Near-fault Motions Research Articles

Dynamic Ruptures on a Frictional Interface with Off-Fault Brittle Damage: Feedback Mechanisms and Effects on Slip and Near-Fault Motion

The spontaneous generation of brittle rock damage near and behind the tip of a propagating rupture can produce dynamic feedback mechanisms that modify significantly the rupture properties, seismic radiation, and generated fault zone structure. In this work, we study such feedback mechanisms for single rupture events and their consequences for earthquake physics and various possible observations. This is done through numerical simulations of in-plane dynamic ruptures on a frictional fault with bulk behavior governed by a brittle damage rheology that incorporates reduction of elastic moduli in off-fault yielding regions. The model simulations produce several features that modify key properties of the ruptures, local wave propagation, and fault zone damage. These include (1) dynamic generation of near-fault regions with lower elastic properties, (2) dynamic changes of normal stress on the fault, (3) rupture transition from crack-like to a detached pulse, (4) emergence of a rupture mode consisting of a train of pulses, (5) quasi-periodic modulation of slip rate on the fault, and (6) asymmetric near-fault ground motion with higher amplitude and longer duration on the side with reduced elastic moduli. The results can have significant implications to multiple topics ranging from rupture directivity and local amplification of seismic motion to near-fault tremor-like signals.

Effects of near-fault ground motions and equivalent pulses on Large Crossing Transmission Tower-line System

The focus of this paper is the seismic response of Large Crossing Transmission Tower-line System (LCTL) to near-fault ground motions, and whether simplified pulses are capable of representing the effects of the ground motion pulses present in near-fault ground motions on seismic response. The effects of forward-directivity pulses and fling-step pulses on the response of near-fault LCTL were assessed. Results showed that near-fault pulse-like ground motions impose a larger seismic response to LCTL compared to far-field ground motions. The response of LCTL to near-fault motions shows higher scatter than the response to far-field ground motions when correlated with simple intensity measures such as PGA. Moreover, the seismic responses increase with the pulse period of near-fault ground motions. The response of LCTL to the forward-directivity ground motions and fling-step ground motions were reproduced using two new equivalent pulse models. It is shown that the equivalent pulse models can capture the important response characteristics of the near-fault record. There are significant differences between the responses of LCTL subjected to forward-directivity ground motions and the responses subjected to fling-step ground motions. Finally, the tower-line coupling of LCTL subjected to near-fault pulse-like ground motion was investigated. Results show that the effect of tower-line coupling on seismic responses of LCTL to far-field ground motions is more obvious than the responses to near-fault ground motions.

Influence of Near-Fault Ground Motions on the Response of Base-Isolated Reinforced Concrete Buildings considering Seismic Pounding

Performance of base-isolated buildings subjected to near-fault ground motions containing long-period pulses is of increasing concern, because these ground motions have the potential to impose large seismic demands on structures. A review of previous studies on the performance evaluation of base-isolated reinforced concrete (RC) buildings under near-fault ground motions shows that these studies lack in the consideration of seismic pounding and the use of lower bound and upper bound values of isolator properties according to the current state of practice. Accordingly, in this study the performance of a typical four-story base-isolated RC building is evaluated using a three-dimensional nonlinear finite element model, considering bounding values of isolator properties, to investigate the influences of (i) pulse-like nature of near-fault ground motions and (ii) seismic pounding with retaining walls at the base. Two sets of ground motions containing 14 far-fault non-pulse-like ground motions and 14 near-fault pulse-like ground motions, representing the risk-targeted maximum considered earthquake (MCER), are used. It is found that the response indicators of the building under near-fault motions are significantly larger than those under far-fault motions. Analysis results reveal that the building response indicators are significantly increased due to seismic pounding. Nonetheless, if a bounding analysis is conducted, consideration of seismic pounding in the analysis does not have appreciable consequences on the prediction of damage to structural elements and drift-sensitive nonstructural components, while dramatic increase in floor accelerations due to pounding is critical for acceleration-sensitive nonstructural components.

Probabilistic sensitivity analysis of suspension bridges to near-fault ground motion

The sensitivities of a structural response due to variation of its design parameters are prerequisite in the majority of the algorithms used for fundamental problems in engineering as system uncertainties, identification and probabilistic assessments etc. The paper presents the concept of probabilistic sensitivity of suspension bridges with respect to near-fault ground motion. In near field earthquake ground motions, large amplitude spectral accelerations can occur at long periods where many suspension bridges have significant structural response modes. Two different types of suspension bridges, which are Bosporus and Humber bridges, are selected to investigate the near-fault ground motion effects on suspension bridges random response sensitivity analysis. The modulus of elasticity is selected as random design variable. Strong ground motion records of Kocaeli, Northridge and Erzincan earthquakes are selected for the analyses. The stochastic sensitivity displacements and internal forces are determined by using the stochastic sensitivity finite element method and Monte Carlo simulation method. The stochastic sensitivity displacements and responses obtained from the two different suspension bridges subjected to these near-fault strong-ground motions are compared with each other. It is seen from the results that near-fault ground motions have different impacts stochastic sensitivity responses of suspension bridges. The stochastic sensitivity information provides a deeper insight into the structural design and it can be used as a basis for decision-making.

Behaviour of liquid storage tanks with VCFPS under near-fault ground motions

Earthquake response of slender and broad liquid storage steel tanks isolated with variable curvature friction pendulum systems (VCFPSs) is investigated under near-fault motions. The tanks isolated with VCFPS are idealised with three-degrees-of-freedom associated with convective, impulsive and rigid masses. The frictional forces mobilised at the interface of the VCFPS are assumed to be velocity independent. The governing equations of motion of isolated tank are derived and solved in the incremental form using Newmark's method. For comparative study, the seismic response of liquid storage tanks with the VCFPSs is compared with that of same liquid storage tanks isolated using the friction pendulum systems (FPSs). The seismic response of isolated liquid storage tanks is also compared with that of the non-isolated tanks. Further, a parametric study is carried out to critically examine the behaviour of liquid storage tanks isolated with the VCFPSs. The important parameters considered are the friction coefficient of VCFPS, the fundamental period at the centre of the sliding surface of VCFPS and the tank aspect ratio. It is observed that under near-fault ground motions, the VCFPS is quite effective in controlling the seismic response, viz. the base shear, the sloshing displacement and the impulsive displacement, of liquid storage tanks.

Effects of moment magnitude, site conditions and closest distance on damping modification factors

Damping modification factors (DMF) are used to adjust response spectral values corresponding to damping 5% of critical to other damping levels. Ground motions recorded are orderly grouped according to moment magnitude, site conditions and closest distance. Near-fault motion records with closest distance closer than 10 km are not included in this paper. Based on the classification, the effects of the three seismological parameters on the median DMF are investigated. Consequently, the influence of site class reduces with increasing earthquake magnitude, and the effect of closest distance generally can be neglected with closest distance closer than 100 km except for rock sites. Except for soft soil sites, moment magnitude has a more significant effect than closest distance and site conditions, and the median DMF from acceleration spectra are most sensitive to seismological parameters. For soft soil sites, the median DMF only vary a little with moment magnitude and closest distance.

Investigation of effect of near-fault motions on substandard bridge structures

The objective of this study was to investigate the effects of near-fault ground motions on substandard bridge columns and piers. To accomplish these goals, several large scale reinforced concrete models were constructed and tested on a shake table using near- and far-field ground motion records. Because the input earthquakes for the test models had different characteristics, three different measures were used to evaluate the effect of the input earthquake. These measures are peak shake table acceleration, spectral acceleration at the fundamental period of the test specimens, and the specimen drift ratios. For each measure, force-displacement relationships, strains, curvatures, drift ratios, and visual damage were evaluated. Results showed that regardless of the measure of input or response, the near-fault record generally led to larger strains, curvatures, and drift ratios. Furthermore, residual displacements were small compared to those for columns meeting current seismic code requirements.

Seismic Performance of Benchmark Highway Bridge with Variable Friction Pendulum System

Dynamic response of a benchmark highway bridge isolated with variable friction pendulum system (VFPS) is investigated under six earthquake motions. The study is based on the finite-element model of the benchmark highway bridge in California. The seismic response of bridge isolated with VFPS is compared with the conventional friction pendulum system (FPS). A parametric study is carried out to study the effects of the maximum coefficient of friction, initial time period and isolation period of VFPS. The response of bridge is also compared with the corresponding uncontrolled case, with FPS, and that controlled by alternate sample control strategies. It is concluded that with the installation of VFPS, the seismic response of the bridge under near-fault motions can be controlled significantly. The VFPS is quite effective in reducing the peak response quantities of the bridge to a level comparable to / better than that of the sample controllers.

Near-Fault and Far-Field Strong Ground-Motion Simulation for Earthquake Engineering Applications Using the Specific Barrier Model

Codes for aseismic design may require use of recorded ground motions as input in dynamic analysis. When records are not available, motions must be simulated. The specific barrier model (SBM) is particularly useful in this context because (1) it provides the most complete, yet parsimonious, self-consistent description of the earthquake faulting processes that are responsible for the generation of high-frequency radiation; (2) it has been calibrated to earthquakes of three different tectonic regions; and (3) its key parameter, the barrier interval, is related to the duration of the near-fault pulses (NFP), the most damaging feature of near-fault motions. We carry out “blind” (i.e., using the minimum amount of a priori source information) simulations of strong motions of well-recorded earthquakes of magnitudes between 6.2–7.6. We assess the quality of fit of the simulated time histories to the recorded motions and show that the simulations exhibit close to zero bias over frequencies of 0.1–20 Hz for the data set used. This exercise illustrates that the method will provide earthquake motions that can be used with confidence in aseismic design.

Strength reduction factors for near-fault forward-directivity ground motions

The seismic design of structures located close to active faults must account for the special characteristics of ground motions in the near-fault region, in particular when these motions contain forward directivity (FD) effects. FD effects result in short duration motions with intense velocity pulses. In this study, the response of nonlinear-single-degree-of-freedom (SDOF) systems to FD motions is characterized by ductility and period-dependent strength reduction factors. This study compares reduction factors from two near-fault data sets, one containing 82 pulse-type motions with FD effects and the other containing 63 motions without FD effects. A statistical analysis was performed to determine if differences between reduction factors for the two sets were statistically significant. Results show that reduction factors for FD and non-FD near-fault motions are different, particularly in the range of periods of FD pulses ( T = 0.5 to 3 s). Strength reduction factors for FD motions were shown to be dependent on earthquake magnitude and, to a lesser degree, site conditions. This magnitude dependence was not observed in non-FD motions. These conclusions indicate that common design specifications based on non-pulse ground motions cannot be directly applied to pulse-type, FD motions.

The [formula omitted] Ölfus earthquake at 15:45 UTC on 29 May 2008 in South Iceland: ICEARRAY strong-motion recordings

At 15:45 UTC on 29 May 2008 a M w 6.3 earthquake occurred in the western part of the South Iceland Seismic Zone (SISZ). Preliminary results indicate that the first motion originated approximately 6.5 km east–southeast of the town of Hveragerdi on what aftershocks appear to identify as an almost 10 km long north–south trending fault. However, most aftershocks outlined another almost 20 km long north–south trending fault less than 2 km from the town of Hveragerdi, which happens to be the location of the recently deployed ICEARRAY, the first small-aperture strong-motion array in Iceland. The ICEARRAY produced high-quality recordings on 11 stations over an aperture of ∼ 1.9 km and a ∼ 50 m minimum interstation distance. The recordings are characterized by strong motion of short duration and high intensity, manifested by the geometric mean of horizontal peak ground acceleration ranging between 0.44 and 0.87 g. Moreover, a prominent long-period near-fault velocity pulse is observed both along the strike-normal and strike-parallel directions. Its apparently repetitive behavior may reflect the complex source effects of the two-fault system. The linear response spectra of the ICEARRAY data indicate that the long-period energy of the velocity pulse seen along the strike-normal direction is not present in the strike-parallel direction. However, the long-period spectral energy seen along the strike-parallel direction is most likely caused by permanent tectonic displacement. Along the strike-normal direction, the Eurocode 8 “Type 2” design spectrum combined with a design spectrum for near-fault motion with distinct pulses appears to capture well the overall spectral composition of the ICEARRAY response spectra. We believe that these globally unique ICEARRAY recordings are of importance to the fields of earthquake engineering and engineering seismology.

Explicit Directivity-Pulse Inclusion in Probabilistic Seismic Hazard Analysis

Probabilistic seismic hazard analysis (PSHA) is widely used to estimate the ground motion intensity that should be considered when assessing a structure's performance. Disaggregation of PSHA is often used to identify representative ground motions in terms of magnitude and distance for structural analysis. Forward directivity–induced velocity pulses, which may occur in near-fault (or near-source) motions, are known to cause relatively severe elastic and inelastic response in structures of certain periods. Here, the principles of PSHA are extended to incorporate the possible occurrence of a velocity pulse in a near-fault ground motion. For each magnitude and site-source geometry, the probability of occurrence of a pulse is considered along with the probability distribution of the pulse period given that a pulse does occur. A near-source “narrowband” attenuation law modification to predict ground motion spectral acceleration ( Sa) amplitude that takes advantage of this additional pulse period information is utilized. Further, disaggregation results provide the probability that a given level of ground motion intensity is caused by a pulse-like ground motion, as well as the conditional probability distribution of the pulse period associated with that ground motion. These extensions improve the accuracy of PSHA for sites located near faults, as well as provide a rational basis for selecting appropriate near-fault ground motions to be used in the dynamic analyses of a structure.

진동대 실험을 통한 원형 합성 기둥의 내진 성능 평가

합성 교각의 설계에서 요구 내진성능을 만족하기 위한 철근상세 규정이 명확하지 않은 측면이 있다. 합성 교각은 단면치수를 감소시키고 지진하중하에서 기둥의 연성을 개선하기 위해 제안되었다. 이 논문에서는 400mm 직경을 가진 단일 강재를 콘크리트에 매입한 합성기둥 부재를 5기 제작하여 합성기둥의 내진성능을 연구하였다. 진동대 실험과 유사동적 실험이 수행되었는데 근단층지반운동을 고려한 축소모형의 구조적 거동이 평가되었다. 실험 변수는 횡철근의 간격, 주철근의 겹침이음, 매입 강재 단면으로 설정하였다. 진동대 실험에 의해 평가된 변위연성도가 유사동적 실험에 비해 적게 나타났고 한정연성설계, 주철근의 겹침 이음 50%를 가진 부재가 기준 부재에 비해서 낮은 연성도를 보였다. 강재비는 극한강도에 영향을 미치고 겹침이음과 횡철근 비의 감소는 변위능력을 감소시켰다. 합성 교각의 상세에 따른 에너지 소산능력의 차이는 뚜렷하게 나타나지 않았다. For the design of composite bridge piers, detail requirements for the reinforcements is not clear to satisfy the required seismic performance. Composite bridge piers were suggested to reduce the sectional dimensions and to enhance the ductility of the columns under earthquake loadings. In this paper, five specimens of concrete encased composite columns of 400mm diameter with single core steel were fabricated to investigate the seismic performance of the composite columns. Shaking table tests and a Pseudo-Dynamic test were carried out and structural behavior of small-scaled models considering near-fault motions was evaluated. Test parameters were the pace of the transverse reinforcement, lap splice of longitudinal reinforcement and encased steel member sections. The displacement ductility from shaking table tests was lower than that from the pseudo-dynamic test. Limited ductile design and 50% lap splice of longitudinal reinforcement reduced the displacement ductility. Steel ratio showed significant effect on the ultimate strength. Lap splice and low transverse reinforcements reduced the displacement capacity. The energy dissipation capacity of composite columns did not show significant difference according to details.

Optimum lead–rubber isolation bearings for near-fault motions

Analytical seismic response of multi-storey buildings isolated by lead–rubber bearings (LRB) is investigated under near-fault motions. The superstructure is idealized as a linear shear type flexible building. The force–deformation behaviour of the LRB is modelled as bilinear with viscous damping. The governing equations of motion of the isolated structural system are derived and the response of the system to normal component of six recorded near-fault motions is evaluated by step-by-step numerical method. The variation of top floor absolute acceleration and bearing displacement of the isolated building is plotted under different system parameters such as superstructure flexibility, isolation period and bearing yield strength. The comparison of results indicated that for low bearing yield strength there is significant displacement in the bearing under near-fault motions. In addition, there also exists a particular value of the yield strength of the LRB for which the top floor absolute acceleration of the building attains the minimum value. Further, the optimum bearing yield strength is derived for different system parameters under near-fault motions. The criteria selected for optimality are the minimization of both the top floor acceleration and the bearing displacement. The optimum yield strength of the LRB is found to be in the range of 10%–15% of the total weight of the building under near-fault motions. In addition, the response of bridge seismically isolated by the LRB is also investigated and found that there exists a particular value of the bearing yield strength for which the pier base shear and deck acceleration attain the minimum value under near-fault motions.

Design spectra including effect of rupture directivity in near-fault region

In order to propose a seismic design spectrum that includes the effect of rupture directivity in the near-fault region, this study investigates the application of equivalent pulses to the parameter attenuation relationships developed for near-fault, forward-directivity motions. Near-fault ground motions are represented by equivalent pulses with different waveforms defined by a small number of parameters (peak acceleration, A, and velocity V; and pulse period, Tv). Dimensionless ratios between these parameters (e.g., ATv/V, VTv/D) and response spectral shapes and amplitudes are examined for different pulses to gain insight on their dependence on basic pulse waveforms. Ratios of ATv/V, VTv/D, and the ratio of pulse period to the period for peak spectral velocity (Tv-p) are utilized to quantify the difference between rock and soil sites for near-fault forward-directivity ground motions. The ATv/V ratio of recorded near-fault motions is substantially larger for rock sites than that for soil sites, while Tv-p/Tv ratios are smaller at rock sites than at soil sites. Furthermore, using simple pulses and available predictive relationships for the pulse parameters, a preliminary model for the design acceleration response spectra for the near-fault region that includes the dependence on magnitude, rupture distance, and local site conditions are developed.

Seismic Site Response for Near-Fault Forward Directivity Ground Motions

Forward directivity effects in the near-fault region produce pulse-type motions that differ significantly from ordinary ground motions that occur at greater distances from the causative fault. Current code site factors are based on empirical observations and analyses involving less intense nonpulse ordinary ground motions. Nonlinear site response analyses with bidirectional shaking are performed using representative site profiles to quantify seismic site response effects for intense near-fault motions resulting from forward directivity. Input rock motions are represented with simplified velocity pulses that characterize the amplitude and period of forward directivity motions. Results indicate that site response affects both the amplitude and period of forward directivity pulses, and hence, local site conditions should be considered when evaluating seismic designs in the near-fault region. Stiff soil sites tend to amplify the peak ground velocity and increase the period of pulse-type motions, particularly, when the period of the rock motion coincides with the degraded period of the site. Amplification is limited at soft soil sites by the dynamic strength of the weak soil, so attenuation occurs for intense input motions. This nonlinearity is not reflected in the site factors in current building codes. Guidance is provided for estimating the amplitude and pulse period for velocity pulses at soil sites.

Response of base-isolated liquid storage tanks to near-fault motions

Seismic response of the liquid storage tanks isolated by the elastomeric bearings and sliding systems is investigated under near-fault earthquake motions. The fault normal and parallel components of near-fault motion are applied in two horizontal directions of the tank. The continuous liquid mass of the tank is modeled as lumped masses known as sloshing mass, impulsive mass and rigid mass. The corresponding stiffness associated with these lumped masses has been worked out depending upon the properties of the tank wall and liquid mass. It is observed that the resultant response of the isolated tank is mainly governed by fault normal component with minor contribution from the fault parallel component. Further, a parametric study is also carried out to study the effects of important system parameters on the effectiveness of seismic isolation for liquid storage tanks. The various important parameters considered are: aspect ratio of tank, the period of isolation and the damping of isolation bearings. There exists an optimum value of isolation damping for which the base shear in the tank attains the minimum value under near-fault motion. The increase of damping beyond the optimum value will reduce the bearing and sloshing displacements but increases the base shear. A comparative performance of five isolation systems for liquid storage tanks is also studied under normal component of near-fault motion and found that the EDF type isolation system may be a better choice for design of isolated tank in near-fault locations. Finally, it is also observed that the satisfactory response can be obtained by analysing the base-isolated tanks under simple cycloidal pulse instead of complete acceleration history.

Assessment of current nonlinear static procedures for seismic evaluation of buildings

An essential and critical component of evolving performance-based design methodologies is the accurate estimation of seismic demand parameters. Nonlinear static procedures (NSPs) are now widely used in engineering practice to predict seismic demands in building structures. While seismic demands using NSPs can be computed directly from a site-specific hazard spectrum, nonlinear time-history (NTH) analyses require an ensemble of ground motions and an associated probabilistic assessment to account for aleatoric variability in earthquake recordings. Despite this advantage, simplified versions of NSP based on invariant load patterns such as those recommended in ATC-40 and FEMA-356 have well-documented limitations in terms of their inability to account for higher mode effects and the modal variations resulting from inelastic behavior. Consequently, a number of enhanced pushover procedures that overcome many of these drawbacks have also been proposed. This paper investigates the effectiveness of several NSPs in predicting the salient response characteristics of typical steel and reinforced concrete (RC) buildings through comparison with benchmark responses obtained from a comprehensive set of NTH analyses. More importantly, to consider diverse ground motion characteristics, an array of time-series from ordinary far-fault records to near-fault motions having fling and forward directivity effects was employed. Results from the analytical study indicate that the Adaptive Modal Combination procedure predicted peak response measures such as inter-story drift and component plastic rotations more consistently than the other NSPs investigated in the study.

Response Spectra for Multicomponent Structural Analysis

This paper aims to investigate the response spectra characteristics of the principal components of seismic motion, which are required for accurate multicomponent structural analysis. Mean spectra were determined for the three principal uncorrelated acceleration components for an ensemble of 97 earthquake records. The average inclination of the quasi-vertical component from the vertical axis is found to be 11.4°, with a standard deviation of 9.9°. The ratio of the minor and the major quasi-horizontal spectra varies between 0.63 and 0.81, which are lower than the value of 1 commonly used in design codes. Greater differences are found for near-fault motions in the intermediate-period range. The ratio of the quasi-vertical and the major quasi-horizontal spectra varies between 0.34 and 0.69 for far-fault and between 0.3 and 1.33 for near-fault motions, depending on vibration period. Smoothed spectra of the three principal components that can be used in modern multicomponent structural analysis methods are presented.

Characteristics of frequency content of near-fault ground motions during the Chi-Chi earthquake

The frequency content of earthquake ground motion is very important because it affects the dynamic response of structural systems. In the paper, we use five scalar parameters (the response spectral predominant period Tp, smoothed spectral predominant period To, Fourier amplitude spectral mean period Tm, equivalent pulse period Tv, and the pseudo-velocity spectral predominant period Tpv) reflecting the characteristics of frequency content of strong ground motion to examine the near-fault three-component motions during the 1999 Chi-Chi earthquake. The result indicates that the frequency content of near-fault motions at the Hanging wall is less than that at the foot wall; Tp shows a smaller value than that of To and Tm and it emerges a reverse relation of three-component motions as compared with that of To and Tm; Tv and Tpv of the near-fault motions at the north end of the rupture display a similar trend to that generated by the rupture directivity effect of strike-slip faulting. We therefore conclude that these observations are useful in the formulation of near-fault design spectra for seismic codes and in zoning studies in seismic risk.