Spatial Variability Of Ground Motion Research Articles

Seismic motion incoherency effects on SSSI responses of nuclear buildings

AbstractThe spatial variation of ground motion may significantly aggravate the seismic damage of large‐scale structures such as nuclear buildings. Establishing the seismic wave field with spatial features is the basis and key to seismic analysis of large‐scale complex structures. For the nuclear island buildings, which are arranged close to each other, the influence of structure–soil–structure interaction (SSSI) and incoherent motion on the structural seismic response should be considered. Taking a domestic nuclear power project as an example, the seismic response analyses under incoherent seismic motion and coherent seismic motion are carried out respectively. The seismic motion incoherency effects on the nuclear island buildings on the common raft and the adjacent seismic category I structure are explored by comparing the results of the in‐structure response spectrum (ISRS) at key elevations. The results indicate that the seismic motion incoherency does not change the trend of the response spectrum curve and the dominant frequency. Both the peak acceleration and zero‐period acceleration of ISRS are changed to some extent. Overall, seismic motion incoherency has a more significant impact on the ISRS of nuclear buildings with smaller volumes.

Seismic excitation model in probabilistic risk assessment of civil infrastructure

AbstractRealistic risk assessment of civil infrastructure subjected to catastrophic events plays a crucial role in strategic asset management. In this process, visualization of the asset and its environment always possesses some epistemic uncertainty associated with the modeler's simplification/abstraction. The present paper aims at providing a critical perspective into the characterization of the seismic excitation model for risk assessment of civil infrastructures and identifying gaps in the present knowledge. For this purpose, the main factors that influence the development of a comprehensive framework for the simulation of seismic excitation are highlighted. The coupling effect of the spatial variability of ground motions and the substructure's footprint on the realization of the seismic excitation model is illustrated. Finally, the necessity of direct incorporation of the load analysis subtask in the probabilistic seismic risk assessment of civil infrastructures is discussed. In addition, simplified probability density functions are proposed for the simulation of the seismic excitation model.

MLPER: A Machine Learning-Based Prediction Model for Building Earthquake Response Using Ambient Vibration Measurements

Deep neural networks (DNNs) have gained prominence in addressing regression problems, offering versatile architectural designs that cater to various applications. In the field of earthquake engineering, seismic response prediction is a critical area of study. Simplified models such as single-degree-of-freedom (SDOF) and multi-degree-of-freedom (MDOF) systems have traditionally provided valuable insights into structural behavior, known for their computational efficiency facilitating faster simulations. However, these models have notable limitations in capturing the nuanced nonlinear behavior of structures and the spatial variability of ground motions. This study focuses on leveraging ambient vibration (AV) measurements of buildings, combined with earthquake (EQ) time-history data, to create a predictive model using a neural network (NN) in image format. The primary objective is to predict a specific building’s earthquake response accurately. The training dataset consists of 1197 MDOF 2D shear models, generating a total of 32,319 training samples. To evaluate the performance of the proposed model, termed MLPER (machine learning-based prediction of building structures’ earthquake response), several metrics are employed. These include the mean absolute percentage error (MAPE) and the mean deviation angle (MDA) for comparisons in the time domain. Additionally, we assess magnitude-squared coherence values and phase differences (Δφ) for comparisons in the frequency domain. This study underscores the potential of the MLPER as a reliable tool for predicting building earthquake responses, addressing the limitations of simplified models. By integrating AV measurements and EQ time-history data into a neural network framework, the MLPER offers a promising avenue for enhancing our understanding of structural behavior during seismic events, ultimately contributing to improved earthquake resilience in building design and engineering.

Quantification of the effects of the spatial variation of ground motions on the seismic response of highway bridges

The stochastic responses of highway bridges to spatial variation of ground motions (SVGM) are analysed in this paper. A model of spatially varying ground motions is used to investigate the relative importance of the incoherency effect, the wave passage effect and the site effects on the stochastic dynamic response of an asymmetrical R.C box girder highway bridge with variable inertia. In this study, the incoherency effect is investigated using two widely used models while the wave-passage effect is incorporated using various wave velocities. Then, the random vibration theory is applied to study the effect of the non-uniform seismic excitations on the bridge structure. The bridge response is evaluated in terms of the mean values of the maximum displacements and the bending moments. Analyses of both stationary and transient response are performed. The results show that the stochastic dynamic responses related to site effects are mostly much greater than those calculated using uniform, delayed and incoherent seismic excitation assumptions. As a result, analytical models used for the stochastic dynamic analysis of long span highway bridges should take into account all the SVGM components, particularly the site-response effects.

Seismic response of a single-layer reticulated dome: Fault-to-structure simulation

Seismic response of large-span spatial structures is particularly susceptible to soil-structure interaction and spatial variability of ground motions. Yet existing seismic analysis fail to properly account for the foundation soil, and the ground motions are commonly determined based on empirical formulas. Physics-based 3D seismic simulation has recently been developed to provide site-specific ground motions, and the fault-to-structure simulation facilitates the incorporation of seismic wave field and soil-structure interaction into the analysis of engineering structures. This article innovatively implements the fault-to-structure simulation on a single-layer reticulated dome. Structural response of fault-to-structure model and two commonly used models is compared and discussed. Results show that the damping of soil reduces the stress and horizontal acceleration of the reticulated dome, while the soil movement leads to an increase in displacement. Noticeably, the seismic propagation characteristics increase vertical acceleration of the inner rings. Compared with fault-to-structure model, the multi-support excitation model and uniform excitation model reduce the vertical displacement of 29.1% and 50.1%, respectively, and reduce the vertical acceleration of the inner rings of 59.9% and 69.0%, respectively.

Multi-unit seismic probabilistic risk assessment: A Bayesian network perspective

The Fukushima Dai-ichi highlighted multi-unit accidents in nuclear power plant (NPP) probabilistic risk assessment (PRA). This paper focuses on multi-unit seismic PRA because seismic events are contributors to NPP site risk. This paper integrates works on modeling ground motion spatial variability at a NPP site and dependent seismic failures into a coherent and rigorous framework using Bayesian networks, and explores the relative importance of different types of correlation on NPP site-based risk metrics. A study of a hypothetical dual unit NPP site considered 15 cases with varying degrees of correlation in spatial ground motion (GM) and seismic failures. Based on the results, the current assumption of perfect GM correlation across a NPP site underestimates risk metrics that consider either unit experiencing core damage (CD) (site CD), which is not conservative. The risk metric that considers both units experiencing CD (concurrent CD) shows differences between the partial and perfect GM correlation assumptions when multiple component failures contribute to the CD in each unit. Within a GM correlation assumption, the site CD increases as the fragility correlation increases, a counterintuitive trend. The risk assessment case outlined in this paper provides more realistic results than the cases reflecting the current state of practice.

Spatial variation of strong ground motions in a heterogeneous soil site based on observation records from a dense array

The difference in local sediment thickness and soil properties has a significant impact on the spatial variation mechanism of seismic ground motion in the engineering scale. Due to the scarcity of observation data of dense arrays, the existing theoretical studies are mostly developed by numerical simulation methods, including human factors and a large number of assumptions. In view of this, based on the multistation observation records of the Luxian MS 6.0 earthquake and Yibin MS 5.1 earthquake obtained using a Zigong dense array, the study quantitatively analyzes the spatial characteristics of ground motion in heterogeneous soil sites by integrating a theoretical model with numerical analysis. In this study, many popular approaches including root-mean-square acceleration, horizontal-to-vertical spectral ratio (HVSR) of microtremor and strong motion records, and lagged coherency are comprehensively utilized to make the conclusion accurate and reliable. The results show that local soil conditions could affect the attenuation of coherence function with distance. The station-pairs with similar HVSR characteristics generally present a higher coherence level when the difference of the interstation distance is less than 100 m. In addition, the coherency function between stations will be greatly reduced when the H/V spectral ratio characteristics differ greatly, which is also obvious in the low-frequency part below 5 Hz. Finally, a lagged coherency model that considers the influence of heterogeneous soil is constructed in this study. The model has a definite physical meaning and can better represent the spatial variation of ground motion at nonbedrock sites.

Influence of uncertainty of structural parameters on the stochastic dynamic response of asphaltic lining dam-foundation systems to non-stationary random seismic excitation

The present work aims toward the effect of uncertain structural parameters on the stochastic dynamic response of an asphaltic lining dam-foundation system subjected to stationary as well as non-stationary random excitation. Uncertain structural parameters of interest are shear modulus and mass density, modeled using the lognormal distribution. The stochastic seismic response of the dam-foundation system to the random loads with uncertain structural parameters is carried out with the Monte Carlo simulation method. The spatial variability of ground motion is considered with incoherence and wave passage effects as stationary as well as non-stationary random excitation. A time-dependent frequency response function is used throughout the study for non-stationary responses. Obtained results indicate that the variability of shear modulus and mass density can be neglected in stochastic dynamic analysis of an asphaltic lining dam-foundation system. Also, it is seen from the results that stationarity is a reasonable assumption for asphaltic lining dams to typical durations of strong shaking.

Assessment of the seismic behavior of short-to-medium overall length girder bridges under asynchronous ground motions

Proper seismic analysis and design of bridges are critical, especially in locations prone to high seismic activities where critical infrastructure is at a high risk of seismic damage leading to direct and indirect losses. The seismic performance of bridges has been widely investigated in the literature, but the effect of asynchronous ground motions for short-to-medium overall length girder bridges is a phenomenon that has not been adequately addressed by bridge codes and thus it warrants further research. For such bridges, the often-employed envelope response spectrum is not generally applicable for asynchronous ground motions as this is a multiple-support excitation problem where potential local demand concentrations are critical. This study investigates the effects of asynchronous ground motion on the seismic response of short-to-medium overall length girder bridges with reinforced concrete columns considering crustal, subcrustal and subduction earthquakes. The main source of spatial variability of ground motions considered was the variation in the local soil conditions at the foundation of 3-span (short) and 7-span (medium) overall length prototype girder bridges. Soil classes A (rocky soil) and E (softer soil) were considered to establish different combinations of soil distribution in the foundation and then compared to baseline models where site class C was applied to all the supports of the structure to study the effects of asynchronous ground motions. It was found that the variation of site class in the foundations for such structures could produce detrimental effects on the dynamic response of the structure. The presence of softer soil in most of the structure's foundations elongated the vibration period of the structure and resulted in higher displacement demands. Results also showed that critical demands are concentrated at locations where the soil conditions change, indicating increased sensitivity to seismic effects at certain locations, which would not be captured by the typical code-prescribed procedure in the presence of asynchronous ground motion effects.

Seismic Response Analysis of Bongo Bridge Subjected to Multiple Support Excitation due to Spatial Variation of Ground Motion

The spatial variation of ground motion (SVGM) may cause detrimental effects on extended structures such as bridges. The SVGM is mainly caused by three phenomena: the incoherence effect, wave passage effect, and the local site effect, which leads the structure to undergo multiple support excitation. In most cases, in the Philippines, the conventional response spectrum analysis per DPWH Bridge Seismic Design Specifications (BSDS) 2013 is used to perform seismic analysis of bridges. However, the method assumes that the ground motion is spatially uniform, thus ignoring the potential effect of SVGM. The study focuses on the seismic response analysis of a conventional existing highway bridge in the Philippines subjected to multiple support excitation accounting for the effect of SVGM. The researcher used the Bongo Bridge located in Ilocos Norte, originally designed using the conventional response spectrum. A method for generating a modified response spectrum that accounts for the effect of SVGM based on the fundamental principle of random vibration theory was also presented to provide guidelines for bridge engineers in the Philippines. Three cases were performed where each seismic response of the existing bridge was obtained and evaluated to determine if the application of multiple support excitation due to SVGM will cause a significant effect on the existing bridge. Based on the results, the effect of multiple support excitation to Bongo Bridge amplified the seismic response of some pier columns and de-amplified some. The study also shows that the effect of multiple support excitation decreased the displacement demand significantly on the Bongo Bridge. Since the study is limited to Bongo Bridge and the seismic response can be affected by many factors, the result may vary to other bridges. Therefore, it is still recommended to perform a parametric analysis accounting for multiple support excitation especially if the structure is situated on a varying ground type or bridges with multiple and longer spans, to determine if this phenomenon will cause a significant effect on the bridge.

Effect of Spatial Variation of Earthquake Ground Motions on Seismic Vulnerability of Urban Road Network Considering Building Environment

Evaluating the seismic damage of urban road infrastructure systems is of vital importance in reducing the earthquake hazard risk of cities. Urban road infrastructure systems are commonly represented as a spatial network that covers the whole city, and in its seismic damage analysis the spatial variation of earthquake ground motions cannot be ignored. However, the effect of the spatial variation of ground motions on the seismic vulnerability of urban road networks is unknown. In this article, based on the case study on Datong, China, it is shown that the spatially variation of ground motions may increase the damage risk of urban road networks and should be considered in the seismic vulnerability analysis. This was achieved by developing a method to evaluate the performance of a post-earthquake road network considering the structural damage of road components and the building environment. This work is anticipated to be a starting point for more sophisticated studies on the earthquake risk analysis of spatially extended urban lifeline systems.

Analysis of the 2019 Mw 5.8 Silivri Earthquake Ground Motions: Evidence of Systematic Azimuthal Variations Associated with Directivity Effects

Abstract The main Marmara fault (MMF) extends for 150 km through the Sea of Marmara and forms the only portion of the North Anatolian fault zone that has not ruptured in a large event (Mw>7) for the last 250 yr. Accordingly, this portion is potentially a major source contributing to the seismic hazard of the Istanbul region. On 26 September 2019, a sequence of moderate-sized events started along the MMF only 20 km south of Istanbul and were widely felt by the population. The largest three events, 26 September Mw 5.8 (10:59 UTC), 26 September 2019 Mw 4.1 (11:26 UTC), and 20 January 2020 Mw 4.7 were recorded by numerous strong-motion seismic stations and the resulting ground motions were compared to the predicted means resulting from a set of the most recent ground-motion prediction equations (GMPEs). The estimated residuals were used to investigate the spatial variation of ground motion across the Marmara region. Our results show a strong azimuthal trend in ground-motion residuals, which might indicate systematically repeating directivity effects toward the eastern Marmara region.

Effects of Spatial Variation of Ground Motion (SVGM) on Seismic Vulnerability of Ultra-high Tower and Multi-tower Cable-stayed Bridges

ABSTRACT This study focused on the influence of SVGM on the vulnerability of an ultra-high tower cable-stayed bridge. Components and system vulnerability analysis of the bridge with longitudinal constraint and floating structural systems was conducted, considering nine combinations of wave passages, sites and incoherence effects. Results generated by these combinations showed that bridge components and systems suffered the greatest damage under conditions where the soil changes from firm to soft and with the lowest incoherence coefficient. Moreover, compared with uniform excitation, the damage probabilities in a longitudinal constraint structure decreased, while a floating structure was found to be more threatened.

2D Geotechnical site-response analysis including soil heterogeneity and wave scattering

This study describes an approach for modeling wave scattering and the spatial variability of ground motion in geotechnical site-response analysis by modeling soil heterogeneity through 2D correlated random fields. Importantly, the required site-specific inputs to apply the proposed approach in a practical setting are the same as those associated with conventional 1D site-response analysis. The results, which are affected by wave scattering attenuation, are compared to those from conventional laterally homogeneous 1D site-response analyses and 1D analyses with randomized velocity profiles extracted from heterogeneous 2D velocity model realizations. A sensitivity study, involving 5400 2D model realizations, investigates the influence of random field input parameters on wave scattering and site response. The computed ground surface acceleration waveforms and transfer functions show that this method is capable of scattering seismic waves. Multiple ground-motion intensity measures are analyzed to quantify this influence and distinguish between the effects of 1D vertical heterogeneities and averaging across many nodes and realizations, from the effects of wave scattering and 2D ground-motion phenomena. The redistribution of ground-motion energy across wider frequency bands and scattering attenuation of high-frequency waves in the 2D analyses resemble features observed in empirical transfer functions computed in other studies. While analyses with 1D randomized velocity profiles are able to replicate median results from 2D analyses for some low-frequency intensity measures (e.g. transfer functions at [Formula: see text] Hz, and spectral acceleration at the fundamental period), medians and standard deviations of high-frequency intensity measures (e.g. transfer function at [Formula: see text] Hz, [Formula: see text], and Arias intensity), which are influenced by wave scattering, are not appropriately captured. Given the equivalent input information requirements as conventional 1D analysis, and the availability of large computational resources, we advocate that the proposed 2D (and eventually 3D) approach is a fruitful path forward to improve the modeling of site-response physics and realize improved predictive capabilities.

A hybrid boundary method for seismic wave propagation problems in slopes

The seismic behavior of a slope site has a critical impact on its stability and the seismic response of structures located on or near the slope. In this paper, a hybrid boundary method(HBM) is proposed for the site response analysis of slopes. Viscous and viscous-spring boundaries are applied in the HBM and equivalent loads are calculated separately on the bottom and the lateral boundary. A horizontally extended model is used to verify the HBM. The results show that the maximum error of the HBM is only 2.010%. On the ground surface of the slope site, amplification occurs at the crest side and de-amplification occurs at the toe side under various cases. The ratio of the maximum and minimum PGA on the ground surface could exceed 2. Therefore, great attention should be paid to the spatial variability of ground motion on slope sites. The HBM is also compared with four conventional approaches. The maximum errors of all the conventional approaches exceed 70%. These examples demonstrate that the HBM has much higher accuracy in practice than conventional approaches.

A Bayesian Network Approach for Modeling Dependent Seismic Failures in a Nuclear Power Plant Probabilistic Risk Assessment

The importance of modeling dependency between seismic failures of multiple components in a nuclear power plant (NPP) probabilistic risk assessment (PRA) has been discussed since the 1980s. In NUREG/CR-7237, Budnitz et al. found the Reed-McCann method to be the most promising method for modeling dependent seismic failures in NPP PRA. However, there are issues with the Reed-McCann method's quantification of the seismic fragility of a system of multiple components. To address this issue and to facilitate an overall realism increase in modeling dependencies in seismic PRA, this paper proposes a Bayesian network (BN) approach to model dependent seismic failures. To illustrate the proposed approach, we calculate the fragility of a parallel system and a series system using the Reed-McCann method, the BN approach, the First-Order Reliability Method (FORM) and Monte Carlo simulation (MCS). Then, we compare the system fragility results from these four approaches/methods to the lower and upper bounds of the system fragility. We found that the BN approach performed better than the Reed-McCann method with respect to providing results that stay within the lower and upper bounds of the system fragility. Further, the BN approach gives similar results to FORM and MCS. This paper proposes a BN approach because, in combination with our previous work about extending a probabilistic seismic hazard analysis to account for the spatial variability of ground motion at an NPP hard-rock site, it can be used to simultaneously and realistically account for dependent seismic failures and spatial variability of ground motion in both single-unit and multi-unit seismic PRAs.

Challenges in the definition of input motions for forensic ground-response analysis in the near-source region

This article scrutinizes the determination of input motions for forensic ground-response analysis in the near-source region, based on recorded surface ground motions at strong-motion station sites, from the same event. The first part of the article draws upon observed ground motions from the 22 February 2011 6.2 Mw Christchurch earthquake to discuss key challenges of the problem associated with the strong spatial variation of ground motion in the near-source region. Effects from the complexity of the rupture, propagation of seismic waves through complex geological structures, and site characteristics are explored. It is argued that, because of the strongly varying source-path “signature” on near-source ground motions, “reference” input motions for ground-response analysis must be specific to, and have similar signature characteristics (be “compatible”) with, the target site which is subject to the analysis. The second part of the article presents a four-step procedure for the derivation of site-specific input motions involving (1) determination of the reference layer where the input motion is to be applied in the analysis, (2) record selection considering the appropriateness of the recording station site for deconvolution and its compatibility with the target site, (3) deconvolution of the selected record to remove local site effects from the recorded ground motion, and (4) scaling of the deconvolved motion to account for differences in the source-to-site distance between the recording station and the target site. As part of the proposed procedure, a novel (amplitude-duration) scaling method is presented. Results from one-dimensional (1D) effective-stress analysis of two target Christchurch sites using input motions from the proposed procedure are used to critically evaluate the procedure and discuss essential requirements for its successful application.

Inelastic response of cable-stayed bridges subjected to non-uniform motions

This paper studies for the first time the effect of the spatial variability of ground motions (SVGM) with large intensities on the inelastic seismic response of the pylons which are responsible for the overall structural integrity of cable-stayed bridges. The svgm is defined by the time delay of the earthquake at different supports, the loss of coherency of the seismic waves and the incidence angle of the ground motion. An extensive study is conducted on cable-stayed bridges with ‘H’- and inverted ‘Y’-shaped pylons and with main spans of 200, 400 and 600 m. The svgm is most detrimental to the pylon of the 200-m span bridge owing to the large stiffness of this bridge compared to its longer counterparts. The stiff configuration of the inverted ‘Y’-shaped pylon makes it more susceptible against the multi-support excitation than the flexible ‘H’-shaped pylon, especially in the transverse direction of the response. Finally, the earthquake incidence angle is strongly linked with the svgm and should be included in the seismic design of cable-stayed bridges.

Effects of spatial variability of ground motions on collapse behaviour of buildings

The seismic damage simulation of buildings has become a research focus because of the rapid urbanisation and the increase in the building density. However, there is no consensus regarding whether the spatial variability of ground motions at the local sites must be considered in collapse simulations. Thus, in the present study, a numerical model based on the finite element method that can effectively predict the collapse behaviour of reinforced concrete buildings is developed. Then, field ground motion records are selected from the measurement database. Finally, a building neighbourhood comprising four typical three-storey reinforced concrete frame structures is established as a numerical example, and the effects of the spatial variability and intensity of ground motions on the structural response and collapse behaviour of buildings are comprehensively investigated. The results indicate that the spatial variability of the ground motions significantly affects the structural responses and collapse behaviours of the buildings within a certain range of the ground motion intensity. The collapse behaviour of a single building exhibits significant differences with different ground motion intensities. These differences are attributed to the coupling of the nonlinearity of the structures and the randomness of the ground motions. It is concluded that the spatial variability of ground motions should be fully considered in the collapse simulation of buildings.

Network analysis of earthquake ground motion spatial correlation: a case study with the San Jacinto seismic nodal array

SUMMARYThe spatial correlation of earthquake ground motion intensity can be measured from strong motion data; however, the data used in past studies is sparsely sampled in space, and only the interstation distance was considered as a correlation variable. These limitations mean that we have only weak constraints on the true correlation structure of ground motion and that potentially important aspects of spatial correlation are unconstrained. In this study, we combine a large-N seismic array and graph analytics to explore this issue at a local scale using small local and regional earthquakes. Our result suggests site conditions, and how they interact with the incident seismic wavefield, strongly condition the spatial correlation of ground motion. Future progress in characterizing ground motion spatial variability will require dense wavefield measurements, either through nodal deployments, or perhaps distributed acoustic sensing measurements, of seismic wavefields. Aftershock sequences of major earthquakes would provide particularly data-rich targets of opportunity.