Characterization Of Earthquake Ground Motion Research Articles

Variability of physics-based simulated ground motions in Thessaloniki urban area and its implications for seismic risk assessment

An accurate characterization of earthquake ground motion and its variability is crucial for seismic hazard and risk analysis of spatially distributed portfolios in urban areas. In this work, a 3D physics-based numerical approach, based on the high-performance spectral element code SPEED (http://speed.mox.polimi.it/), is adopted to generate ground shaking scenarios for strong earthquakes (moment magnitude MW=6.5–7) in the Thessaloniki area (Northern Greece). These simulations account for kinematic finite-fault rupture scenarios and a 3D seismic velocity including the two main geological structures present in the area (Thessaloniki and Mygdonia basins). The numerical model is successfully validated by comparing simulated motions, on the one hand, with the recordings of a real small-magnitude (MW4.4) earthquake and, on the other, with empirical Ground Motion Models for the historical MW6.5 1978 earthquake. The sensitivity of results to the velocity model, anelastic attenuation, and non-linear soil effects is evaluated. The variability of the ground motion intensity measures in Thessaloniki as a function of the finite-fault rupture realizations (causative fault, magnitude, hypocenter location) is explored to gain insight into its potential impact on seismic risk assessment in urban areas.

Seismic risk assessment at urban scale from 3D physics-based numerical modeling: the case of Thessaloniki

The main aim of this work is to present a prototype of seismic risk assessment study at urban scale which incorporates next generation tools for hazard assessment, with application to the city of Thessaloniki, Northern Greece. The key ingredient of the proposed approach is the characterization of earthquake ground motion by means of three-dimensional broadband physics-based numerical simulations, which explicitly account for the fault rupture, the propagation path in heterogeneous media and complex geological conditions. The seismic damage of contemporary reinforced concrete building stock of the city of Thessaloniki has been evaluated using the capacity spectrum method, for a scenario corresponding to the destructive historical earthquake of June 20th 1978 (MW 6.5). Although the vulnerability model considered in these analyses reflects the contemporary building stock and, hence, differs from the situation at the time of the earthquake, the soundness of our damage estimates has been successfully verified through the comparison with the post-1978 earthquake damage observations. Results of this study demonstrate that 3D physics-based simulations can provide a more accurate and detailed characterization of earthquake ground motion and of its spatial variability, as compared to standard empirical approaches, and can be effectively used to improve seismic risk studies for strategic urban areas.

Analytical case study on the seismic performance of a curved and skewed reinforced concrete bridge under vertical ground motion

Rational representation of seismic demand based on realistic characterization of earthquake ground motion is essential for seismic bridge design. Realization of the impact of vertical ground motion has led to an initiative to better characterize and simulate the seismic scenario for typical bridges in near-fault regions, such that structural design practices may improve. Current structural design provisions either do not account for effects of vertical ground motion or specify a simplistic equivalent-static-load based methodology. An assessment of the effect of vertical ground motions on a horizontally skewed and curved highway bridge is presented for a moderate-to-high seismic region. A numerical model of a skewed and curved, three-span bridge located in Tacoma, Washington is subjected to a suite of horizontal and vertical ground motions using non-linear time-history analysis. The ground motions selected are composed of a range of near-fault earthquake records with varying component characteristics such as site condition, fault distance, and vertical-to-horizontal acceleration ratios. The scenario represents a bridge with a short vibrational period in a moderate seismic zone where vertical ground motion effects may be applicable, yet are not considered by structural codes. The results of the numerical simulations depict a considerable impact from vertical ground motion to the substructure and superstructure, including behavior typically not documented in existing studies. The numerical simulations show amplification from the vertical component of ground motion in the demand on components of the substructure, particularly for vertical-to-horizontal peak ground accelerations exceeding one. The effects of vertical ground motion on skewed and curved bridges are also evaluated against a straight counterpart with comparable overall configuration. For structural designers, the results of this study indicate that excluding the vertical ground motion component from analyses may impose a larger margin of risk than previously perceived. This is particularly the case for moderate-to-high seismic regions where shorter period bridges may resonate with predominant frequencies of vertical ground motion.

Characterization of earthquake ground motion of multiple sequences

Multiple acceleration sequences of earthquake ground motions have been observed in many regions of the world. Such ground motions can cause large damage to the structures due to accumulation of inelastic deformation from the repeated sequences. The dynamic analysis of inelastic structures under repeated acceleration sequences generated from simulated and recorded accelerograms without sequences has been recently studied. However, the characteristics of recorded earthquake ground motions of multiple sequences have not been studied yet. This paper investigates the gross characteristics of earthquake records of multiple sequences from an engineering perspective. The definition of the effective number of acceleration sequences of the ground shaking is introduced. The implication of the acceleration sequences on the structural response and damage of inelastic structures is also studied. A set of sixty accelerograms is used to demonstrate the general properties of repeated acceleration sequences and to investigate the associated structural inelastic response.

Empirical Equations for the Prediction of the Significant, Bracketed, and Uniform Duration of Earthquake Ground Motion

Abstract The complete characterization of earthquake ground motion includes the length of the interval of strong shaking as well as the amplitude and frequency content of the time series. There are relatively few published equations available for the prediction of strong-motion duration from earthquakes, which may in part be a consequence of the fact that the duration of shaking has generally not been considered in structural engineering. Recognizing that there are many applications for which an estimate of the duration of ground motion is needed, this study presents new empirical predictive equations for a number of definitions of strong-motion duration using the records from the Next Generation of Attenuation (NGA) global database of accelerograms from shallow crustal earthquakes. The equations can be used to estimate ground-motion durations from shallow crustal earthquakes of magnitude between M w 4.8 and 7.9 at distances up to 100 km from the source.

Estimations of the S-wave velocity structures in Chia-Yi City, Taiwan, using the array records of microtremors

AbstractShear-wave velocities (VS) have been widely used for the site characterization of earthquake ground motion. We report here our investigation of theS-wave velocity structures of Chia-Yi City, Taiwan using the array records of microtremors at seven sites. The dispersion curves at these sites were first calculated using the F-K method proposed by Capon (1969); theS-wave velocity structures in Chia-Yi City were then estimated by employing the surface wave inversion technique (Herrmann, 1991). At frequencies lower than about 1 Hz, the propagation directions are concentrated in the northwest and southwest quadrants. The generation of these may be attributed to the ocean waves of the Taiwan Strait. The harder site (CBA) has higher phase velocities, while the softer sites (CWB, SHP and YRU) have lower phase velocities, especially at frequencies between 1 and 5 Hz. The shallow velocity structures (0–1,500 m) can be roughly divided into four to five layers. The depth of the alluvium gradually increases from east to west and from north to south.

Ground Motion Intensity Measures for Liquefaction Hazard Evaluation

The requirements of performance-based earthquake engineering place increasing importance on the optimal characterization of earthquake ground motions. With respect to liquefaction hazard evaluation, ground motions have historically been characterized by a combination of peak acceleration and earthquake magnitude, and more recently by Arias intensity. This paper introduces a new ground motion intensity measure, CAV5, and shows that excess pore pressure generation in potentially liquefiable soils is considerably more closely related to CAV5 than to other intensity measures, including peak acceleration and Arias intensity. CAV5 is shown to be an efficient, sufficient, and predictable intensity measure for rock motions used as input to liquefaction hazard evaluations. An attenuation relationship for CAV5 is presented and used in an example that illustrates the benefits of scaling bedrock motions to a particular value of CAV5, rather than to the historical intensity measures, for performance-based evaluation of liquefaction hazards.

Correlations between duration and number of effective cycles of earthquake ground motion

An important aspect to include in comprehensive characterization of earthquake ground motion is the duration of the shaking, which can be represented either by an interval of time during which the motion fulfils some specified criteria or else the number of effective cycles of motion. Several different definitions have been proposed for measuring both of these parameters and analysis of empirical predictive equations and a large strong-motion dataset show that durations calculated using different definitions are generally poorly correlated. Numbers of cycles of motion calculated using different definitions can show much better correlations in some cases. In nearly all cases the correlations between calculated durations and numbers of cycles are very poor, with the exception of uniform duration and number of cycles obtained using rainflow range counting, both using absolute rather than relative thresholds. The scatter in this correlation can be reduced by adding an appropriate measure of the period content of the motion as an explanatory variable.

Site‐dependent models of earthquake ground motion

AbstractRandom vibration analyses of structural systems subjected to seismic loading are dependent upon the characterization of earthquake ground motion as a stochastic process. The response of structural systems to earthquakes is dependent strongly on the local geological conditions, which should be incorporated into seismological models of ground motion. In the study presented herein, three previously developed ground‐motion models are adapted to incorporate site‐dependent characteristics. Records obtained from two recording stations in California are used as a basis for the ground‐motion models. Single‐degree‐of‐freedom (SDOF) oscillators are subjected to ensembles of accelerograms generated from these models, and both elastic and inelastic response are considered. Response statistics are compared to those generated by the analysis of structural response to ensembles of recorded motion from the two sites. The important features of the ground motion for effective reproduction of response statistics are identified, and observations are made on the sensitivity of specific response parameters to site‐dependent characteristics of the ground motion.