Field Ground Motion Research Articles

Dimension-reduction representation of stochastic ground motion fields based on wavenumber-frequency spectrum for engineering purposes

It is of great necessity to achieve the efficient and accurate representation of ground motion fields considering the spatial variation of which possess paramount importance to the seismic safety and reliability evaluation of lifeline systems. To circumvent the difficulties caused by the decomposition of cross power spectral density (PSD) matrix and the interpolation between discretized spatial points in the spectral representation method (SRM) for uni-dimensional multi-variate (1D-nV) stochastic vector process (regarded as discrete stochastic field), a wavenumber-frequency SRM (WSRM) for simulating multi-dimensional uni-variate (mD-1V) spatial-temporal stochastic field (regarded as continuous stochastic field) has been developed for years. In this paper, an updated WSRM (UWSRM), which can reduce the number of elementary random variables, is proposed for simulating the 2D-1V stochastic ground motion fields. Compared with the conventional WSRM (CWSRM) which belongs to the Monte Carlo simulation schemes, merely two elementary random variables are required to realize the precise modeling of ground motion fields. For the sake of facilitating the calculation, the UWSRM embedded in the Fast Fourier Transform (FFT) technique is applied to simulate the stochastic ground motion fields. Numerical results including the comparison of the accuracy and efficiency with the CWSRM verify the effectiveness of the UWSRM. Moreover, to illustrate the application prospect of the UWSRM, the refined random seismic response analysis of a buried pipeline network is implemented by combining with the probability density evolution method (PDEM). The analysis results preliminarily demonstrate the applicability of the UWSRM in lifeline systems.

Multi-hazard earthquake-tsunami loss estimation of Kuroshio Town, Kochi Prefecture, Japan considering the Nankai-Tonankai megathrust rupture scenarios

The Nankai-Tonankai Trough is the primary source of major subduction earthquakes in western and central Japan, and the preparedness against future Nankai-Tonankai megathrust subduction events is of critical importance. This study develops a novel multi-hazard catastrophe model for the future Nankai-Tonankai megathrust subduction earthquake and tsunami based on a stochastic earthquake source modeling approach, and applies it to Kuroshio Town, Kochi Prefecture, Japan, that faces significant seismic-tsunami hazards due to the Nankai-Tonankai subduction earthquakes. The earthquake-tsunami catastrophe model incorporates the stochastic rupture sources, spatially correlated ground motion fields, tsunami inundation simulations, detailed building portfolio data, seismic and tsunami fragility models, and building damage cost estimation. It produces the multi-hazard and single-hazard loss distributions, accompanied by detailed earthquake rupture scenarios, shaking-tsunami hazard intensity distributions, and building damage distributions. The earthquake-tsunami loss estimation results for Kuroshio Town indicate that the tsunami loss tends to increase with the magnitude more significantly than the shaking loss and contributes more towards the increase in the multi-hazard total loss with the earthquake magnitude. Importanly, the new multi-hazard tool facilitates the identification of critical multi-hazard loss scenarios and leads to integrated hazard-risk maps that are particularly useful for disaster risk reduction and management purposes.

Effect of Linear Soil Condition on Seismic Inputs

Seismic inputs to structures in terms of risk consistent response spectrum and seismic hazard curves are developed at bedrock level considering ten independent seismic source zone in the vicinity of the Kathmandu valley. The seismic hazard curve is derived by assuming temporal occurrence of earthquakes to follow Poisson model. Response spectrum is developed using an empirical relationship of spectral ordinates with magnitude of earthquakes and epicentral distance. The seismic risk factor is introduced in response spectrum using conditional probabilities. Power spectral density function consistent with response spectrum is derived and ground acceleration time histories are derived from power spectral density function using Monte Carlo technique. To obtain free field hazard curves and ground motion parameters, one dimensional wave propagation analysis is used for two different underlying soil conditions.

Empirical fragility assessment using conditional GMPE-based ground shaking fields: application to damage data for 2016 Amatrice Earthquake

Recent earthquakes have exposed the vulnerability of existing buildings; this is demonstrated by damage incurred after moderate-to-high magnitude earthquakes. This stresses the need to exploit available data from different sources to develop reliable seismic risk components. As far as it regards empirical fragility assessment, accurate estimation of ground-shaking at the location of buildings of interest is as crucial as the accurate evaluation of observed damage for these buildings. This implies that explicit consideration of the uncertainties in the prediction of ground shaking leads to more robust empirical fragility curves. In such context, the simulation-based methods can be employed to provide fragility estimates that integrate over the space of plausible ground-shaking fields. These ground-shaking fields are generated according to the joint probability distribution of ground-shaking at the location of the buildings of interest considering the spatial correlation structure in the ground motion prediction residuals and updated based on the registered ground shaking data and observed damage. As an alternative to the embedded coefficients in the ground motion prediction equations accounting for subsoil categories, stratigraphic coefficients can be applied directly to the ground motion fields at the engineering bedrock level. Empirical fragility curves obtained using the observed damage in the aftermath of Amatrice Earthquake for residential masonry buildings show that explicit consideration of the uncertainty in the prediction of ground-shaking significantly affects the results.

Simulation of seismic ground motion fields via object-oriented spatial statistics with an application in Northern Italy

This work offers a novel methodological framework to address the problem of generating data-driven earthquake shaking fields at different vibration periods, which are key to support decision making and civil protection planning. We propose to analyse the entire profiles of spectral accelerations and project their information content to unsampled locations in the system, based on the theory of Object Oriented Spatial Statistics. The proposed methodology combines a non-ergodic ground motion model with a fully functional model for the residual term, the latter consisting of (i) the spatially-varying systematic effects due to source, site and path, and (ii) the remaining aleatory error. The proposed methodology allows to generate multiple shaking scenarios conditioned on the data, jointly and consistently for all the vibration periods, overcoming the intrinsic limitations of existing multivariate approaches to the problem. The approach is tested on a vast dataset of ground motion records collected in the study-area of the Po Plain (Northern Italy), for which a region-specific fully non-ergodic GMM was previously calibrated. Our validation tests demonstrate the potentiality of the approach, which is capable to effectively simulate spectral acceleration profiles, while keeping the ability to capture the main physical features of ground motion patterns in the region.

A novel framework for conditional simulation of fully nonstationary spatially varying ground motion field

SummaryExisting procedures for conditionally generating the spatially varying ground motion (SVGM) through the coherency‐based methods can be broadly categorized into two approaches: (i) cross‐spectral density (CSD)‐based procedures and (ii) auto‐spectral density (ASD)‐based procedures (reported by the authors elsewhere). CSD‐based procedures consider only the phase variability, whereas ASD‐based procedures account for phase as well as the amplitude variability. However, the ASD‐based framework and most of the CSD‐based frameworks simulate the stationary SVGM, and nonstationarity is imparted through the uniform modulation, ignoring the temporal variation of the frequency content. This paper proposes an evolutionary power spectral density (EPSD)‐based framework for the simulation of nonstationary SVGM. This framework is developed in two steps. First, a new methodology based on Hilbert transform is proposed to compute the nonstationary ground motion given the EPSD, modulating functions, and phase. Second, the ASD‐based framework reported by authors elsewhere is extended to account for the EPSD, and the associated nonstationary ground motion is simulated using the proposed methodology. Nonstationary ground motions thus simulated are assessed against SMART1 array recordings. The close resemblance between the simulated accelerogram and the recorded data indicates that the proposed EPSD based framework accounts not only for the phase and amplitude variabilities but also for the nonstationarity.

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.

Pile induced filtering of seismic ground motion in homogeneous soil

The foundation input motion (FIM) that a structure experiences during an earthquake, is known to be different from the free field ground motion due to soil structure interaction (SSI) effects. Kinematic interaction in a single pile can also introduce a rotational component to the FIM. Conventionally, soil structure interaction is performed by applying the free field ground motion to the structure ignoring the effects of kinematic interaction. Deep foundation elements such as piles are known to suppress certain frequencies of ground motion which in turn induces kinematic bending moments in them. In this study, kinematic soil pile interaction is simulated using 3D numerical models using a coupled finite element-boundary element method. Single pile, group pile and piled raft models in a homogeneous soil profile are analysed for vertically propagating shear waves. Three earthquake time histories with varying frequency content are considered in this study. Transfer functions are then plotted together to analyse the effects of pile induced filtering of ground motion. The ratio of response spectrum at the foundation level and free field ground, for the pile group considered, is found to closely follow the behaviour of a fixed headed single pile. It is found that embedment of the pile cap, as in the case of a piled raft can result in further filtering of ground motion.

Seismic fragility assessment of existing reinforced concrete buildings in Patna, India

Assessment of seismic fragility of reinforced concrete (RC) buildings requires appropriate consideration of uncertainty in aleatory and epistemic sources. Aleatory uncertainty is typically attributed to randomness in ground motion records, while epistemic is due to the uncertainties in structural modelling. The present work focuses on obtaining the variability functions and fragility curves by considering those uncertainties for low, mid and high rise RC buildings in Patna, India, which is a region of high seismicity.Modelling uncertainties requires consideration of material, geometrical, and design as random parameters. These parameters mainly comprise material strength, storey height, plan, typology, design provisions, and various irregularities. In this study, the selection of the random parameters is purely based on the investigation of buildings in the Patna region. Considering the multiple combinations of random parameters, the buildings are modelled and analysed using nonlinear static (NLS) pushover analysis. The monotonic load in a triangular pattern is applied for NLS analysis to get pushover curves. The pushover curves are subsequently converted into capacity curves considering the dynamic characteristic of the first mode of vibration. The obtained capacity curves are utilised to derive the variability in the capacity spectrum for different categories of RC buildings. Subsequently, variability due to randomness in ground motion records is determined by generating several artificial (comprising near field and far field) ground motions considering the seismotectonic map of the region.Thence, by accounting for both the uncertainties, lognormal variability functions are determined, and seismic fragility assessment is performed for different building categories in Patna, India. Furthermore, the seismic vulnerability of existing buildings can be determined based on the obtained fragility curve.

Seismic response analysis of the reef-seawater system under obliquely incident P and SV waves

Despite the increasing focus on the seismic safety of reef engineering sites, the current studies on this topic are still based on the hypotheses of uniform seismic excitations or the vertical incidence of seismic waves. In this study, a two-dimensional seismic analysis model for the reef-seawater system under obliquely incident P and SV waves is developed, which considers the fluid-soil-interaction and the wave radiation effects of the fluid and the solid domain. The oblique input of the seismic wave into the reef-seawater model is accomplished through the boundary substructure method. A series of parametric studies are conducted, the ground motion fields on the reef sites are discussed, and the effects of several fundamental factors on the seismic response are comprehensively studied. The results show that under most circumstances, the least favorable load case for the reef site seismic response is the vertical incidence of seismic waves. However, for the horizontal ground motion of the reef site under the P wave, the least favorable incident angle is approximately 30°. In addition, the largest horizontal seismic response usually appears near the downstream side of the reef flat, and the peak velocity and acceleration ratios can reach up to 4 in this area.

Study on Ground Motion Intensity Measures for Seismic Response Evaluation of Circular Tunnel

Ground motion intensity measure (IM) is a key link between the seismic hazard analysis and structural seismic response analysis, which play an important role in the performance-based earthquake engineering framework. In order to find suitable IM for seismic response evaluation of circular tunnel, nonlinear dynamic time-history analyses are conducted on a two-dimensional finite element model of tunnel-surrounding rock interaction in this study. The near field ground motions without velocity pulse and far field ground motions as recommended by FEMA-P695 are used as the input ground motions for the numerical model. Besides, the overall lining damage indices in compression and in tension are used as engineering damage measures (DMs) to estimate the lining damage state. The applicability of 20 existing ground motion IMs commonly used in engineering practice are investigated in this study. Statistically regression analysis is carried out for results obtained from nonlinear time-history analyses. Based on the criteria of efficiency and sufficiency, Arias intensity is identified as the optimal IM for seismic response prediction of circular tunnel structure.

A stochastic ground motion model for the urban environment

Nowadays stochastic ground motion models used for the seismic analysis and design of structures take into account the soil deposit only, disregarding the presence of existing buildings nearby. However, it is well known that ground motion in urban environment is modified by the presence of buildings, mainly due to the radiation energy emitted from a vibrating structure in the soil that alters the seismic free field motion. This study is a first attempt to propose a stochastic ground motion analytical model able to take into account the influence of the urban environment. A simplified discrete model is developed so to consider the influence of the radiated wave field into the free field ground motion. Comparison in terms power spectral density functions and peak ground acceleration determined from the proposed ground motion model and those determined through conventional approaches are carried out. Numerical results clearly show the efficiency of the proposed model to capture this complex phenomenon in the stochastic seismic analysis of structures by improving the accuracy of the estimation of the peak response of above 30% . Limits of the proposed formulation are also discussed.

Post-Earthquake Damage Assessment after the 6.5 Mw Earthquake on December, 7th 2016 in Pidie Jaya, Indonesia

ABSTRACT This paper presents a post-earthquake damage assessment of the 6.5 Mw earthquake in Pidie Jaya, Indonesia which occurred on December, 7th 2016. The earthquake killed 112 people and caused the collapse of about 3000 buildings. The assessment started on the day after the earthquake. The survey covered nine sub-districts of Pidie Jaya regency in the Province of Aceh, northern Sumatra. The post-event damage assessment was conducted by a team of researchers from Syiah Kuala University and local authorities. The damage survey assessed the damage levels of residential houses, school buildings and markets. The most common construction damages reported in this article are failures of reinforced concrete, confined masonry and timber structures. The ground motion field was modeled using Openquake application and the site response employed Vs30 values inferred from topography. This calculated ground motion was compared to data from post-earthquake field assessment reports. The highest percentage of heavy damage was recorded in the area of the fault rupture and along the shorelines area. The types of failure observed are summarized in the context of Indonesian construction practice, with particular reference to the seismic resilience of the Rumoh Aceh traditional wood construction.

Seismic response of concrete gravity dams under near field and far field ground motions

Near field ground motions trigger higher damage to the structures due to the presence of distinct velocity pulses which are absent in far field ground motions. The present study deals with the time history analysis of dam-reservoir-foundation coupled system using finite element technique under near field and far field ground motions. Koyna dam, which is a concrete gravity dam, is taken as case study for the present investigation. Six ground motions, recorded near to the ruptured fault are selected as near field ground motions. Whereas, six other ground motions, recorded at other stations located far away from fault plane, of the same earthquake events, are considered as far field ground motions. Along with the comparative study of seismic response of Koyna dam, qualitative assessment of the seismic performance of the dam are also evaluated. The results obtained from the analysis demonstrate the different impact of near field and far field ground motions on the seismic response of concrete gravity dams. Further, the study also ascertain the importance of selection of far field ground motions on seismic safety of concrete gravity dams.

POD-based dimension reduction representation of stochastic ground motion fields

In this paper, two kinds of proper orthogonal decomposition (POD) representations for simulating the non-stationary stochastic vector processes are derived based on the basic theory of POD, namely the orthogonal-random-variables-based POD representation and the random-phase-angles-based POD representation. It is clarified that the random-phase-angles-based scheme is a special case of the orthogonal-random-variables-based scheme, and the differences between them are analyzed from the number of elementary random variables and the constraint conditions. It is found that the randomness degree (the number of elementary random variables) of the non-stationary stochastic vector processes could be greatly reduced by introducing some appropriate constraint conditions. To this end, the efficient dimension reduction simulation of the non-stationary stochastic vector processes with just two elementary random variables is realized through introducing more rigorous constraints on the orthogonal random variables in the orthogonal-random-variables-based scheme. Also, the complete probability information of the stochastic processes is fully emerged. Through the numerical analysis of the ground motion stochastic field simulation, it is obtained that the relative errors of the mean and standard deviation, evolutionary power spectrum and coherence function of the non-stationary ground motion acceleration vector process are all in good agreement with the corresponding target values, which shows that the proposed method is effective and accurate. Meanwhile, in order to further verify the applicability of this method in terms of engineering characteristics upon ground motion, according to the five classes of the typical soil sites divided in “Seismic ground motion parameters zonation map of China”, 920 strong motion records are selected for comparing and analyzing the response spectrum and the Fourier amplitude spectrum of ground motion acceleration, respectively. The research shows that the simulation results of the proposed method have good consistency with the strong motion records, which lays a solid foundation for this proposed method applied in engineering applications.

A Hybrid Empirical Green’s Function Technique for Predicting Ground Motion from Induced Seismicity: Application to the Basel Enhanced Geothermal System

A method is described for the prediction of site-specific surface ground motion due to induced earthquakes occurring in predictable and well-defined source zones. The method is based on empirical Green’s functions (EGFs), determined using micro-earthquakes at sites where seismicity is being induced (e.g., hydraulic fracturing and wastewater injection during shale oil and gas extraction, CO2 sequestration, and conventional and enhanced geothermal injection). Using the EGF approach, a ground-motion field (e.g., an intensity map) can be calculated for a potentially felt induced event originating within the seismic zone. The approach allows site- and path-specific effects to be mapped into the ground-motion field, providing a local ground-motion model that accounts for wave-propagation effects without the requirement of 3D velocity models or extensive computational resources. As a test case, the ground-motion field for the mainshock (ML = 3.4, M = 3.2) resulting from the Basel Enhanced Geothermal System (EGS) was simulated using only seismicity recorded prior to the event. We focussed on peak ground velocity (PGV), as this is a measure of ground motion on which Swiss norms for vibration disturbances are based. The performance of the method was significantly better than a previously developed generic ground-motion prediction equation (GMPE) for induced earthquakes and showed improved performance through intrinsic inclusion of site-specific effects relative to predictions for a local GMPE. Both median motions and the site-to-site ground-motion variability were captured, leading to significantly reduced misfit relative to the generic GMPE. It was shown, however, that extrapolation beyond units of a couple of magnitude leads to significant uncertainty. The method is well suited to a real-time predictive hazard framework, for which shaking estimates are dynamically updated in light of newly recorded seismicity.

Seismic reliability evaluation of gas supply networks based on the probability density evolution method

An accurate and efficient reliability evaluation is necessary for network systems with uncertain conditions, especially for the lifeline networks subjected to earthquakes. This study takes gas supply networks as examples and establishes an approach to evaluate the seismic reliability of networks on the basis of the probability density evolution method. First, a nonlinear finite element model is established to simulate buried pipe networks that are subjected to earthquakes. Then, the seismic stresses of the pipes are derived, and the von Mises stresses of the pipes are calculated and used to judge pipe failure. Second, a connectivity index is defined to describe the connectivity between the source and the terminal. Third, on the basis of the probability density evolution method, network reliability is obtained after introducing a physically-based model to simulate ground motion field. Finally, two networks are used as examples to demonstrate the proposed approach, and the results are validated by the Monte Carlo simulation method and compared with a selective recursive decomposition algorithm.

Inferring Earthquake Ground‐Motion Fields with Bayesian Networks

Bayesian Networks (BNs) have the ability to perform inference on uncertain variables given evidence on observed quantities, which makes them relevant mathematical tools for the updating of ground-motion fields based on strong-motion records or macroseismic observations. Therefore the present article investigates the use of BN models of spatially correlated Gaussian random fields as an accurate and scalable method for the generation of ground-motion maps. The proposed BN model is based on continuous Gaussian variables, as opposed to discrete variables as in previous formulations, and it is built to account for cross-correlated ground-motion parameters as well as macroseismic observations. This approach is validated with respect to the analytical solution (i.e., conditional multivariate normal distributions) and it is also compared to the USGS ShakeMap method, thus demonstrating a better ability to model jointly the inter- and intra-event error terms of ground-motion models. The scalability of the approach, i.e. its capacity to be applied to large grids, is ensured by a grid sub-division strategy, which appears to be computationally efficient and accurate within an error rate of a fraction of percent. Finally, the BN implementation is demonstrated on a real-world example (the Mw 6.2 Kumamoto, Japan, 2016 foreshock), where vector-valued shake-maps of cross-correlated intensity measures are generated, along with the integration of macroseismic observations.

13.15: Seismic design of steel moment resisting frames: Modal strength reduction factors including strength deterioration and panel zone effects

ABSTRACTTraditional seismic design of framed structures uses a single constant value for the strength reduction factor. In this work, explicit expressions for different values of strength reduction factors for the first significant modes of steel moment resisting frames including strength deterioration and panel zone effects are developed. These factors are functions of modal periods and damage and are designed for four performance levels in a performance – based seismic design framework. The above factors are obtained through extensive parametric studies involving 20 steel frames and 100 far – field ground motions and following a two steps procedure: First an equivalent linear structure with the same mass and elastic stiffness of the original non – linear one is constructed based on equivalent modal damping ratios, which substitute the non‐linearities; then use of these equivalent modal damping ratios in conjunction with modal damping reduction factors leads to modal strength reduction factors. Using these factors, one can obtain the seismic design base shear of a frame through response spectrum analysis. Thus, a more accurate and rational seismic design method is established. Numerical examples are presented to illustrate the method and demonstrate its advantages against conventional seismic design methods.

From Source to Building Fragility: Post-Event Assessment of the 2013 M7.1 Bohol, Philippines, Earthquake

We use ground-motion simulations of the 2013 Bohol, Philippines, earthquake along with a new post-disaster exposure/damage database to constrain building fragility and vulnerability. The large number of damaged buildings (>70,000) and the wide spread of seismic intensities caused by this earthquake make it an ideal candidate for such a study. An extensive survey was conducted leading to a robust description of over 25,000 damaged and undamaged structures. Ground-motion fields were simulated using ground-motion prediction equations and stochastic modeling, and the estimated and observed values were compared. The finite source model used in the simulation was based on the analysis of aftershocks and SAR data. The ground motions were associated with the empirical database to derive fragility and vulnerability models. Results indicate that the pattern of damage is best captured in the stochastic simulation. Constraints were placed on seismic building fragility and vulnerability models, which can promote more effective implementation of construction regulations and practices.