GENERATION OF SYNTHETIC EARTHQUAKE ACCELEROGRAMS USING SEISMOLOGICAL MODELLING: A REVIEW

A stochastically based seismological model is described in this paper and used to generate synthetic accelerograms that are considered representative of intraplate earthquake events recorded on rock. The model, whilst well publicised in the seismological literature, is discussed and reviewed in this paper from an engineering perspective. The key factors influencing the frequency properties of the Fourier spectrum of the earthquake ground motion are presented and the differences observed between regions such as between eastern and western USA axe discussed. A procedure to generate representative artificial accelerograms from the Fourier spectrum is described. Average response spectra derived from these synthetic accelerograms are then compared with a selection of response spectra derived from recorded accelerograms to test and substantiate the model. The application of the seismological modelling approach to regions outside North America and the implications of the Displacement Based prinicples on the future development of seismological modelling have been addressed and discussed in the paper.     

Generation of Multiple Earthquake Accelerograms Compatible with Spectrum Via the Wavelet Packet Transform and Stochastic Neural Networks

The principal purpose of this article is to present a novel methodology based on wavelet packet transform techniques and stochastic neural networks to generate more artificial earthquake accelerograms from available data, which are compatible with specified response spectra or the design spectra. The proposed method uses the decomposing capabilities of wavelet packet transform on earthquake accelerograms, and the learning abilities of stochastic neural network to expand the knowledge of the inverse mapping from response spectrum to coefficients of wavelet packet transform of earthquake accelerogram. This methodology results in a stochastic ensemble of wavelet packet transform coefficients of earthquake accelerograms and, they are used to the generate accelerograms applying the inverse wavelet packet transform. Finally, an interpretive example is presented which uses an ensemble of recorded accelerograms to train and test the neural network, aiming at the demonstration of the method effectiveness.     

Estimation of Strong Ground Motions in Southwest Western Australia with a Combined Green's Function and Stochastic Approach

As only a very limited number of earthquake strong ground motion records are available in southwest Western Australia (SWWA), it is difficult to derive a reliable and unbiased strong ground motion attenuation model based on these data. To overcome this, in this study a combined approach is used to simulate ground motions. First, the stochastic approach is used to simulate ground motion time histories at various epicentral distances from small earthquake events. Then, the Green's function method, with the stochastically simulated time histories as input, is used to generate large event ground motion time histories. Comparing the Fourier spectra of the simulated motions with the recorded motions of a ML6.2 event in Cadoux in June 1979 and a ML5.5 event in Meckering in January 1990, provides good evidence in support of this method. This approach is then used to simulate a series of ground motion time histories from earthquakes of varying magnitudes and distances. From the regression analyses of these simulated data, the attenuation relations of peak ground acceleration (PGA), peak ground velocity (PGV), and response spectrum of ground motions on rock site in SWWA are derived.     

Empirical Green's Functions Modified by Attenuation for Sources Located at Intermediate and Far Distances from the Original Source

We present a scheme to modify empirical Green's functions by attenuation considering: (1) geometrical spreading; (2) decay in high frequency; (3) regional attenuation; and (4) phase of the signal. The accelerograms computed with the proposed simulation method are compared, in time and frequency domains, with strong ground motions from subduction and intermediate-depth earthquakes recorded in Mexico. It is shown that this simple empirical Green's functions technique can synthesize both the shape and amplitude of the response spectra in the site, considering a postulated seismic source located at different distances from the original one.     

ON THE CHARACTERISATION OF THE PHASE SPECTRUM FOR STRONG MOTION SYNTHESIS

A new approach has been presented to characterise phase spectra for simulating realistic nonstationary characteristics in synthetic accelerograms. The phase characteristics of the recorded earthquake accelerograms have been studied for this purpose and it has been found that the phase curve/unwrapped phases exhibit a monotonic downward trend which allows the problem of phase characterisation to be cast as a constrained nonlinear programming problem. The phase spectrum is first characterised by matching mean and variance of the generated distribution of relative phases with those obtained from recorded motions. As a practical application, it is shown how phase spectra can be characterised for an ensemble of synthetic accelerograms so as to maximise the severity of sample realisations.     

Nonlinear Response of RC Framed Buildings with Isolation and Supplemental Damping at the Base Subjected to Near-Fault Earthquakes

The nonlinear seismic response of base-isolated framed buildings subjected to near-fault earthquakes is studied to analyze the effects of supplemental damping at the level of the isolation system, commonly adopted to avoid overly large isolators. A numerical investigation is carried out with reference to two- and multi-degree-of-freedom systems, representing medium-rise base-isolated framed buildings. Typical five-story reinforced concrete (RC) plane frames with full isolation are designed according to Eurocode 8 assuming ground types A (i.e., rock) and D (i.e., moderately soft soil) in a high-risk seismic region. The overall isolation system, made of in-parallel high-damping-laminated-rubber bearings (HDLRBs) and supplemental viscous dampers, is modeled by an equivalent viscoelastic linear model. A bilinear model idealizes the behavior of the frame members. Pulse-type artificial motions, artificially generated accelerograms (matching EC8 response spectrum for subsoil classes A or D) and real accelerograms (recorded on rock- and soil-site at near-fault zones) are considered. A supplemental viscous damping at the base is appropriate for controlling the isolator displacement, so avoiding overly large isolators; but it does not guarantee a better performance of the superstructure in all cases, in terms of structural and non structural damage, depending on the frequency content of the seismic input. Precautions should be taken with regard to near-fault earthquakes, particularly for base-isolated structures located on soil-site.     

Site Effects at Long Periods from Digital Strong Motion Records of the KiK-net,Japan

The high-quality digital records of the Japanese KiK-net were examined, with the aim of studying the influence of local site conditions on the displacement spectral ordinates at long periods. The attention was limited to those records for which the velocity profiles up to 100–200 m depth were known, and corresponding surface and borehole accelerograms were available. Based on the available records and with the support of 1D numerical simulations, different aspects that may have an influence on the amplification of long period spectral ordinates were studied, including the bedrock velocity profile, the site classification using V_s,30, and the earthquake magnitude.

Small amplification factors at long periods were found, ranging from 1 to 1.3, with median value from 1.05 to 1.1, for Eurocode 8 site classes B and C, respectively. Only for two records on soft soils (at the boundary between Eurocode 8 classes C and D), from small magnitude earthquakes, large amplification factors were obtained, up to about 4. A good correlation was found of the amplification levels with the response spectral ratio D(T₀)/D(10), where D(T₀) and D(10) are displacement spectral ordinates of the input signal at bedrock, at the fundamental period T₀ of the soil profile and at T = 10 s, respectively.     

EMPIRICAL EVALUATION OF SITE EFFECTS BY MEANS OF H/V SPECTRAL RATIOS AT THE LOCATIONS OF STRONG MOTION ACCELEROMETERS IN ISRAEL

Acceleration data from local and regional earthquakes is of prime importance in evaluating the seismic hazard. Consequently, strong motion accelerometers are currently installed at more than 60 locations in Israel. We have explored the possibility of site amplification effects at 10 sites where local earthquakes triggered strong motion accelerometers by integrating empirical and analytical estimations. Implementing H/V spectral ratio techniques using 15 accelerograms from nine earthquakes, 105 seismograms shear-wave records of 35 local and regional earthquakes and seismograms of microtremors were used in the empirical evaluations. The subsurface models were constructed by integrating available geological and geophysical information at the analysed site with empirically evaluated site response functions. Amplification effects of factor 3-6 are observed at various frequencies in the 0.8-6.0 Hz band. Through the analysis process it became evident that the instant availability of many useful time windows of microtremors provides systematic estimations of the fundamental resonance frequency of each site and their associated amplification levels, which are similar to those obtained from H/V spectral ratios of seismograms and accelerograms and to those inferred from the subsurface geology. Analytical transfer functions should be reviewed with respect to empirical site response evaluations. Estimations that are based on only one approach may be totally misleading.     

Relationships between Modified Mercalli Intensity and Engineering Ground-Motion of the Earthquakes in Persia

New empirical relationships between modified Mercalli intensity (MMI) and engineering ground acceleration (PGA) of the earthquakes were generated for the Iranian territory. MMI and PGA values were gathered from 16 large earthquakes. We derived two sets of relationships which predict MMI in terms of PGA; each developed using both horizontal and three components of accelerograms. The first set is independent of earthquake magnitude and epicentral distance. Better results were observed when magnitude and epicentral distance conditions were included into equations. They might be used for both rapid assessments of ground shaking and also mapping damage potential in Iran and neighboring area.     

THE EFFECTIVE DURATION OF EARTHQUAKE STRONG MOTION

The duration of strong ground shaking during earthquakes can play an important role in the response of foundation materials and structures, particularly when strength or stiffness degradation is encountered. A thorough seismic hazard assessment should therefore include an estimation of the expected duration of strong motion, which first requires criteria to define the part of an accelerogram considered to represent the duration of strong ground motion. Some 30 different definitions of strong motion duration are reviewed and classified into generic groups. Problems that arise with the use of these definitions for duration are highlighted. A new definition of duration is presented using a previously unexplored option which identifies the part of the record where the main energy is contained and constrains this strong shaking phase by absolute criteria. This new definition is shown to give consistently meaningful durations for strong earthquake accelerograms from an engineering viewpoint. The correlations between the new definition of duration and magnitude, soil conditions and distance are explored as a first step towards the development of predictive equations.     

Design Earthquake Ground Motion Prediction for Perth Metropolitan Area with Microtremor Measurements for Site Characterization

Perth is the largest city in Western Australia and home to three-quarters of the state's residents. In recent decades, there have been a lot of earthquake activities just east of Perth in an area known as the South-West Seismic Zone. Previous numerical results of site response analyses based on limited available geology information for PMA indicated that Perth Basin might amplify the bedrock motion by more than 10 times at some frequencies and at some sites. Hence, more detailed studies on site characterization and amplification are necessary. The microtremor method using spatial autocorrelation (SPAC) processing is a useful tool for gaining thickness and shear wave velocity (SWV) of sediments and has been adopted in many previous studies. In this study, the response spectrum of rock site corresponding to the 475-year return period for PMA is defined according to the probabilistic seismic hazard analysis (PSHA) based on the latest ground motion attenuation model of Southwest Western Australia. Site characterization in PMA is performed using two microtremor measurements, namely SPAC technique and H/V method. The clonal selection algorithm (CSA) is introduced to perform direct inversion of SPAC curves to determine the soil profiles of representative PMA sites investigated in this study. Using the simulated bedrock motion as input, the responses of the soil sites are estimated using numerical method based on the shear-wave velocity vs. depth profiles determined from the SPAC technique. The response spectrum of the earthquake ground motion on surface of each site is derived from the numerical results of the site response analysis, and compared with the respective design spectrum defined in the Australian Earthquake Loading Code. The comparison shows that the code spectra are conservative in the short period range, but may slightly underestimate the response spectrum at some long period range.     

SEISMIC HAZARD ASSESSMENT IN FLORENCE CITY ITALY

A seismic hazard analysis of Florence city was performed in the frame of a project concerning the dynamic behaviour of cable-stayed bridges. Both a probabilistic approach and a methodology based on the use of a local macroseismic catalogue were applied. A local catalogue was expressly compiled for this purpose, to collect the macroseismic intensities actually observed at the site as a result of past earthquakes. This sort of catalogue is an independent tool to verify the assumptions of the probabilistic approach (seismic zoning, earthquake recurrence relation, attenuation model), though it can supply results in terms of macroseismic intensity only and reflects the effective seismic history at the site, without taking into account any variability. The Cornell' methodology was used to assess probabilistic hazard in terms of macroseismic intensity, peak ground acceleration, peak ground velocity, and pseudovelocity uniform response spectra. The local catalogue points out level VII of the Mercalli-Cancani-Sieberg scale (MCS) as the maximum intensity historically observed in Florence. The probabilistic approach leads to the consideration of intensity VIII MCS as the maximum credible for the city. The probabilistic analysis in terms of ground motion was performed using attenuation relations estimated for alluvium sites, since the geology of Florence area is represented by fluvial and lacustrine deposits of various thickness. Peak ground acceleration values with 90% non exceedence probability in 50 and 500 years are respectively 145 and 219 cm/s's for a shallow alluvium site, and 95 and 157 cm/s's for a deep alluvium site; the corresponding peak ground velocity values for sites located on alluvium are 6.41 and 11.76 cm/s. Uniform response spectra are provided for shallow and deep alluvium sites, according to frequency-dependent attenuation relations estimated from strong Italian earthquakes.     

博物馆文物的防震保护研究（二）——设防地震动输入及结构地震动反应

通过对上海及邻近地区地震危害性分析,结合地震研究工作结果,根据上海博物馆文物防震保护的具体要求,研究确定了该博物馆文物防震保护的设防水准及地震动参数。场地地震动输入峰值加速度水准确定为50年超越概率3％,地震动反应谱采用上海市标准《建筑抗震设计规范》(DGJ08-9-92)中给出的地震影响系数曲线(阻尼比0.05),并将输入峰值加速度调整为180gal。研究采用了目前通用的拟合反应谱的方法,通过拟合选定的地震动反应标准谱,合成了6条人造地震波时程曲线,拟合精度控制在相对误差不大于5％;同时结合上海市《建筑抗震设计规范》(DGJ08-9-92)中所附的一条强震记录,共同作为上海博物馆结构地震反应分析的输入地震动。又根据上海博物馆建筑的结构特征分析,结合建筑结沟动力特征的实测结果,通过采用层模型结构时程分析计算方法,计算并获得了在遭遇设防地震时,上海博物馆建筑物每一层的地震动反应,表明上海博物馆建筑各楼层的地震动反应具有明显的楼层放大作用。对各楼层地震动反应的统计分析,给出了上海博物馆建筑物每一层的地震动参数,以此可作为今后文物地震危害性分析,及研究设计隔震装置等防震措施的依据。     

SEISMIC AMPLIFICATION AT A SOFT SOIL SITE WITH LIQUEFIABLE LAYER

It is well known that local soil conditions play a key role in the amplification of earthquake waves. In particular, a liquefiable shallow soil layer may produce a significant influence on ground motion during strong earthquakes. In this paper, the response of a liquefiable site during the 1995 Kobe earthquake is studied using vertical array records, with particular attention on the effects of nonlinear soil behaviour and liquefaction on the ground motion. Variations of the characteristics of the recorded ground motions are analysed using the spectral ratio technique, and the nonlinearity occurring in the shallow liquefied layer during earthquake is identified. A fully coupled, inelastic finite element analysis of the response of the array site is performed. The calculated stress-strain histories of soils and excess pore water pressures at different depths are presented, and their relations to the characteristics of the ground motions are addressed.     

Earthquakes and archaeology

This article examines the use of archaeological evidence for the assessment of historical earthquakes in the Eastern Mediterranean region and Middle East, long before the advent of modern seismology. We ask the questions when and where have large earthquakes happened in the past? How can this evidence contribute to our scientific understanding of earthquake activity? Is it possible on literary and archaeological grounds to distinguish between earthquake damage and damage from other causes? It is found that archaeological evidence for an earthquake is not always clear or unambiguous and that there is a need for collaboration between archaeologists, historians, geologists, engineering seismologists and workers in other disciplines, to evaluate the traces of earthquakes in excavations, both for understanding their effects at the site and for the information they can provide about the nature of the earthquake implicated.     

ARTIFICIAL ACCELEROGRAMS REPRESENTATIVE OF THE DAMAGE DISTRIBUTION ON A LOW-RISE SHEAR WALL

This study attempts to reproduce in an artificial way, at a given magnitude-distance couple, the statistical characteristics of damage caused by real accelerograms. The structure adopted is a low-rise shear wall, modelled as a nonlinear, one degree of freedom system with a degrading frequency as a function of a non-cumulative damage variable. Strong-motion records, contained in a large database, are characterised in terms of seismological and seismic parameters. Artificial accelerograms are generated from response spectra representative of real accelerograms belonging to different magnitude-distance zones. Although the mean damage is consistent, the low dispersion of damage caused by the artificial accelerograms with respect to that caused by the real accelerograms is highlighted. In order to reproduce the damage dispersion in addition to the mean damage, a generation method of representative artificial accelerograms is proposed. This method introduces the standard deviation drawn from attenuation relationships into a dispersion pre-process with respect to the regressed spectrum at a given magnitude-distance point. The method turns out to be capable of reproducing the statistical features of damage produced by real strong-motion records.     

SEGMENTAL CROSS-SPECTRUM AS A NEW TECHNIQUE IN SITE RESPONSE ESTIMATION USING SPECTRAL RATIO ANALYSIS

Different aspects of spectral analysis for site response evaluation are investigated in this study. The segmental cross-spectrum is proposed in spectral analysis of earthquake ground motions. The performance of segmental cross-spectrum in contrast with the conventional methods is investigated through the mathematical modelling, numerical analysis and application to earthquake data recorded at Chiba and Shinfuji downhole arrays in Japan. In analysis of earthquake data, the soil amplification function is identified using both uphole/downhole (U/D) and H/V spectral ratios. The advantage of seg-mental cross-spectrum is assessed by comparing identified amplification functions using different spectral methods and theoretical soil response. The reliability of site response estimations obtained by H/V spectral ratio using segmental cross- and Fourier spectra is also examined by means of cross-validation with the U/D spectral ratio of earthquake motion and theoretical soil response. Furthermore, the application of segmental cross-spectrum in nonlinear soil response is examined by comparing the amplification function of weak and strong motions for both methods. The results validate the advantage of segmental cross-spectrum in both linear and nonlinear soil response, particularly, when it used with H/V technique.     

SIMULATION OF THE FOURIER PHASE SPECTRUM FOR THE GENERATION OF SYNTHETIC ACCELEROGRAMS

This study presents the application of a new method for generating synthetic accelerograms based on statistical distributions for Fourier phase differences and Fourier amplitudes as functions of earthquake magnitude, hypocentral distance and site geology. Two important characteristics of the methodology are that it requires a small number of input parameters and that ground motion time histories can be simulated without any specific modulation function. Two areas with different tectonic patterns (North-Eastern and Central Italy) were selected for the application. The results of our analysis are reliable in the case of Central Italy because the data set is large and quite uniformly distributed, while for North-Eastern Italy our results should not be used for distances greater than 30 km.     

SYSTEM IDENTIFICATION OF LANDFILL SEISMIC RESPONSE

Assessment of landfill seismic response necessitates the availability of reliable dynamic material properties. During the past decade, geophysical surveys and computational studies have been conducted to investigate the seismic response of the Operating Industries, Inc. (OII) landfill in Southern California. In this paper, a survey and summary of available research results is presented. In addition, a set of Oil input-output seismic records during six earthquakes is thoroughly analysed. Spectral analyses are conducted to shed light on the landfill dynamic response characteristics. A simple shear beam model is found to be useful in modelling the landfill resonant behaviour. System identification techniques are employed to estimate the landfill stiffness and damping properties. These properties are defined by minimising the difference between computed and recorded acceleration response spectra at the landfill top. The identified stiffness properties are found to be near the lower bound of those documented through geophysical measure-ments. Identified damping of about 5% (at resonance) is within the range of earlier investigations. Comparisons of the computed and recorded accelerations show: (I) effectiveness of a linear viscous shear beam model in simulating the landfill dynamic behaviour, for the recorded small to moderate levels of dynamic excitation (up to 0.26 g peak lateral acceleration), and (ii) potential of the employed system identification procedure for analysis of input-output seismic motions.