Variation Of Seismic Ground Motions Research Articles

Method for Determining Effective Frequency Range of Coherency Based on Seismic Energy

When using data recorded by dense instrument arrays to fit the coherency model of spatial variation of seismic ground motions, the selected frequency range may affect the fitting parameters, which also affects the synthesized time history of the ground motion field. In this study, the acceleration of the 5th and the 45th Earthquake of the SMART-1 array was selected. The Abrahamson and Loh models were used to perform the parameter fitting of the coherency for frequency ranges of 0–8, 0–16 and 0–24[Formula: see text]Hz, and the obtained results were different. The smaller the frequency range, the lower the fitted lagged coherency. The influence of frequency range on fitting parameters of Loh model is much greater than that of Abrahamson model. Based on the relative relationship of ground motion energy distribution in each frequency band represented by the power spectra, from the perspective of power spectrum energy, the energy ratio concept was introduced, i.e. the ratio of the power spectra for a specific frequency range to the total power spectra. Based on the energy ratio, a method for determining the effective frequency range of the coherency model was developed. Through the comparison and analysis of the horizontal-component acceleration of the SMART-1 array, it was found that when the frequency range was 0–5[Formula: see text]Hz, the energy ratio exceeded 95%, when the frequency range was 0–8[Formula: see text]Hz, the energy in the frequency band reached approximately 99% of the total energy. Therefore, it is recommended that when the parameters of the coherency model are fitted, the frequency corresponding to an energy ratio of 95% can be used as the effective frequency. Within this range, the contamination of high-frequency components on the fitting results is minimized.

Response of a high-speed train travelling over a long and high-pier viaduct during moderate earthquakes

This contribution presents the results of numerical simulations in which a high-pier viaduct under lateral and vertical non-stationary spatially variable seismic ground motions is crossed by an articulated high-speed train at several speeds. The spectral representation method is employed to generate the required number of acceleration time-history series at each support location of the railway viaduct studied. Wave passage, local soil conditions and loss of coherency effects have been accounted for in the simulated ground-motion time histories, with several peak ground accelerations. Train and bridge responses have been obtained by means of a non-linear dynamic interaction multibody and finite element model. An analysis of train running safety indices has been carried out. The properties of earthquakes and train speeds which may cause problems to traffic safety are detailed, as is the train derailment probability. The numerical simulation outcomes show that it is not safe for the train to travel over the studied viaduct in earthquakes with peak ground acceleration equal to 0.03 g and for train speeds above 280 km/h.

On the combined effect of topographic irregularities and wave passage on the spatial variation of seismic ground motion

On the combined effect of topographic irregularities and wave passage on the spatial variation of seismic ground motion

Frequency-dependent site factors for the Icelandic strong-motion array from a Bayesian hierarchical model of the spatial distribution of spectral accelerations

The earthquake ground motions of over 1700 earthquakes recorded on a small-aperture strong-motion array in south Iceland (ICEARRAY I) that is situated on a relatively uniform site condition characterized as rock, exhibit a statistically significant spatial variation of ground-motion amplitudes across the array. Both earthquake and microseismic horizontal-to-vertical spectral ratios (HVSR) have been shown to exhibit distinct and in some cases, bimodal peaks in amplification, indicating site resonance at periods of 0.1–0.3 s, a phenomenon that has been attributed to a surface layer of lava rock lying above a sedimentary layer, a structure that is then repeated with depth under the array. In this study, we implement a Bayesian hierarchical model (BHM) of the seismic ground motions that partitions the model residuals into earthquake event term, station term, and event–station term. We analyzed and compared peak ground acceleration (PGA) with the 5% damped pseudo-acceleration response spectrum (PSA) at oscillator periods of T = 0.05–1.0 s. The results show that the event terms, dominate the total variability of the ground-motion amplitudes over the array. However, the station terms are shown to increase in the period range of 0.1–0.3 s on most stations and to different extents, leading to an increase in the overall variability of ground motions at those periods, captured by a larger inter-station standard deviation. As the station terms are a measure of how much the ground motions at those stations deviate from the array average, they act as proxies for localized site effects and amplification factors. These results, improve our understanding of the key factors that affect the variation of seismic ground motions across the relatively small area of ICEARRAY I. This approach can help to improve the accuracy of earthquake hazard assessments on local scales, which in turn could contribute to more refined seismic risk assessments and engineering decision-making.

Effect of spatial variability of seismic strong motions on long regular and irregular slab-girder RC bridges

Long bridges and other lifeline systems are particularly susceptible to the spatial variability of earthquake actions. Even though it is a matter that has been substantially studied in the past decades, there are still plenty of issues with the engineering application of earthquake spatial variability. One of the difficulties is generating displacement time-histories with zero initial and small residual displacement values. This is essential to obtain results that have good quality in terms of the pseudo-static effects. Another difficulty is the uncertainty associated with the simulation of the spatial variability of the seismic ground motions in engineering applications (Zerva et al. in: 16th European conference on earthquake engineering, Thessaloniki, 2018). In this study, the conditional method for simulation of spatially variable ground motions is used and the respective displacement time-histories are generated. These time-histories are then applied to perform seismic analysis of a set of long irregular RC bridges, with different irregularity layouts and different total lengths. The goal of this study is to ascertain the impact of spatial variability on the seismic behaviour of these structures, according to their length, irregularity and to the coherency model of the strong motions and comparing the results with the case without spatial variability. The relative importance between dynamic and pseudo-static effects is also ascertained for each case-study. Finally, the definition of the cases for whom the effects of the spatially variable seismic ground motions are more detrimental than those of the uniform ground motions.

Vs30 değerine bağlı koherans modeli

Strong ground motion caused by earthquakes at every point of extended structures would not be same. This difference in ground movement has an important effect on the design of these types of structures. Meanwhile, the seismic resistant design has been lead to investigate the variability of earthquake ground motion over last decades. This variability of strong ground motion can define in terms of frequency or time. In this study, frequency domained variability named coherency is considered. Several coherency models have been proposed without considering soil effect. In this context, spatial variation of seismic ground motion based on the average shear wave velocity over the upper 30 m of depth, Vs30 is analyzed. Initially, coherency values are calculated using data triggered during six earthquakes recorded by the Istanbul Earthquake Rapid Response System. Lagged coherency data is considered in the process to get the coherency model. Nonlinear regression analysis is used for the model to obtain a good-fit to observed data. A coefficient is defined based on Vs30 values of the station-pairs. The cohereny model based on this coefficient of Vs30 is derived for EW and NS components. It is expected that coherency function decreases with the increase of frequency and separation distance. The decrease in the coefficient of Vs30 causes decrease in coherency. The variance in the coherency model between EW and NS components is small. This coherency model is used to simulate spatial variable ground motion for the accurate seismic design of elongated structures for the future studies.

A new solution to estimate the time delay on the topographic site using time domain 3D boundary element method

This study focuses on investigating spatial variation of ground motion that has great influence on the dynamic behavior of the large structures located on the surface topography. One of the most effective parameters on the spatial variation of ground motion is the difference between the arrival time of seismic waves in different points located on the abutments. In this research, a three-dimensional model of the Pacoima Dam site is prepared. The time domain 3D boundary element method is used to apply non-uniform excitation at the dam supports. This model is subjected to vertically propagating incident SH and P waves. The time delay can be characterized by calculating the value of the time delay for which the cross-correlation between two records is maximized. Finally, to obtain the time delay in a topographic site, a function considering effective parameters such as the height from the canyon base, wave velocity and predominant frequency, is presented. Furthermore, a code was developed for generating the spatially variation of seismic ground motions. The results show that the proposed functions have an acceptable accuracy in estimating the time delay to generate non-uniform ground motion.

Seismic Analysis of a Curved Bridge Considering Soil-Structure Interactions Based on a Separated Foundation Model

A separated foundation model was proposed in order to reduce the calculation scale of the numerical model for analyzing soil-bridge structure dynamics. The essence of the wave input analysis model considering soil-structure interaction was analyzed. Based on the large mass method, a one-dimensional time-domain algorithm of the free field was derived. This algorithm could simulate the specified ground motion input well. The displacement expansion solution of the free wave field was solved based on the propagation law of waves in a medium. By separating the soil foundations around the pile foundations of the bridge, the ground motion was transformed into an equivalent load applied on an artificial boundary. The separated foundation model could consider the incoherence effect and soil-structure interaction simultaneously; the number of model elements were reduced, and the computational efficiency was improved. In order to investigate the seismic response of a curved bridge considering soil-structure interaction under spatially varied earthquakes, a curved bridge with small radius was adopted in practical engineering. Spatially correlated multi-point ground motion time histories were generated, and the nonuniform ground motion field was simulated based on the wave input method on an artificial viscoelastic boundary. The effects of different apparent wave velocities, coherence values, and site conditions on the seismic response of the bridge were analyzed. The results showed that the spatial variation of seismic ground motion had a considerable effect on the bending moment and the torsion of the girder. The site effect had great influence on the bending moment of the pier bottom. When considering soil-structure interaction, the spatial variation of ground motion should be fully considered to avoid underestimating the structural response.

Analysis and modeling of ground motion coherency at uniform site conditions

In this work, nonstationary ground motion coherency analysis and modeling are performed using wavelet analysis and relevance vector machine regression. Earthquake ground motion data from four events recorded at dense seismograph SMART-1 array in north-south and east-west horizontal directions are used to investigate the lagged coherency behavior. Wavelet transform is used to compute the nonstationary lagged coherency that characterizes the space-time variation of seismic ground motion. Lagged coherency behavior with frequency, distance, and time is examined. It is shown that the lagged coherency is a time-varying quantity. The results implied that the lagged coherency on uniform soil is relatively insensitive to the earthquake events, and depends mainly on time, frequency, and separation distance. Relevance Vector Machines (RVM) regression is used to develop a model for nonstationary lagged coherency that characterizes the space-time variation of seismic ground motion. Earthquake ground motion in the in the north-south and the first principal directions is used to determine the lagged coherency model. The proposed model does not require a fixed parametric functional form. The developed model can estimate the lagged coherency for different separation distances at different time instants and frequencies.

Viaduct seismic response under spatial variable ground motion considering site conditions

The evaluation of the seismic hazard for a given site is to estimate the seismic ground motion at the surface. This is the result of the combination of the action of the seismic source, which generates seismic waves, the propagation of these waves between the source and the site, and site local conditions. The aim of this work is to evaluate the sensitivity of dynamic response of extended structures to spatial variable ground motions (SVGM). All factors of spatial variability of ground motion are considered, especially local site effect. In this paper, a method is presented to simulate spatially varying earthquake ground motions. The scheme for generating spatially varying ground motions is established for spatial locations on the ground surface with varying site conditions. In this proposed method, two steps are necessary. Firstly, the base rock motions are assumed to have the same intensity and are modelled with a filtered Tajimi-Kanai power spectral density function. An empirical coherency loss model is used to define spatial variable seismic ground motions at the base rock. In the second step, power spectral density function of ground motion on surface is derived by considering site amplification effect based on the one dimensional seismic wave propagation theory. Several dynamics analysis of a curved viaduct to various cases of spatially varying seismic ground motions are performed. For comparison, responses to uniform ground motion, to spatial ground motions without considering local site effect, to spatial ground motions with considering coherency loss, phase delay and local site effects are also calculated. The results showed that the generated seismic signals are strongly conditioned by the local site effect. In the same sense, the dynamic response of the viaduct is very sensitive of the variation of local geological conditions of the site. The effect of neglecting local site effect in dynamic analysis gives rise to a significant underestimation of the seismic demand of the structure.

Layer-effect on variable seismic ground motions of SV-wave scattered by a double-stratified circular-arc crossing by layer-interface

This study is to theoretically investigate and quantify the variability of the seismic surface motions of a double-stratified half space with a circular-arc canyon by the SV wave incidence. Compared with the SH-wave induced scattering by the canyon with the single-layer (ideally homogeneous) medium, the challenge is to simultaneously satisfy the boundary conditions at the ground surface and layer-interface and deal with the relatively complex wave transmission on layer-interface. Theoretical solution for the SV-wave induced scattering by a double-stratified half space with circular-arc canyon is derived explicitly. The present method is validated by reducing the proposed solution with that in the homogenous medium. Then, an example is considered to further verify the derived results by simultaneously satisfying the continuity and zero-stress boundary conditions. The effects of the layer medium, the incident frequencies and incident angles on the ground motion of the double-stratified medium are investigated parametrically and compared with that of the homogeneous case. Results show that the derived solution is theoretically reliable and the layer-effect on the variability of the canyon ground motions is obvious.

Comparative evaluation of foundation modeling for SSI analyses using two different ABC approaches: Applications to dams

The numerical modeling of the unbounded domain is one of the most challenging issues both for site response and soil-structure interaction analyses. The viscous-type absorbing boundary conditions (ABCs) are commonly used for the modeling of the unbounded domain due to their low computational cost and simplicity. However, in cases, their application can lead to substantial errors. In this paper, a time-domain viscoelastic perfectly matched layer (PML) is implemented for modeling the unbounded domain in the dynamic response evaluation of a concrete gravity dam. The performance of the viscoelastic PML and the viscous ABCs are examined through numerical examples under different types of dynamic excitations including uniform and spatially variable seismic ground motions. The numerical results highlight the excellent performance of the PML approach to model the unbounded domain under all types of excitations. It is also shown that the viscous-type ABCs perform poorly under spatially variable excitations and reflected waves from the structures, as they cannot properly absorb inclined waves.The nonlinear response of a concrete gravity dam subjected to deconvolved seismic excitations at depth resulting from five surface seismic records was then evaluated. The effect of the finite foundation depth on the dam response was also examined by considering a shallow (approximately equal to the height of the dam) and a deep (approximately double the height of the dam) finite foundation length. The results indicate that the PML is capable of maintaining its accuracy even if the size of the computational finite domain is reduced. On the other hand, it was shown that the shallow finite foundations modeled with viscous ABCs substantially underestimate the dam response under uniform excitations, whereas the use of a deep finite foundation model with viscous ABCs can lead to more conservative results. An important conclusion of this study is that the use of foundation models with viscous-type ABCs for the dynamic analysis of dams, and, consequently, other long structures, subjected to spatially variable excitations, should be avoided as it can significantly underestimate the crest displacement and the total damage in the dam.

Site effect estimation on two Icelandic strong-motion arrays using a Bayesian hierarchical model for the spatial distribution of earthquake peak ground acceleration

Earthquake recordings on two small-aperture (covering ∼1.2 km2 each) strong-motion arrays in Iceland (ICEARRAY I and II) exhibit considerable variations in the spatial distribution of ground-motion amplitudes. To better understand this spatial variability, we use a Bayesian Hierarchical Model (BHM) that incorporates ground motions models (GMMs) for peak ground accelerations (PGA) developed from ground motion databases recorded by the two arrays, respectively. The posterior distributions of the model parameters are then determined using Markov Chain Monte Carlo simulations in the context of Bayesian statistical methods. The BHM allows the partitioning of a GMM into event, station, and event-station terms, which in turn allows the relative contributions of source, path, and site effects to be quantified. The results indicate that site effects can dominate the spatial distribution of ground-motion parameters (e.g., PGA) observed across both ICEARRAY I and II. Although the site conditions across ICEARRAY I have been classified as uniform (i.e., “rock” with a relatively flat topography), station terms contribute ∼13% to the total variability in the amplitudes of predicted ground motions across the array. In contrast to ICEARRAY I, the variation of the geologic profiles and topography is much greater across ICEARRAY II. As a result, the inter-station variability is shown to contribute up to ∼57% of the total variability in the amplitudes of predicted ground motions across the array, with the contributions being less constrained for ICEARRAY II than ICEARRAY I due to the relative sizes of the recorded ground motion databases. These results facilitate our understanding of the key factors that affect the variation of seismic ground motions across a relatively small area. Such a detailed microzonation is of great importance for earthquake hazard assessment on a local scale and has practical implications for engineering decision making.

Relevance vector machines modeling of nonstationary ground motion coherency

Conventional coherency models do not consider the nonstationary character of ground motions. Accurate modeling of nonstationary coherency can motivate accurate simulation of spatially incoherent ground motions. In this work, wavelet transform and Relevance Vector Machines are used to develop a regression model for nonstationary lagged coherency that characterizes the space-time variation of seismic ground motions. Earthquake data from four events recorded at dense seismograph SMART-1 array is used in the analysis. The nonstationary lagged coherency is computed using the wavelet transform of strong motion S-wave window, and then fed to the Relevance Vector Machine as training data. A homogeneous isotropic field is assumed. The proposed model does not require a fixed parametric functional form. It has been shown that the proposed model can estimate the lagged coherency for different separation distances at different time instants.

A directionally-dependent evolutionary lagged coherency model of nonstationary horizontal spatially variable seismic ground motions for engineering purposes

A novel directionally-dependent evolutionary lagged coherency model for nonstationary horizontal spatially variable seismic ground motions is developed for engineering purposes. Evolution of the model is achieved by varying the values of the parameters in time. First, based on the generalized harmonic wavelets, the approach for estimating the evolutionary lagged coherency of realistic seismic ground motions is presented. Then, the evolutionary lagged coherency model of spatially variable seismic ground motions is formulated. This proposed model is based on the analytic form of the cross correlation function for a plane wave in a homogeneous random medium. The parameters of the model are treated as piecewise constant variables for describing the evolutionary characteristics of the estimated coherency. Finally, the evolutionary lagged coherency of the spatially variable ground motion accelerations recorded at the SMART-1 array, Taiwan, during Event 45, is investigated and modeled. The ground motions are divided into three sections: a build-up portion, a high-intensity portion and a decaying portion. The model parameters are identified in these three sections respectively. The comparisons between the actual estimated coherency and the model indicate that the proposed evolutionary lagged coherency model can well reflects the time-dependent correlation of the realistic spatially variable seismic ground motions.

Spatially Variation of Seismic Ground Motions and Foundation Ground Stability of Inhomogeneous Layered Ground Considering Nonlinearity

Spatially Variation of Seismic Ground Motions and Foundation Ground Stability of Inhomogeneous Layered Ground Considering Nonlinearity

Spatial Variation of Seismic Ground Motions and Foundation Ground Stability of Inhomogeneous Layered Ground Considering Nonlinearity - Due to the Difference of Ground Inhomogeneous Distribution Property and Input Ground-Motion Characteristics

Spatial Variation of Seismic Ground Motions and Foundation Ground Stability of Inhomogeneous Layered Ground Considering Nonlinearity - Due to the Difference of Ground Inhomogeneous Distribution Property and Input Ground-Motion Characteristics

A Fast Computational Framework for Seismic Random Response of Multi-support Structures

This paper presents a fast computational framework for seismic random response of multi-support structures, of which computational efficiency handers its application to practical engineering. First of all, after structural modelling based on the finite element method, the large mass method is used to exert the load on the supports in the form of seismic acceleration history, which ensures that the equations of motion of the system are solved directly. Secondly, the impulsive load is exerted on the supports and corresponding dynamic analysis is carried out. On the basis of the impulsive response, a generalized frequency response function (FRF) can be obtained via the Fourier transform. Thirdly, according to classic formulation, auto power spectrum density (PSD) functions of structural response may be calculated, which involves the generalized FRFs and the PSD function matrix of the seismic accelerations at the supports. The proposed framework is verified by comparing its computational results with the analytical solution of a single DOF system. A 5-span bridge during earthquakes is taken as a case study. The results show that three factors influencing spatial variation of seismic ground motion should be taken into consideration to get safer random response of a multi-support structure.

Revised Seismic Intensity Parameters for Middle-Field Horizontal and Rocking Strong Ground Motions

AbstractThis paper presents a new method for determining rocking ground motion considering the effects of both time delay and loss of coherency in spatially variable seismic ground motions. Acceleration, velocity, and displacement response spectra resulting from the rocking component are also derived. Next, new seismic intensity parameters are proposed to estimate the combined action of the middle-field horizontal and rocking motions on the earthquake excitation of structures. These seismic intensity parameters are revised forms of the widely used ones for characterizing translational components, namely, peak ground velocity, peak ground displacement, cumulative absolute displacement, acceleration response spectrum, acceleration spectrum intensity, Fajfar intensity, and Kappos spectrum intensity. In addition, an expression for the estimation of the effect of the base tilt due to the rocking ground motion on the structural response is presented. The main advantage of the proposed relations is that they can...

Spatially Varying Ground Motion Effects on Seismic Response of Adjacent Structures considering Soil-Structure Interaction

Spatial variation of seismic ground motions is caused by incoherence effect, wave passage, and local site conditions. This study focuses on the effects of spatial variation of earthquake ground motion on the responses of adjacent reinforced concrete (RC) frame structures. The adjacent buildings are modeled considering soil-structure interaction (SSI) so that the buildings can be interacted with each other under uniform and non-uniform ground motions. Three different site classes are used to model the soil layers of SSI system. Based on fast Fourier transformation (FFT), spatially correlated non-uniform ground motions are generated compatible with known power spectrum density function (PSDF) at different locations. Numerical analyses are carried out to investigate the displacement responses and the absolute maximum base shear forces of adjacent structures subjected to spatially varying ground motions. The results are presented in terms of related parameters affecting the structural response using three different types of soil site classes. The responses of adjacent structures have changed remarkably due to spatial variation of ground motions. The effect can be significant on rock site rather than clay site.