Major earthquakes in Mexicali Valley, Mexico

Bulletin of the Seismological Society of America,Vol.94,No.6,pp.2186–2197,December2004Quantification of Hanging-Wall Effects on Ground Motion: Some Insights from the1999Chi-Chi Earthquakeby Tsui-Yu Chang,Fabrice Cotton,Yi-Ben Tsai,and Jacques Angelier Abstract Accelerometric records of the Chi-Chi earthquake from sites on the hanging wall exhibit larger acceleration than those from the footwall.Based on ground accelerations recorded at79near-field stations(10hanging-wall stations and 69footwall stations,respectively)and precise mapping of fault-rupture traces,the hanging-wall/footwall effects of the Chi-Chi earthquake have been fully studied.We show that the hanging-wall effects cannot be simply accounted for by a proper choice of distance metric.The closest distance to the rupture plane(D rup)is then selected to develop an empirical ground-motion model by using the data collected during the mainshock of the Chi-Chi earthquake that struck Taiwan.With the exception of some sites immediately next to the rupture traces(D rupՅ5km),the acceleration residuals between this empirical model and the recorded data at the footwall stations are closeto zero for stations in the distance range from5to50km.On the other hand,the average acceleration amplification on the hanging wall is equal to the natural loga-rithmic values of0.64ע0.4for all hanging-wall sites within20km of D rup.The hanging-wall/footwall effects have also been evaluated for several response spectral periods.It is observed that both the horizontal and vertical components of spectral acceleration are apparently amplified for sites on the hanging wall at a distance from 5to20km for spectral periods0.02to0.5sec,whereas the vertical component has less amplification than the horizontal in all the spectral periods considered.The horizontal component of spectral acceleration at the hanging-wall sites also shows a larger value for the long-period motion,relative to the footwall,for periods larger than1.0sec.The hanging-wall effects are relatively constant,at low frequencies,as the distance extends to about20km.This observation cannot be explained by the simplified empirical model.Rather,it suggests that waves trapped in the hanging-wall wedge may have been involved.IntroductionThe Chi-Chi earthquake of M W7.6,was the greatest thrust event in the twentieth century to strike the Taiwan area(Ma et al.,2001).Owing to strong shaking and exten-sive surface ruptures,more than100,000buildings were de-stroyed,and more than2500people lost their lives in this event.A large set of records has been acquired by one of the densest and most advanced seismic and geodetic networks in the world(e.g.,Shin et al.,2000;Yu et al.,2001).The high quality and quantity of these data have led to numerous studies on mechanical and kinematical characteristics of the earthquake process(e.g.,Chung and Shin,1999;Ma et al., 2001;Oglesby and Day,2001;C.Wu et al.,2001;Lin,2001; Zeng and Chen,2001;Wang et al.,2001;Yoshioka,2001; Dalguer et al.,2001).The accelerometric data of this earthquake have shown that ground motion on the eastern side of the Chelungpu fault,a major north–south-trending and east-dipping thrust fault whose rupture caused the Chi-Chi earthquake,exhib-ited dramatically high accelerations in comparison to the re-cords of footwall stations on the western side of the Che-lungpu fault(Fig.1).Most of the damaged buildings were located without exception at the hanging wall.Prior to a rigorous quantification of the hanging-wall effects,ground-motion differences between the hanging-wall and footwall sites can be qualitatively observed from the time series of raw accelerometric records.Figure2shows sample acceler-ograms of the hanging-wall and footwall records,respec-tively.It shows a clear,long-period dispersive Love-like mo-tion as waves propagating from the source to the footwall (station CHY101);however,the hanging-wall site(station TCU089)exhibits a steady high-frequency motion from20 (the triggered timing)to50sec.The obvious conclusion is2186Quantification of Hanging-Wall Effects on Ground Motion:Some Insights from the1999Chi-Chi Earthquake2187Figure1.Map of central Taiwan,showing the near-field recording stations of the Chi-Chi earth-quake used in this study,the epicenter,and the sur-face-rupture trace.Distribution of the peak ground ac-celeration(PGA)of the Chi-Chi’s mainshockis Figure2.Examples of the acceleration-time his-tory of the Chi-Chi earthquake,recorded on the hang-ing wall and footwall,respectively.that the footwall stations recorded long-period oscillations of surface waves that are not seen on recordings from the hanging-wall stations.Based on empirical approaches,recent studies have pro-posed that peak ground accelerations(PGA s)from thrust earthquakes can be20%–30%larger than from strike-slip earthquakes(e.g.,Campbell,1993;Sadighn et al.,1993; Abrahamson and Somerville,1993,1996;Oglesby et al., 1998,2000).It has also been shown that the hanging wall systematically exhibits amplified near-field ground motion in comparison to the footwall(e.g.,the1971San Fernando earthquake,the1980El Asnam,Algeria,earthquake,and the1994Northridge earthquake[Nason,1973;Ruegg et al., 1982;Oglesby et al.,1998,2000]).However,the hanging-wall effects have not been accurately quantified thus far be-cause of a lack of data recorded at hanging-wall stations. Thefirst quantitative studies on hanging-wall effects were done by Abrahamson et al.(e.g.,Abrahamson and Somer-ville,1993,1996;Somerville et al.,1996;Abrahamson and Silva,1997),whose primary data were from the records of the1994Northridge earthquake(Bommer et al.,2003). Their empirical model indicates a more than50%increase in peak horizontal accelerations on the hanging wall relative to the median attenuation at all sites over a distance of10 to20km.Their quantification of the hanging-wall effects was for buried faults.In the case of the Chi-Chi earthquake,detailed mapping of the surface-rupture traces byfield surveys immediately after the occurrence of the earthquake(Central Geological Survey of Taiwan,1999;Chen,2000),provides an excellent opportunity to conduct a quantitative study of the hanging-wall effects.With a good reconstruction of the geological setting in the fault zone by thefield surveys,the hanging-wall sites can be clearly defined as the stations on the eastern side of the Chelungpu fault.The empirical ground-motion-attenuation equation as a function of distance is deduced from several different distance metric definitions.Finally, the distance closest to the rupture is chosen to estimate the hanging-wall effects based on the Chi-Chi earthquake’s rec-ords.The amplification factor of the hanging-wall ground motion is determined by the residuals of this empirical ground-motion model.This amplification factor is also cal-culated in the spectral domain.Taking advantage of a large number of available records and the precise mapping of rup-ture traces acquired from the Chi-Chi earthquake,the present study aims at(1)better understanding the factors that may control the hanging-wall effects,(2)quantifying these ef-fects,and(3)discussing ways to take these effects into ac-count in ground-motion calculation.Fault Geometry and Ground-Motion DataThe epicenter of the Chi-Chi earthquake was located at the southern part of the Chelungpu fault,with a focal depthillustrated by multiple gray shadows.2188T.-Y.Chang,F.Cotton,Y.-B.Tsai,and J.AngelierFigure 3.Acceleration-attenuation relationsas three functions of geometrical-spreading distance.Scheme of source-to-site distance measures is illustrated at upper left.Seismo-genic depth is indicated by gray shadow.At lower left,three sites with zero value of D jb are supplemented in the plot of acceleration atten-uation with a pseudo-distance of 0.06km to better describe the PHA distribution tendency at the near field.See equation (1)for explana-tion of PHA .of about 7km (Figs.1and 3).This rupture reactivated a major thrust of the fold-and-thrust belt of Taiwan (for de-tailed reports about the Chi-Chi earthquake,see e.g.,Lee,et al.,1999;Shin and Teng,2001).The near-field area of this earthquake comprises the Foothills and Coastal Plain regions of central Taiwan.According to geological surveys and focal-mechanism solutions,the fault plane of this reactivated rup-ture can be regarded as a simplified plane delineated by the surface fault traces with an approximate strike of N5ЊE and a dip of 29Њto the east.The surface trace of this fault rupture has been relocated by means of mobile GPS instruments after the Chi-Chi earthquake with a precision of about 1m (Cen-tral Geological Survey of Taiwan,1999).In this study the ground-acceleration data were obtained from an extensive seismic instrumentation program com-pleted in 1996by the Seismology Center of the Central Weather Bureau (TSMIP/CWB )(Liu et al.,1999;Lee et al.,1999;Shin et al.,2000).Excluding a few stations in lique-faction areas,a total of 69stations have been selected on the footwall side,and 10stations on the hanging-wall side (Fig.1;Table 1).Ground Motion in Different Distance–MetricDefinitionsGround-motion-attenuation relations used in engineer-ing seismology provide simple models in terms of a limited number of relevant parameters,such as magnitude,distance,and site conditions.Some of the variations in the derived attenuation relations are,in fact,due to difference in dis-tance–metric definition for various geometrical spreading models (epicentral,hypocentral,closest rupture distance,distance to seismogenic part of the rupture plane,closest horizontal projection to the vertical projection of the rupture,etc.)(Abrahamson and Shedlock,1997;Scherbaum et al.,2003).On the basis of simple geometrical considerations,sites located above the fault rupture are expected to show larger ground motion than sites at the same rupture distance located on the footwall,because the hanging-wall sites are closer to a larger area of the inclined fault plane than the footwall site.This geometrical effect is usually thought to be a primary factor for causing the hanging-wall effects.It is therefore essential to determine which one of the various distance–metric definitions will better account for the hanging-wall effects.One popular distance–metric definition in earthquake engineering is the Joyner–Boore distance (D jb ).The D jb dis-tance is defined as the distance to the vertical projection of the fault surface to the Earth’s surface.Thus,D jb is zero when the site is directly over the ruptured portion of a fault.Another definition is D rup ,the nearest distance between the station and rupture surface.Similar to D rup ,D seis is the near-est distance to a location on the fault believed to be strong enough to generate strong motion,which only considers parts of the fault that are deeper than a specified depth (gen-erally 3–5km)when determining the nearest distance to the site.Taking advantage of a large number of acceleration records at the near field of the Chi-Chi earthquake and the well-reconstructed fault geometry,we compare the observed peak ground acceleration (PGA )with respect to these differ-Quantification of Hanging-Wall Effects on Ground Motion:Some Insights from the1999Chi-Chi Earthquake2189Table1List of Accelerometric Stations Used in This StudyHanging-Wall SitesPeak Ground Acceleration(cm/sec2)Station Code V NS EW Lat.(Њ)Long.(Њ) TCU05219443934924.198120.74 TCU06851936250224.278120.7657 TCU07141663951823.986120.7882 TCU07817130244023.8123120.8455 TCU08919022534823.9037120.8565 TCU07227537146524.0392120.8577 TCU07938441758023.8397120.8942 TCU08431242398923.883120.8998 TCU07427036858623.9607120.9617 CHY08071684279223.5972120.6777Footwall SitesPeak GroundAcceleration(cm/sec2)Station Code V NS EW Lat.(Њ)Long.(Њ) CHY0029613510823.7197120.413 CHY02414116227623.7577120.6065 CHY02517015215923.78120.5142 CHY02670667623.7988120.4115 CHY027********.7523120.247 CHY02833675062423.6327120.6055 CHY02915823328323.6143120.5288 CHY0763*******.638120.2217 CHY0827*******.7247120.2995 CHY0921128310223.792120.4782 CHY0933*******.6525120.1478 CHY09441536423.7935120.322 CHY10116239033323.6862120.5622 CHY10413017714323.6698120.4657 TCU0296219415524.5588120.749 TCU0316512311324.562120.7008 TCU0366112213424.4488120.6963 TCU0386614314224.4912120.663 TCU03912213619324.4922120.7838 TCU0407912215924.4502120.635 TCU0428220824824.5547120.8077 TCU0469711614024.4687120.854 TCU0489717611724.1807120.5928 TCU04917824227324.1795120.6902 TCU0508712814324.182120.6328 TCU05111023115724.1608120.6518 TCU05312113222524.194120.6693 TCU05413319014324.1615120.675 TCU05515320825724.1397120.6642 TCU05611714015424.1592120.6238 TCU0578110011124.178120.6105 TCU0596416215724.2688120.5637 TCU0608610119724.225120.6438 TCU0618615413324.1363120.5493 TCU06313313017924.1085120.6155 TCU0648211410924.346120.6095 TCU06723131348924.0917120.7197 TCU0707615724924.1955120.5472 TCU07522425732523.9835120.6778 TCU07627542034023.9078120.6757TCU0879111211924.3482120.7733 TCU1008411110824.186120.6152 TCU10116425420824.2417120.709 TCU10217316929824.2492120.7208 TCU10314214912724.3103120.7158 TCU104908710124.2097120.6017 TCU1056112411124.2387120.5595 TCU10611612215724.0855120.549 TCU1079514412824.2638120.5387 TCU10913315914924.0845120.571 TCU111779412524.1137120.487 TCU11265687824.056120.4235 TCU11380727023.8925120.3863 TCU115761159423.9598120.4697 TCU11611913318523.8568120.5802 TCU1179011312124.133120.4603 TCU1181009211624.0028120.4233 TCU11960536323.924120.3118 TCU12016719422323.98120.613 TCU12223625620723.8127120.6095 TCU1238613214924.0177120.5427 TCU1289016314124.4162120.7607 TCU12933561198323.8783120.6843 TCU13611217116724.2602120.6507 TCU13811020720223.9223120.595 TCU14068537123.9575120.3592 TCU141107898723.8335120.464 TCU145526070023.9418120.337 V,vertical;NS,north–south;EW,east–west.ent distance metrics in order to analyze the hanging-wall effects.As afirst step,these distance–metric definitions have been calculated for each of the accelerographic stations.Fig-ure3shows schematically the different distance–metric def-initions for the case of the Chi-Chi earthquake.In Figure3, it is shown that for all distance definitions considered,the motions recorded on the hanging wall are larger in general than the median estimates calculated with the whole dataset. This hanging-wall amplification effect is observed regard-less of the distance–metric definitions within a distance of 15km.In addition,a near-field saturation is also evident at very short distances(less than5km)with the models,using D jb and D rup.In other words,the footwall acceleration achieves a maximum value at a distance within5km of the fault plane,whereas the hanging-wall sites exhibit high ac-celeration at greater distances.In afirst approximation,the hanging-wall effects cannot be fully accounted for by a proper choice of distance–metric definition of the ground-motion model(Fig.3).In terms of high ground motion at nearfield,site effect is considered an important factor that may largely amplify acceleration;it is therefore necessary to examine the site condition for the studied area to clarify the physical cause of the hanging-wall effect.Site Condition at Near-Field SitesIn the case of the Chi-Chi earthquake,most of the hanging-wall stations are located on conglomerates that be-TCU08212918322124.1475120.6762190T.-Y.Chang,F.Cotton,Y.-B.Tsai,and J.AngelierTable 2Coefficients for use in Equation (1)to Estimate Pseudo-Acceleration for the Horizontal Mean of the Chi-Chi EarthquakeNear-Field Ground MotionModelsabcr ar br cr ln PHAD rup 2.668מ1.33613.919 2.7900.64712.6000.425D jb 1.313מ1.0197.975 1.5130.377 6.8240.394D seis 0.682מ0.861מ0.2770.5860.162 2.1550.360long to various fluvial terraces.This corresponds to Site Class C in the 1997Uniform Building Code (UBC )classi-fication scheme by the International Conference of Building Officials (1997).The footwall sites are located on Quater-nary sediments,which belong to Site Classes D and E ac-cording to the same classification.However,as shown by Y.M.Wu et al.(2001),the PGA site-correction factor de-rived from the observed and predicted values is between 1.25and 4at the Chi-Chi earthquake footwall sites and is between 0.8and 1.25at the hanging-wall sites.According to these results,the near-field ground motion should be especially amplified at the footwall sites,if we take site condition as a main factor for acceleration amplification.The nonlinear site effects of soil were not exceptionally reported at the western footwall area of Class E in the 1997UBC classification scheme;peaks and corresponding ratios of amplification observed at such sedimentary sites were markedly different from those at other sites (Wang et al.,2002).This phenomenon corresponds with observations of some footwall sites where greater ground motion occurred than the elastic–linear prediction called for at intermediate distances.Somewhat remarkable is that two hanging-wall stations (TCU052and TCU068)exhibited low PGA at a lo-cation very close to the northern extremity of surface rup-ture.Nonlinear site effects can occur in some cases for short periods on Class C (firm)soil;these effects have relatively low amplitudes at large magnitudes and short distances (Campbell and Bozorgnia,2003).However,if we take into account the movement character of this event,high velocity and long rise time at the northern tip of the Chelungpu fault (Zeng and Chen,2001),we tend to address the low PGA at these two stations by the particular thrusting process of the Chelungpu fault rather than by the nonlinear site effects.Recent studies show there is no definitive site classifi-cation for the TSMIP/CWB stations so far;even so,we can assess the possible effect by a background knowledge of the geology.C.T.Lee et al.(2001)examined the site condition of TSMIP/CWB stations through multiple approaches:study of geological maps,response spectral shape,and horizontal-to-vertical spectral ratio.They found that the classification of the TSMIP/CWB sites is consistent with the distribution of major geological features in Taiwan as a first approximation.However,they recognized that this consistency failed for a few sites that were either on the edge of specific structures or above severely weathered layers.After re-examining in situ the geomorphologic setting of the stations,they con-cluded that the site condition for the hanging wall of the Chi-Chi earthquake corresponds to Class D of the 1997UBC classification scheme,and corresponds to Classes C,D,and E for the footwall (C.T.Lee et al.,2001).This limited con-trast in site conditions between the hanging wall and the footwall shows that site effects are probably not the main factors that could explain the observed differences between hanging-wall and footwall ground motions.Accordingly,neither a distance-dependent attenuation definition nor an amplification generated from site condi-tions can give a good explanation of the cause of the large ground motion at the hanging wall in the Chi-Chi earth-quake.In the following sections,we extend a series of nu-merical analyses to quantify the hanging-wall amplification factor in this earthquake,which can provide reference for the related study.Regression ModelCompared to the hanging-wall amplification of ground acceleration,the footwall sites,which lie on Quaternary sed-iments,exhibit a distance-dependence feature at the western side of the Chelungpu fault:the PGA amplitude gradually diminishes with increasing distance from the rupture trace of this fault (Fig.1).This pattern provides a clear image of the seismic attenuation in the Chi-Chi earthquake area.We evaluate ground-motion attenuation without reference to site conditions.The ground-motion-estimation equation is given by:ln PHA סa םb ln(d םc ),(1)where PHA is the geometrical mean of the amplitudes of two horizontal components in g ;d is geometrical spreading dis-tance in kilometers.The coefficients to be determined are a ,b ,and c .Here we determine these coefficient values by min-imizing the sum of square errors with respect to the coeffi-cient c ,and a simple numerical search (Press et al.,1992).The corresponding coefficients,using the three different distance–metric models previously mentioned,are listed in Table 2.The resulting regression curves are shown in Figure 3for the three different distance models.It indicates that the hanging-wall effect that causes amplification by an averaged factor of 3appears in all models (D jb ,D rup ,and D seis ).Hanging-Wall and Footwall EffectsWe now choose to examine the empirical ground-motion model with the distance definition D rup in our anal-ysis of the hanging-wall effects.D rup was also the distance–metric adopted in the ground-motion study of Abrahamson and Silva (1997),which allows us to compare their model with the acceleration records of the Chi-Chi earthquake.The residuals from the attenuation equation are pre-sented in Figure 4for the hanging-wall and footwall sites,respectively.Similar to the illustration in Abrahamson andQuantification of Hanging-Wall Effects on Ground Motion:Some Insights from the 1999Chi-Chi Earthquake2191Figure 4.Comparisons of hanging-wall and foot-wall effects on PHA residuals from the Chi-Chi earth-quake.Residuals plotted as functions of distance for all the sites presented in Figure 1.Hanging-wall sta-tion codes are indicated.Somerville’s article (1996),the footwall sites are presented in negative distances to distinguish them from the hanging-wall sites.Figure 4shows the maximum residual,ln(PHA obs /PHA pred ),derived both from the hanging-wall and footwall sites.The maximum residual on the hanging wall attained 1.0.It is worth mentioning that the PHA residuals derived from the hanging-wall sites cannot be described as functions of distance.This implies a complexity of seismic-wave vi-bration between the fault-plane/density-contrast layer and the half-space surface on the hanging wall.In addition,two hanging-wall stations located at the northern tip of the wall show a relatively low acceleration residual in comparison to the other hanging-wall sites (TCU052and TCU068in Fig.4).These two stations were reported to exhibit a long rise time in the rupture process,which resulted in a low dynamic stress drop and moderate PGAs (Zeng and Chen,2001).On the footwall,except for some sites very close to the rupture trace (D rup Յ5km),the residuals are close to zero for distances from 5to 50km.The very small residuals for the footwall sites shown in Figure 4reveal that ground mo-tion on the footwall can be perfectly characterized by a sim-ple distance-dependent empirical model.The average of the PHA residuals for the hanging-wall accelerations is equal to 0.64ע0.4for the sites used in Figure 4.This value is similar to the determination derived from other dipping faults (0.67ע0.14,according to Abra-hamson and Somerville,1996).On the footwall the corre-sponding average residual within the stations over the same distance range (5–20km)is מ0.08ע0.1(Fig.4).Amplification Factors for Different Spectral PeriodsThe same analysis has been applied to several response-spectra periods for the horizontal and vertical components.The response spectra at 5%damping are computed by using the Nigam and Jennings (1969)algorithm for all the stations that were used.Plots of the residuals for the spectral accel-eration,ln(SA obs /SA pred ),at periods of 0.02to 4.0sec are shown in Figure 5for the horizontal and vertical compo-nents.For each motion-period bin selected in this article,the predicted attenuation model was constructed by the corre-sponding spectral acceleration derived from both hanging-wall and footwall sites,similar to the one that is shown in Figure 4.It is observed that both the horizontal and vertical components are apparently amplified from 5to 20km at the hanging-wall sites for periods 0.02to 0.5sec.However,the amplification factor in the vertical component is smaller than the one of the horizontal component for all periods.For pe-riods longer than 0.8sec,the hanging-wall effect apparently tends to be zero for the vertical component.We further take into consideration the spatial variation of the spectral attenuation on the hanging wall.In Figure 5the numbers,in increasing sequence,indicate the station dis-tribution from north to south (numbers from 1to 10).This labeling helps in classifying the hanging-wall stations ac-cording to their location around a north–south direction.Thegoal of this labeling is to detect directivity effects that might affect the stations north of the rupture.However,the data analysis (Fig.5)does not reveal any clear dependency on this north–south classification.Only stations 1and 2(TCU052and TCU068),located close to the rupture,re-vealed smaller residuals both in the horizontal and vertical components,almost in all periods.In the vertical component these northern stations exhibit a low attenuation rate for the long-period motions (say,between 0.8and 4.0sec),which may be inferred from the long rise time and a strong thrust-ing movement at the northern tip of the Chelungpu fault (Zeng and Chen,2001).We summarize the averaged amplification factors for different periods in Figure 6.This figure shows that the hanging-wall effect affects both the horizontal and the ver-tical components,especially for the short-period motions,whereas the horizontal acceleration is more amplified than the vertical at all periods.The largest amplification is ob-served at periods shorter than 1.0sec.For spectral periods larger than 1.0sec,the vertical component of ground motion at the hanging-wall sites is not significantly amplified,whereas the horizontal component shows an average ampli-fication of about 2.11to 4.0sec.Comparison with Empirical Prediction forHanging-Wall EffectAbrahamson and Silva (1997)proposed an empirical seismic-attenuation model,which takes into account the es-timation of the hanging-wall effect derived by Abrahamson and Somerville (1996).We compared the Chi-Chi earth-quake hanging-wall records with their ground-motion mod-2192T.-Y.Chang,F.Cotton,Y.-B.Tsai,and J.AngelierFigure5.Residuals of the spectral accelerations for the sites on the hanging wall(solid circles)and the footwall(open circles).Various periods for(a)the horizontal component and(continued)Quantification of Hanging-Wall Effects on Ground Motion:Some Insights from the1999Chi-Chi Earthquake2193Figure5.(continued)(b)the vertical component are analyzed.Number sequence accords with thestation distribution from north to south,as follows:1,TCU068;2,TCU052;3,TCU072;4,TCU071;5,TCU074;6,TCU089;7,TCU084;8,TCU079;9,TCU078;10,CHY080.Figure6.Summary of the spectral-accelerationresidual as function of oscillator period.els with and without the hanging-wall effect.Figure7a il-lustrates the predicted curves with and without considering the hanging-wall effect derived from four selected stations with distinct distance bins of0.4,5,10.6,and16.2km.The figure shows that the hanging-wall effect becomes less im-portant for sites very close to the rupture plane(Ͻ5km). Figure7b shows two sets of comparisons for all the hanging-wall records,with a trend line connecting the mean value obtained for each period.According to Figure7b,the Abrahamson and Silva (1997)ground-motion model(with or without the hanging-wall factor)tends to underestimate the Chi-Chi earthquake’s long-period ground motions.The long-period hanging-wall amplification remains significant for periods longer than1.0 sec in the case of this earthquake,whereas the corresponding coefficient(a9of equation9in Abrahamson and Silva’s1997 model)of the hanging-wall effect decreases to zero over the period range of0.75to5.0sec.The logarithmic ratio of observed/predicted spectral ac-celeration is about zero for spectral periods shorter than1.0 sec in both predictions with and without consideration of hanging-wall effect.Figure7b also indicates a low ground-motion amplitude for the0.2-sec period of the earthquake’s hanging wall compared to the motion predicted by Abra-hamson and Silva(1997).This discrepancy between the ob-servations and the Abrahamson and Silva(1997)model may be considered as natural earthquake-to-earthquake variation. This discrepancy could also indicate that there is a need to increase the number of hanging-wall records in strong-motion databases used to derive empirical ground-motion models(especially for large earthquakes with surface rup-tures).Characteristics of Near-Field Ground MotionThe foregoing results show that the hanging-wall effect mainly concerns periods of4.0sec or less.In order to im-prove our comprehension of near-field ground motion,weexamine the response spectra in six distance bins.To exploredistribution trends,we only discuss the geometrical averagesof spectrum amplitude,which correspond to different spec-tral periods in each distance bin.Average spectra are plottedagainst period in Figure8for oscillator periods in the range0.02to5.0sec.In thefirst approximation,the mean spectrum on thehanging wall is similar for the sites within distances of about5–10km and10–20km,whereas the band of periods withhigh spectral amplitude is shifted toward longer periods forthe larger-distance bin(Fig.8).It is observed that the short-distance group of hanging-wall sites has a strong spectralacceleration at a short period,whereas the high-frequencyoscillator motion is attenuated for sites at larger distances.It is remarkable that the spectral amplitudes of the fault-zonesites,including the near-field sites(D rupՅ5km)and the hanging-wall stations(TCU068and TCU052),are not aslarge as those of other hanging-wall sites,although thesenear-field sites have comparable PHA.On the other hand,thefootwall sites with a distance larger than5km show a dra-matic diminution of ground-motion energy for all periodsconsidered.The peaks observed on the hanging wall do notappear on footwall spectra at distances larger than5km.Thisagain confirms the asymmetry of seismic energy across thefault geometry(Oglesby et al.,1998,2000;Oglesby andDay,2001).In addition,the rapid decrease of spectral accelerationfrom the hanging wall to the footwall indicates that seismicenergy was concentrated at the hanging-wall block when theChi-Chi earthquake occurred.Discussion and ConclusionsHanging-wall/footwall effects are difficult to quantifyaccurately because of scarcity of ground-motion data at nearfield and incomplete knowledge of fault geometry of blindthrust earthquakes.Three physical mechanisms,distance ef-fect,wave trapping,and directivity effects,are thought toresult in hanging-wall effect(e.g.,Somerville et al.,1997;Bard and Riepl-Thomas,2000).In this study,we have takenadvantage of a large collection of accelerometric records toquantify the hanging-wall amplification in the Chi-Chi earth-quake.The following conclusions and recommendations aremade.1.It isfirst shown that the hanging-wall effect cannot besimply accounted for by choosing an appropriate defini-tion of distance for the attenuation model.This resultshows that the hanging-wall effect cannot exclusively re-sult from a geometrical factor(otherwise an appropriatechoice of the distance–metric definition would suffice).Although our analyses ignored the influence of local siteeffect,our experiences gained from studying the Chi-Chiearthquake show that the site effect may generate prin-cipally the increasing excitation of dispersive long-period。