore, Chennai, Coimbatore, and Vizag city in the Intraplate region using most appropriate input parameters. Initially dynamic properties of shallow bedrock sites are estimated by carrying out experimental studies of Multichannel Analysis of Surface Wave and Standard Penetration Test (SPT). Then new shear modulus (Gmax) versus SPT N correlation has been developed to overcome limitations in the existing similar correlations. Further for response calculation purpose, input layer, dynamic model curves and suitable Gmax correlations for different soil types have been identified by parametric study. Finally, a nonlinear site response has been carried out and amplification factors for shallow bedrock sites are estimated and presented in this thesis. In the first part of this work, preliminary site response analysis of hypothetical shallow bedrock sites has been carried out and mismatching of site classification and amplification factor is highlighted. Further, limitation of routinely used shear modulus correlation for stiffness estimation, dynamic models (shear modulus and damping curves), the input level of site response has been highlighted. Characterization of the subsurface and estimation of dynamic properties requires understanding of site effects and amplification. Even though several destructive earthquakes have caused extensive damages in shallow bedrock sites in Peninsular India, which is part of intraplate region, very limited systematic attempt has been made to estimate the dynamic properties of several sites in these regions. Soil dynamic properties in the form of shear wave velocity or shear modulus are required to estimate site effects and amplification. Experimental studies are carried out in shallow bedrock regions of South Peninsular India to obtain shear velocity and SPT N profiles. For these profiles, average 30 m values as per NEHRP and average till soil column, are estimated. It is observed that these values are different than average values up to rock because of inclusion of rock shear wave velocity values of the soil average values in shallow bedrock sites. Hence, 30 m average concept results in stiffer site class than soil average values. Misinterpretation of site class and following NEHRP provisions in shallow bedrock sites can lead to incorrect site coefficients and hence incorrect design force parameters. Literature review shows that several site response studies are being carried out by estimating shear modulus of soil layers considering SPT N values. Even though SPT N data are widely used for site response and seismic micro zonation, very few studies are available for in situ correlation between shear modulus versus standard penetration test (SPT) N values using field experiments. It is found in this study that many of the currently used Gmax correlations are not suitable for a particular soil type. Hence, in this study available correlations between SPT N and shear modulus are compiled and reviewed. New correlations are proposed by combining author’s data with available old data from each researcher separately in two ways (a) using all the data and (b) eliminating assumed and extrapolated data i.e. measured data. This study shows that correlations using measured data are better than correlations using all the data (including extrapolated). Further, another set of correlations is developed by combining three and more data sets by considering all the data and measured data separately. Three and more data combinations give the best correlation when compared to the original independent correlations and two data combined correlations. A new correlation has been developed considering measured, old and new, data from Japan and India, where N values are measured with hammer energy of 78%. Modification factors for old and new correlations are suggested for the other regions, where SPT N values are measured with different hammer energy. Representative evaluation of soil response requires the input parameters to be close to the physical behavior of soil column in the site. Several site response studies are carried out in South India considering limited representative parameters such as intraplate recorded earthquake data, soil specific shear modulus correlation, best soil dynamic model and input layer. Several site response analyses are being carried out using existing few shear modulus reduction and damping curves without knowing their suitability. Similarly, input ground motion for site response is being given at the depth of rock layer or borehole termination depth or 30 m. As per our knowledge, there is no clear cut guideline regarding the use of suitable Gmax correlation for the specific soil column, best shear modulus reduction and damping curves for typical soil and appropriate input layer in the site response study of shallow bedrock sites. As part of this study, an attempt has been made to identify suitable Gmax correlation for different types of soil column such as sand, clay and gravel alone or the mixture of all (sand, clay, gravel, sandy soil). Sites with earthquake data recorded at the surface, soil profiles along with SPT N values and shear wave velocity are selected from K-NET (Japanese website) data set for this study. Collected earthquake data consist of moment magnitude (Mw) of 5.0 to 9.0, which are recorded at different epicentral distances. Site response analysis has been carried out by considering earthquake data recorded at a rock site as an input ground motion to the soil profiles published in K-NET data site. Surface ground motion and response spectrum are obtained from different Gmax correlations. The results obtained are compared with surface recorded earthquake same event. The study shows that peak ground acceleration (PGA), response spectrums (RS) and amplification factor (AF) obtained from very few Gmax correlations are comparable with the recorded PGA, response spectrum and amplification factor. Over the years, several researchers have presented a variation of shear modulus and damping ratio with shear strain for different materials. Of several modulus and damping curves available for different soil type from existing literature, set of curves are selected such that only few input parameters are required to use this curve in site response analysis. Selected curves are then used for representing corresponding soil type in the evaluation of soil response. Soil profiles of sites having a surface and bedrock motion recordings are selected from the Kiban-Kyoshin Network data (KiK-net, http://www.kyoshin.bosai.go.jp/). Site response study has been carried by giving rock recorded data as input and surface response is evaluated for each site by changing modulus and damping curves. The evaluated surface response is compared with recorded data for different soil types. Comparison concentrated to identify the best match regards the shape of spectral curve and PGA value. Based on analysis, appropriate dynamic model curves for each soil type has been identified. Further an attempt is made to identify the input layer shear wave velocity beyond which change in response is insignificant. For the purpose, soil density and modulus and damping curves were kept constant by a parametric study by giving input at recorded level. Using same Kik-net data, site response analysis is carried out by considering constant properties and changing input level. Estimated response by giving input at different depth is compared with original response, the layer in which response changes are considerable is considered as cutoff layer. Shear wave velocity of cut off layer is almost similar in most of profiles considered in the study. This study shows that the input given below the soil layer of having shear wave velocity 500 (±100) m/s and above is predicting response close to recorded data irrespective of the soil type. Parametric study results obtained in this chapter are used as an input / guideline for site response studies of shallow bedrock sites. Shear stiffness of the column above the input level has been estimated using shear wave velocity profiles discussed previously. It is found that few locations do not have Vs profiles, hence few SPT N profiles are selected and added. In total 64 shallow bedrock sites are considered for analysis. In previous studies on subject, for most of the site response analyses was carried out in the intraplate shallow bedrock sites, considering synthetic data or active region ground motion data were considered. In this study, for first time available intraplate data are compiled and acceleration time histories are selected based on regional seismicity. A total of 13 intraplate motions recorded in stable continental regions is selected and baseline corrected. These motions have PGA varying between 0.05-0.17g and is in accordance with the hazard maps suggested for these regions by various researchers. Site response calculations are done using a one-dimensional non-linear approach in DEEPSOIL v5.1 software. Water table information and Index properties in the study are obtained from soil reports for the corresponding bore logs. In total, 832 analysis has been carried out and surface spectral values are compiled. Amplification factors are calculated using Ratio of Response Spectra (RRS) method from the spectral results for different time period ranges. Initially amplification factors are evaluated considering the period ranges 0.1-0.5 and 0.4-2.0 s which are similar to the International Building Code (IBC) and compared. The study shows that IBC period range does capture variation of the spectrum of intraplate shallow bedrock sites. Hence the new period range has been derived by considering spectral signatures of input and surface response spectrum. Amplification factors are calculated for new period range 0.01-0.06 s and 0.05-1.0 s. Significant amplification is observed in 0.05-1.0 s period range and amplification factor corresponding to this range is proposed as a final result. Soil profiles used in the study are grouped as five groups based on the stiffness of the soil column above input based on similarity in spectral signatures. The five groups are G1 with shear modulus <50 MPa, G2: 50-100 MPa, G3: 100-150 MPa, G4: 150-250 MPa and G5: >250 MPa. Average amplification for each group has been estimated and compared with previous studies. This study shows that amplification of short period range is comparable with PGA ratio amplification factor estimated in the region. Spectral amplification for the period range 0.05-1.0 s is less than short period amplification and IBC values. These values are calculated for each stiffness group and are decreasing with an increase in stiffness, with 3.24 for the group with modulus less than 50 MPa to 1.84 for the group with modulus greater than 250 MPa.

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

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.

Accurately predicting surface ground motions is critical for many earthquake engineering applications. Equivalent-linear (EQL) site response analysis is a numerical technique used to compute surface ground motions from input motions at bedrock using the site-specific dynamic soil properties. The purpose of this study was to investigate the accuracy of EQL site response analysis for stiff soil sites by comparing computed and observed transfer functions and response spectral amplification. The Kiban Kyoshin network (KiK-net) in Japan is a seismograph network consisting of downhole array sites with strong-motion accelerometers located at the ground surface and at depth. Recorded motions and shear wave velocity profiles are available for most sites. Observed transfer functions and response spectral amplification were computed for 930 individual seismic recordings at 11 stiff soil KiK-net sites. Computed transfer functions and response spectral amplification were calculated from EQL site response analysis by specifying the KiK-net base sensor motion as the input motion. Sites were characterized using the measured shear wave velocity profiles and nonlinear soil properties estimated from empirical models. Computed and observed transfer functions and response spectral amplification were compared at different levels of strain for each site. The average difference between the observed and computed response spectral amplification across the 11 sites were compared at different levels of strain. Overall, there is reasonable agreement between the computed and observed transfer functions and response spectral amplification. There is agreement between the computed and observed site periods, but with over-prediction of the computed response at the observed site periods. Higher modes often computed by the theoretical model were not always observed by the recordings. There is very good agreement between the computed and observed transfer functions and response spectral amplification for periods larger than the site periods. There is less agreement between the computed and observed transfer functions and response spectral amplification for periods less than the site periods. There is mostly over-prediction of the response spectral amplification at these periods, although some under-prediction also occurred. Across all 11 sites the predicted spectral amplification is within +/-20% at shear strains less than 0.01%. At shear strains between approximately 0.01 and 0.03%, the spectral amplification is over-predicted for these sites, in some instances by as little as 5% and in other instances by a factor of 2 or more.
text

When an earthquake occurs, seismic waves radiate away from the source and travel rapidly through the earth's crust. The motion recorded at the ground surface of an area could be really different, in terms of duration and frequency content, from the reference outcrop motion due to the following site conditions: sequence of soil layers, velocity contrast between soil layers, thickness of each layer, dynamic behaviour of the soils, topography and geometry of the sub-interface. Concerning the site-effects, it is possible to discuss about one-dimensional (1D) effects, two-dimensional (2D) and three-dimensional (3D) ones. One-dimensional ef-fects are induced in case of horizontally layered deposits with a horizontal ground surface (vertically heterogeneous media). Two-dimensional site-effects are generated in case of a complex stratigraphic sequence (vertically and laterally heterogeneous media) and/or in case of an uneven ground surface. In presence of stratigraphic and topographical surface varying in any direction (vertically, laterally and transversally heterogeneous media), it is necessary to refer to three-dimensional site-effects. Seismic microzonation (SM) studies are used to assess local geological and ge-otechnical site conditions and to identify earthquake characteristics. A SM study can be undertaken according to three different levels of details, as reported in the Italian guidelines. In particular, numerical analyses are requested for a level III SM to quantify the reference motion modification. This work aims to evaluate complex site-effects for a real case study, i.e. the Bo-vino village, located in South of Italy. This case study has been chosen due to the presence of a soft soil valley surrounded by rock outcrop hills. As a consequence complex site-effects are expected. Essential ingredients for predicting site effects are: topography, stratigraphy, interface between soil layers and dynamic soil behaviour. Moreover it is necessary to select the reference seismic event and to define properly the input motion used in the numerical analyses. The present thesis addresses how to perform a site response analysis using the finite element (FE) method. Two different FE codes, in the time domain, have been used: QUAKE/W, based on the equivalent-linear approach, and PLAXIS 3D, which adopts a non-linear Hardening Soil model with small strain stiffness (HSs). At first, some numerical approaches to simulate 1D site response are defined with reference to ideal case studies. These approaches have been validated by compari-son with results obtained with the code EERA, which is based on an equivalent-linear approach in the frequency domain. 1D schemes have also been used to clarify the definition of reference motion and numerical input motion. The numerical simulations of seismic site response for the Bovino case study are then presented. Before discussing the results of these analyses, local geology and topography are described. The geotechnical model is subsequently defined, based on field investigations and laboratory data. The reference outcrop motion is then selected, according to the Italian probabilistic seismic hazard maps. Seven real accelerograms have been selected as reference outcrop motions. The results of the 2D analyses performed using QUAKE/W, with reference to 22 sections, are presented first. The results of the 2D analyses are compared with those of 1D analyses performed with reference to 42 soil columns, extracted along two sections. Finally, results of 1D, 2D and 3D analysis performed with PLAXIS 3D, assuming two reference motions, are discussed. The analyses allowed to identify the effects of dimensional scheme to seismic site response, the dependency of each amplification pattern to the selected reference motion, the ground motion modification due to dif-ferent topography and soil layers interface, the comparison between different consti-tutive approaches to the same problem (i.e. linear equivalent and non-linear).

Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2007.
Source: Dissertation Abstracts International, Volume: 68-11, Section: B, page: 7519. Adviser: Youssef M.A. Hashash. Includes bibliographical references (leaves 244-254) Available on microfilm from Pro Quest Information and Learning.

Earthquake-induced landslides are a significant seismic hazard that can generate large economic losses. Predicting earthquake-induced landslides often involves an assessment of the expected sliding displacement induced by the ground shaking. A deterministic approach is commonly used for this purpose. This approach predicts sliding displacements using the expected ground shaking and the best-estimate slope properties (i.e., soil shear strengths, ground water conditions and thicknesses of sliding blocks), and does not consider the aleatory variability in predictions of ground shaking or sliding displacements or the epistemic uncertainties in the slope properties. In this dissertation, a probabilistic framework for predicting the sliding displacement of flexible sliding masses during earthquakes is developed. This framework computes a displacement hazard curve using: (1) a ground motion hazard curve from a probabilistic seismic hazard analysis, (2) a model for predicting the dynamic response of the sliding mass, (3) a model for predicting the sliding response of the sliding mass, and (4) a logic tree that incorporates the uncertainties in the various input parameters. The developed probabilistic framework for flexible sliding masses is applied to a slope at a site in California. The results of this analysis show that the displacements predicted by the probabilistic approach are larger than the deterministic approach due to the influence of the uncertainties in the slope properties. Reducing these uncertainties can reduce the predicted displacements. Regional maps of seismic landslide potential are used in land-use planning and to identify zones that require detailed, site-specific studies. Current seismic landslide hazard mapping efforts typically utilize deterministic approaches to estimate rigid sliding block displacements and identify potential slope failures. A probabilistic framework that uses displacement hazard curves and logic-tree analysis is developed for regional seismic landslide mapping efforts. A computationally efficient approach is developed that allows the logic-tree approach to be applied for regional analysis. Anchorage, Alaska is used as a study area to apply the developed approach. With aleatory variability and epistemic uncertainties considered, the probabilistic map shows that the area of high/very high hazard of seismic landslides increases by a factor of 3 compared with a deterministic map.
text

The development of a site-specific seismic hazard curve for a soil site requires the incorporation of site effects into the hazard calculation through the use of a site-specific amplification function. This study investigates the effect on the resulting soil hazard curves of different approaches to compute the site-specific amplification function. Amplification functions and their standard deviations can be developed using equivalent linear site response analyses. This study investigates the amplification function predictions of one-dimensional (1D) and two-dimensional (2D) site response analyses. For 1D analysis, one set of analyses are performed using time series (TS) input motions while a second set is performed using random vibration theory (RVT). One-dimensional site response analyses are performed for a shallow and a deep soil site and the results are compared for seismic hazard predictions. The influence of spatial variability introduced through randomization of site shear wave velocity (V[subscript S]) is also investigated. Shear wave velocity profile randomization does not significantly change the predicted amplification functions, except for the RVT analysis near the site period. At these periods, (V[subscript S]) randomization reduces the amplification function predicted by RVT making it more similar to the TS analysis prediction. The surface hazard at a site is dependent on the median amplification factor and its associated standard deviation. Spatial variability and uncertainties in soil properties across a site are often taken into account by modeling multiple 1D profiles in 1D site response analyses. However, this approach assumes that analyzing multiple 1D profiles captures accurately the effects of the true multi-dimensional spatial variability of the soil properties. This study investigates the results of two-dimensional (2D) site response analyses that incorporate spatial variability in the (V[subscript S]) profile through Monte Carlo simulation. Two-dimensional site response analyses are performed for 2D random fields generated with various statistical parameters (i.e. vertical and horizontal correlation distances) to investigate the effect of different levels of spatial variability on surface response across a region of interest (ROI). Two-dimensional site response analyses are performed for a shallow site. Results indicate that horizontal correlation distance has more influence on the analyses results than the vertical correlation distance. As the horizontal correlation distance increases, the median surface response spectrum across the ROI decreases. This reduction in median surface response is more pronounced around the site period. The influence of the vertical correlation distance is more pronounced when the horizontal correlation distance is large. As the vertical correlation distance increases, the median surface response spectrum across the ROI increases, which is more pronounced around the period of the motion. The predictions of 1D and 2D site response analyses modeling the (V[subscript S]) variability are compared. 1D analyses are performed on separately generated 1D (V[subscript S]) profiles (infinite horizontal correlation) and on the (V[subscript S]) profiles across the ROI of each 2D (V[subscript S]) field realization generated for 2D analysis (finite horizontal correlation). The results indicate that both sets of 1D analyses predict lower median response than 2D analyses. The 1D analyses with finite horizontal correlation display comparable levels of variability in the site response, however 1D analyses with infinite horizontal correlation display higher variability.
text

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.

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.

In the present work I infer the 1D shear-wave velocity model in the volcanic area of Pozzuoli-Solfatara using the dispersion properties of both Rayleigh waves generated by artificial explosions and microtremor. The group-velocity dispersion curves are retrieved from application of the Multiple Filter Technique (MFT) to single-station recordings of air-gun sea shots. Seismic signals are filtered in different frequency bands and the dispersion curves are obtained by evaluating the arrival times of the envelope maxima of the filtered signals. Fundamental and higher modes are carefully recognized and separated by using a Phase Matched Filter (PMF). The obtained dispersion curves indicate Rayleigh-wave fundamental-mode group velocities ranging from about 0.8 to 0.6 km/sec over the 1-12 Hz frequency band. I also propose a new approach based on the autoregressive analysis, to recover group velocity dispersion. I first present a numerical example on a synthetic test signal and then I apply the technique to the data recorded in Solfatara, in order to compare the obtained results with those inferred from the MF analysis Moreover, I analyse ambient noise data recorded at a dense array, by using Aki’s correlation technique (SAC) and an extended version of this method (ESAC) The obtained phase velocities range from 1.5 km/s to 0.3 km/s over the 1-10 Hz frequency band. The group velocity dispersion curves are then inverted to infer a shallow shear-wave velocity model down to a depth of about 250 m, for the area of Pozzuoli-Solfatara. The shear-wave velocities thus obtained are compatible with those derived both from cross- and down-hole measurements in neighbour wells and from laboratory experiments. These data are eventually interpreted in the light of the geological setting of the area. I perform an attenuation study on array recordings of the signals generated by the shots. The  attenuation curve was retrieved by analysing the amplitude spectral decay of Rayleigh waves with the distance, in different frequency bands. The  attenuation curve was then inverted to infer the shallow Q inverse model. Using the obtained velocity and attenuation model, I calculate the theoretical ground response to a vertically-incident SH wave obtaining two main amplification peaks centered at frequencies of 2.1 and 5.4 Hz. The transfer function was compared with those obtained experimentally from the application of Nakamura’s technique to microtremor data, artificial explosions and local earthquakes. Agreement among the transfer functions is observed only for the amplification peak of frequency 5.4 Hz. Finally, as a complementary contribution that might be used for the assessment of seismic risk in the investigated area, I evaluate the peak ground acceleration (PGA) for the whole Campi Flegrei caldera and locally for the Pozzuoli-Solfatara area, by performing stochastic simulations of ground motion, partially constrained by the previously described results. Two different methods (random vibration theory (RVT) and ground motion generated from a Gaussian distribution (GMG)) are used, providing the PGA values of 0.04 g and 0.097 g for Campi Flegrei and Pozzuoli-Solfatara, respectively.
Università degli Studi di Napoli Federico II
Published
open

Il presente lavoro consiste in un esperimento pilota per valutare l'efficacia dell'applicazione di reti micro-sismiche a piccola scala in sistemi di early-warning per il monitoraggio di versanti rocciosi instabili e la prospezione in passivo di aree di frana. Il lavoro si articola in tre parti principali: a.) analisi del dato sismico per la caratterizzazione del corpo di frana e dell'area in esame; b.) analisi del dato sismico continuo per la valutazione di eventuali modifiche nei parametri di analisi capaci di rivelare variazioni interne al corpo di frana, e c.) analisi dei transienti, discriminazione e classificazione. Un ulteriore sezione è dedicata al confronto con i dati indipendenti disponibili derivati dalla rete di monitoraggio già presente al sito di acquisizione.

Earthquake is undoubtedly one of the greatest natural disasters that can induce serious structural damage and huge losses of properties and lives. The resulting destructive consequences not only have made structural seismic analysis and design much more important but have impelled the necessity of more realistic representation of ground motions, such as inclusion of ground motion spatial variations in earthquake modelling and seismic analysis and design of structures. Recorded seismic ground motions exhibit spatial variations in their amplitudes and phases, and the spatial variabilities have an important effect on the responses of structures extended in space, such as long span bridges. Because of the multi-parametric nature and the complexity of the problems, the development of specific design provisions on spatial variabilities of ground motions in modern seismic codes has been impeded. Eurocode 8 is currently the only seismic standard worldwide that gives a set of detailed guidelines to explicitly tackle spatial variabilities of ground motions in bridge design, providing both a simplified design scheme and an analytical approach. However, there is gap between the code-specified provisions in Eurocode 8 and the realistic representation of spatially varying ground motions (SVGM) and the corresponding stochastic vibration analysis (SVA) approaches. This study is devoted to bridge this gap on modelling of SVGM and development of SVA approaches for structures extended in space under SVGM. A complete and realistic SVGM representation approach is developed by accounting for the incoherence effect, wave-passage effect, site-response effect, ground motion nonstationarity, tridirectionality, and spectra-compatibility. This effort brings together various aspects regarding rational seismic scenarios determination, comprehensive methods of accounting for varying site effects, conditional modelling of SVGM nonstationarity, and code-specified ground motion spectra-compatibility. A comprehensive, systematic, and efficient SVA methodology is derived for long span structures under tridirectional nonstationary SVGM. An absolute-response-oriented scheme of pseudo-excitation method and an improved high precision direct integration method are proposed to reduce the enormous computational effort of conventional nonstationary SVA. A scheme accounting for tridirectional varying site-response effect is incorporated in the nonstationary SVA scheme systematically. The proposed highly efficient and accurate SVA approach is implemented and verified in a general finite element analysis platform to make it readily applicable in SVA of complex structures. Based on the proposed SVA approach, parametric studies of two practical long span bridges under SVGM are conducted. To account for spatial randomness and variability of soil properties in soil-structure interaction analysis of structures under SVGM, a meshfree-Galerkin approach is proposed within the Karhunen-Loeve expansion scheme for representation of spatial soil properties modelled as a random field. The meshfree shape functions are proposed as a set of basis functions in the Galerkin scheme to solve integral equation of Karhunen-Loeve expansion, with a proposed optimization scheme in treating the compatibility between the target and analytical covariance models. The accuracy and validity of the meshfree-Galerkin scheme are assessed and demonstrated by representation of covariance models for various homogeneous and nonhomogeneous spatial fields. The developed modelling approaches of SVGM and the derived analytical SVA approaches can be applied to provide more refined solutions for quantitatively assessing code-specified design provisions and developing new design provisions. The proposed meshfree-Galerkin approach can be used to account for spatial randomness and variability of soil properties in soil-structure interaction analysis.