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THE USE OF REAL EARTHQUAKE ACCELEROGRAMS ASINPUT TO DYNAMIC ANALYSISJULIAN J. BOMMER a & ANA BEATRIZ ACEVEDO ba Department of Civil and Environmental Engineering , Imperial College London, SouthKensington campus , London, SW7 2AZ, UK E-mail:b ROSE School, Collegio Alessandro Volta , Via Ferrata 17, Pavia, 27100, ItalyPublished online: 04 Sep 2008.

To cite this article: JULIAN J. BOMMER & ANA BEATRIZ ACEVEDO (2004) THE USE OF REAL EARTHQUAKE ACCELEROGRAMS ASINPUT TO DYNAMIC ANALYSIS, Journal of Earthquake Engineering, 8:S1, 43-91, DOI: 10.1080/13632460409350521

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Journal of Earthquake Engineering, Vol. 8, Special Issue 1 (2004) 43-91 @ Imperial College Press

@ Imperial College Press www.icpress.co.uk

THE USE OF REAL EARTHQUAKE ACCELEROGRAMS AS INPUT TO DYNAMIC ANALYSIS

JULIAN J. BOPVIMER

Department of Civil and Environmental Engineering, Imperial College London, South Kensington campus,

London S W7 2AZ, UK j. bomrnerQimperial. ac.uk

ANA BEATRIZ ACEVEDO

ROSE School, Collegio Alessandm Volta, Vza Femata 17, Paviu 271 00, Italy

The increasing availability of strong-motion accelerograms, and t be relative ease with which they can be obtained compared to synthetic or artificial records, makes the use of real records an ever more attractive option for defining the input to dynamic analyses in geotechnical and structural engineering. Guidelines on procedures for the selection of appropriate suites of acceleration time-series for this purpose are lacking, and seis- mic design codes are particularly poor in this respect. Criteria for selecting records in terms of earthquake scenarios and in terms of response spectral ordinates are presented, together with options and criteria for adjusting the selected accelerograms to match the elastic design spectrum. The application of both geophysical and response spectrd search criteria is illustrated using compatible scenarios, and the selected records are analysed and adjusted to produce suites of acceleration time-series suitable for dynamic analyses. The paper concludes with suggestions for making use of real records in engi- neering analysis and design, and recommendations are given for improving the current guidelines provided in seismic design codes.

Keywords: Strong-motion records; dynamic analysis; strong-motion databank; strong- motion database; spectral matching; seismic design codes,

1. Introduction

For earthquake-resistant design and for seismic assessment of existing structures, the earthquake-induced ground shaking is generally represented in the form of a response spectrum of acceleration or displacement. The spectrum used as input to equivalent lateral force or spectral modal methods of analysis is usually obtained by scaling an elastic spectrum by factors that account for, amongst other phenomena, the influence of inelastic structural response. There are, however, situations in which the simulation of structural response using a scaled elastic response spectrum is not considered appropriate, and N l y dynamic analysis is required. These situations may include the following: buildings designed foi a high degree of ductility; structures with configuration in plan or elevation that is highly irregular; structures for which

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higher modes are likely to be excited; critical structures, the failure of which would cause unacceptable harm or disruption; structures with special features, such as base isolation. Faced with these special situations, the engineer will generally have to employ time-history analysis, for which the requirements are an appropriate non- linear model for the structure and a suitable suite of accelerogams to represent the seismic excitation.

A workshop on improving the characterisation of earthquake ground motion held in 1997 [ATC, 19991 reflected the importance of the issue of defining accelero- grams for engineering design in its first conclusion, which recommended to "develop guidelines for generating and selecting time histories that can be used by the prac- tising engineer 272 seismic analysis and design of facilities". Nonetheless, there is relatively little published technical literature on the subject of selecting and scaling real strong-motion records for design, and this paper therefore attempts to present the issues involved and offer some insights as well as some guidance for engineers.

There are three basic options available to the engineer in terms of obtaining acceleration time-series. The first is to use artificial spectrurn-compatible accelero- grams generated using programs such as SIMQKE [Gasparini and Vanmarcke, 19791. The approach employed in SIMQKE is to generate a power spectral den- sity function &om the smoothed response spectrum, and then to derive sinusoidal signals having random phase angles and amplitudes. The sinusoidal mot ions are then summed and an iterative procedure can be invoked to improve the match with the target response spectrum, by calculating the ratio between the target and actual response ordinates at selected hequencies; the power spectral density function is then adjusted by: the square of this ratio, and a new motion generated.

The attraction of such an approach is obvious because it is possible to obtain ac- celeration time-series that are almost completely compatible with the elastic design spectrum (Fig. I), which in some cases will be the only information available to the design engineer regarding the nature of the ground motions to be considered. How- ever, it is now widely accepted that the use of such artificial records, particularly f& non-linear analyses, is problematic. The basic problem with spectrum-compatible artificial records is that they generally have an excessive number of cycles of strong motion and consequently they possess unreasonably high energy content. Here it is necessary to discuss terminology, since the adjective "artificial" is also applied (sometimes with the additional qualifier of "intelligent7') to the outcome of applying selective adjustments to real accelerograms, using techniques that are discussed in Sec. 4.2. In this paper, the term "artificial" is used exclusively for records such as those shown in Fig. 1. These types of records are not considered to be suitable for use in non-linear analyses. In addition to the problems associated with how these artificial records are generated, there can also be difficulties that arise from match- ing the acceleration time-series to the entire elastic design spectrum. The latter will generally be a uniform hazard spectrum (UHS), including in seismic design codes, obtained from probabilistic seismic hazard assessment (PSHA), and there- fore enveloping the ground motions from several seismic sources [e-g. Reiter, 1990;

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The Use of Real Earthquake Acceierugrams 45

Period (seconds)

I

Time (seconds)

Fig. 1. Artificial accelerograms generated to match the S 1 soil category elastic response spectrum from the French seismic design code; the uppermost pIot compares the average ordinates of the three spectra with the code spectrum.

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46 J . J. Bommer & A. B. Acevedo

Bommer et al., 20001. Naeim and Lew (19951 assert that: "To generate an accelera- tion time-history to be compatible to a PSHA-generated design spectrum is neither reasonable nor realistid'. Certainly it is the case that if the UHS is strongly influ- enced by more than one source of seismicity, for example by small, local earthquakes and by distant, large magnitude events, spectrum-compatible artificial records will tend to be particularly unrealistic. .

The second category of ground-motion records available to the engineer is syn- thetic accelerograrns generated from seismological source models and accounting. for path and site effects. These models range from point source stochastic sirnula- tions through their extension to finite sources, to fully-dynamic models of stress release, although the latter are still under development. Programs for some of the many methods of ground-motion generation that have been developed [e.g. Zeng et aL, 1994; Beresnev and Atkinson, 1998; Boore, 20031 are freely available, but their application, in terms of defining the many parameters required to characterise the earthquake source, will generally require the engineer to engage the services of specialist consultant in engineering seismology. The determination of the source parameters for previous earthquakes invariably carries a high degree of uncertainty, and the specification of these parameters - to which the resulting ground motions can be highly sensitive - for future earthquake scenarios can involve a significant degree of expert judgement.

The third category of records is real accelerograms recorded during earthquakes, which by definition are free from the problems associated with artificial spectrum- compatible records. Real strong-motion records are now easily accessible in large numbers and their retrieval and manipulation is relatively straightforward, whence the design engineer will often be able to prepare a suite of records without the ser- vices of an engineering seismologist. This paper provides an overview of the issues involved in preparing suites of real records for use in dynamic analyses, and exam- ines different procedures for the selecting and scaling of the records. The following section provides an overview of the sources from which strong-motion data are now available, including some assessment of the coverage of different earthquake scenar- ios and the ease with which each source allows the user to perform searches. The two sections that follow deal with the issues of how records are selected and how they can be scaled to match, in some specified sense, the elastic design spectrum. The penultimate section of the paper then explores all of these issues through two approximately compatible searches, one using an earthquake scenario and the other using direct matching to a code spectrum, and through the application of differ- ent selection and scaling procedures to the suites of accelerograms obtained from each search. The paper closes with simple guidelines for the selection and scaling of real strong-motion records, and discusses how these might be incorporated into seismic design codes.

Before closing this section, mention should be made of synthetic accelerograms generated using empirical Green's functions, which are effectively a hybrid of the second and third categories of acceleration time-series.

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The Use of Real Earthquake Accelerograrns 47

2. Availability of S trong-Motion Accelerograms

The operators of strong-motion recording networks generally produce reports presenting the records obtained born their accelerographs, either on a periodic basis or following a significant earthquake. Others have produced compendia of records from one or more networks in a given region or country. One of the first examples of such reports was the CALTECH (California Institute of Technology) volumes produced after the 1971 San Fernando earthquake in California. These volumes, which accompanied magnetic tapes on which the digitised records and associated response spectra were distributed, were an important landmark in mak- ing strong-motion records more widely available to both researchers and practicing engineers. A usehl distinction can be made between a collection of digitised ac- celerograms and a catalogue of associated information about the earthquakes and the recording stations from which the accelerograms were obtained, as well as about the records themselves: the former is a strong-motion databank, the latter a strong- motion database [Bommer and Ambraseys, 19921. To facilitate the use of strong- motion records in engineering analysis and design, the practicing engineer requires both an extensive databank of accelerograms and access to a database of reliably determined parameters in order to select appropriate recordings.

2.1. Global databanks

Amongst the first efforts to compile a global catalogue of earthquake accelerograms was the databank presented in a series of reports by Leeds [1992]. The reports listed about 400 horizontal component records from shallow earthquakes in western USA and a similar number from earthquakes in Alaska, the Cascadia subduction zone and the rest of the world, mainly coming £tom Japan and Mexico. In or- der to facilitate selection, the accelerograms were organised into bins according to magnitude ranges, focal depth and site classification, the latter distinguishing only between hard and soft sites. Epicentral and hypocentral distances were reported for each record. Another extensive catalogue of strong-motion records, almost ex- clusively horn North America (including Mexico) was presented by Naeim and Anderson [1996]. The report presented 1470 horizontal component records and 527 vertical components, but the focus adopted by the authors was to make the records accessible to engineers by providing listings of strong-motion parameters, includ- ing peak amplitudes, elastic and inelastic spectral ordinates, and durations; the only seismological parameters given were magnitude (for which the largest value reported by the USGS was given, which resulted in a mixture of Mw, M, and M L , amongst others), focal depth and hypocentral distance.

In recent years, several strong-motion databanks and databases have been is- sued and distributed on CD-ROM, which has been another important development in making accelerograms more widely available to end-users. Several strong-motion network operators have produced CD-ROM collections of their own records, includ- ing agencies in Japan and Mexico, and others have been issued with the recordings

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born a particular earthquake, such as the 1999 Chi-Chi event in Taiwan [Lee e t al., 2001). The USGS (United States Geological Survey) issued an important collection of almost 1500 accelerograms horn 500 earthquakes recorded by ground-level in- struments in North and Central America between 1933 and 1986, with information regarding the earthquake and recording station provided in the header of each com- ponent file. The project was presented a . a n update and consolidation of the work originally made available via the CALTECH volumes [Seekins et al., 19921. The global strong-motion databank compiled by the National Geophysical Data Center (NGDC), which was distributed as a 3-volume CD-ROM in 1996, contains 15000 individual component records from about 1000 earthquakes around the world up to 1994 [Row, 19961. Although the NGDC databank makes a very large number of records available to users, the parameters in the database have not been uniformly re-evaluated and this limits its use as a selection tool: only epicentral distances are given for the records and various magnitudes are reported, with a quarter of the earthquakes having either a magnitude of zero (presumably implying no value is available) or a value on an unspecified scale.

In this respect, some smaller collections of data may be of greater use simply because they provide more complete and more uniformly determined source, path and site parameters for the accelerograms. Work has been ongoing for many years to determine uniform parameters associated with strong-motion records from Europe and the Middle East [Ambraseys and Bommer, 1990,1991] and in 2000 a CD-ROM of European Strong-Motion Data was issued and distributed as a result of a Eu- ropean Union-funded project [Ambraseys et al., 2000). The CD-ROM includes just over 1000 accelerograms from more than 400 earthquakes, with a database of associ- ated parameters including uniformly calculated Joyner-Boore distances [Abraharn- son and Shedlock, 19971 for nearly all records fkom earthquakes of magnitude 6 or greater; for smaller earthquakes, generally only epicentral distance is provided, but for such events the two measures are comparable. The style-of-faulting is known for more than half of the records, and the site classification for more than 80% of the records, although the reliability of the information on which the latter is based is highly variable. The CD-ROM allows the user to search records in terms of different combinations of parameters such as magnitude, &tame and site classification, and peak ground acceleration (PGA) can also be used as a search parameter. Another useful collection, in which the data (exclusively from soft rock and stiff soil sites in western US) has effectively been pre-searched to be presented in magnit ude-distance bins, is available on the CD-ROMs accompanying NUREG/ CR-6728 [McGuire et aL, 20011.

2.2. Internet sites

The most significant development in strong-motion data dissemination in recent years is the creation of several Internet sites from which users can search and down- load accelerograms in digital form [Wald, 19971. Many operators of accelerograph

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The Use of Real Earthquake Accelerogmm 49

networks maintain their own web sites, in some cases allowing users to download digitised records. An excellent Internet site for obtaining Japanese strong-motion data is the K- Net site at http://www.k-net. bosaz.go.jp/k-net/zndex-en.shtml. There are also a number of sites that provide data from several networks, although these vary in the degree of access that they actually provide to the digital strong-motion records. The NGDC site (http://wwzu.ngd~.noaa.~ov) allows users to search the database mentioned in the previous subsection, but the records can only be o b tained korn the CD-ROM collection. The databank of accelerograrns from Eu- rope and the Middle East, containing almost three times as many records as were available on the CD-ROM described above, can now be searched via the In- ternet Site for European Strong-Motion Data (ISESD) launched in March 2002 (http://www.isesd. cv. ic. ac.uk) [Ambraseys et al., 20031. Figure 2 shows the distri- bution of the strong-motion records in the European Internet site with respect to magnitude, distance and site classification; it can be appreciated that although the databank is extensive, the majority of the records actually correspond to earth- quakes that are unlikely to be of engineering significance, given that the threshold magnitude considered worthy of consideration for engineering purposes is generally taken to be about 5.

Two other important websites for accessing strong-motion data are COSMOS and PEER. The COShlOS website (http://db.cosmos-eq. org) contains a databank of more than 4000 freely available records &om around the world, 40% of which are from western US, 20% fiom Japan and about 18% from New Zealand, the main objective of the website being to make as many records as possible available to users [Stepp, 20001. Simple searches can be performed in terms of ranges of mag- nitude, distance and PGA, as well as by region. Moment magnitudes are provided for almost half of the earthquakes in the database; distances can be searched as hypocentral or distance from the fault rupture, but the latter is provided for a much smaller proportion of the data. Advanced searches can be performed in terms of several other parameters, including mechanism, rake angle, site geology, peak ground velocity (PGV), and spectral ordinates at a few response periods, although these parameters are not provided for all records.

The PEER databank (http://peer. berkeley. edu/smcat) includes 1557 records from 143 earthquakes in tectonically active regions, for which the time- histories and response spectra for different damping ratios can be downloaded. The distribu- tion of the records in the PEER databank with respect to magnitude, distance and site classification is shown in Fig. 3. The PEER database reports &Iw, Ms and ML for earthquakes, with 90%, 85% and 78% of the records having a value on each of the scales, respectively. Distances are reported using three different metrics, these being the closest distance to the fault rupture, hypocentral distance and the Joyner- Boore distance. The proportions of the records for which each distance is given are 80% for &,,, 15% for Rhyp and 48% for Rjb (see Abrahamson and Shedlock [I9971 for distance definitions). The site geology at the recording stations is classified ac- cording to two different schemes, one attributed to the USGS, using four classes

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50 J+ J . Bornmer & A . B. Aceuedo

o m j A a A 1 A rock

O urn o stiff soil 0 alluvium

unknown

Distance (km) Fig. 2. Distribution of European strong-motion databank with respect to magnitude, source-te site distance, and site classification.

whose limits are defined by Vs,30 values of 750, 360, and 180 m/s, which are the values used by Boore et al. [1997] based on the NEHRP classification scheme, and the other being either the Geomatrix scheme or the CWB classification for stations in Taiwan; the Geomatrix scheme includes five categories, the stiffest with shear wave velocities above 600 m/s, the softest those with less than 150 m/s [Abraham- son and Silva, 19971. 65% of the records are classified in terms of USGS scheme, and 84% in terms of the Geomatrix or CWB schemes.

The PEER database lists some earthquakes for which the digitised records are not actually available at the site, most of these corresponding to European events.

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The Use of Real Earthquake A c c e i e r o g m 51

intermediate

+ unknown

Distance (km) Fig. 3. . Distribution of f EER strong-motion databank with respect to magnitude, source-tesite distance, and site classification. Magnitude is assigned according to the following order of priority: M w , M,, ML, and distances as RrUp, Rjb, Rhyp. Rock sites are those classified as USGS or Geomatrix class A or CWB class 1, intermediate sites are class B or C or 2 in the CWB scheme, and others are soft.

The COSMOS site also includes very few accelerograrns from Europe, which makes the ISESD a useful complement to the COSMOS and PEER sites.

Using the PEER database, searches can be performed in terms of magnitude, distance, site classification, rupture mechanism, PGA, PGV and peak ground dis- placement (PGD) , or alternatively in terms of the maximum spectral acceleration in a user-specified period range. In terms of search capabilities, provided one has access to a large databank, the optimum approach - as illustrated in Sec. 5 - can be to use both seismological and response spectral criteria simultaneously. In a follow-up to the ISESD website, a new CD-ROM is to be distributed in early 2004, which will allow users to execute searches using a wide range of possible combinations of parameters related to the characteristics of the earthquake source, the source-to-site path, and the site itself, as well as in terms of strong-motion parameters, including response spectral ordinates [Ambraseys et al., 20041.

A final point worthy of note with regard to the strong-motion records that can be obtained either from CD-ROM collections or downloaded from Internet sites, is with respect to the processing applied to the signals. The problems associated with distortion of high-frequency cornponenk of motion due to instrument response, and more importantly with baseline errors and long-period noise in digitised analogue strongmotion recordings, are well known [Trifunac et al., 1973; Hudson, 19791.

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32 J. J. Bommer 13 A. B. Acevedo

The problems are reduced with digitally recorded accelerograms, but by no means eliminated [e.g. Boore e t a/., 20021. For any application that requires &placement time-series, such as asynchronous analysis of bridges, and the analysis of long- period structures, the issues of baselme errors and long-period noise can become particularly sigdicant. The COSMOS site offers records as contributed by net- work operators, in uncorrected and/or corrected format, but often no details are provided of the correction procedures applied. Important exceptions to this are records supplied by the USGS and CSMIP (Californian StrongMotion Instrumen- tation Program), for which processing details are contained in the record headers. Nearly all records on the ISESD site are available in both uncorrected and cor- rected formats, except for those cases where only corrected records were provided by the network operators. The remaining records have all been corrected by the subtraction of a linear baseline and band-passed filtered using an elliptical filter [Sunder and Connor, 19821 with cut-off kequencies of 0.25 and 25 Hz; no instru- ment correction has been applied. Where uncorrected records are available, users may apply their own preferred correction procedures. The accelerograrns available at the PEER web site are all in corrected format, with most of the records having been individually processed by Dr. Walter Silva, using a causal Butterworth filter, with cut-off frequencies based on inspection of the Fourier amplitude spectrum and the integrated displacement time-series, and a correction for instrument response. For applications where long-period response and ground displacements are impor- tant, the PEER records are likely to be an attractive choice since there is some degree of confidence in the displacement records (reflected in the fact that PGD is offered as a search parameter), which may not be true for the corrected records from the COSMOS and ISESD sites.

3. Criteria for Selecting Strong-Motion Records

The way in which records can be chosen is to a large extent dictated by the infor- mation available to the engineer regarding the seismic hazard or the design ground motions at the site of interest. Figure 4 provides an overview of the different options that are available.

Guidance on this topic in the literature is very limited, and in seismic design codes very little useful guidance is given on how appropriate records should be selected. Bornmer and Ruggeri (20021 considered 33 current or recent codes for the seismic design of buildings, and identified that only eight of these specify time- history analysis to be compulsory (under certain specified circumstances); most codes allow dynamic analysis and even those that require it often specify the use of spectral modal analysis rather than direct integration techniques. This partially explains why guidelines on preparing ground-motion input for full dynamic analysis are not well developed. Indeed, some codes seem to consider this an issue outside their scope: the 1984 Indian code simply informs the user of the option of using "time-history analysis based on expected ground motion for which special studies are

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The Use of Real Earthquake Accelerogmrns 53

Synthetics 4

Selection in terms of Selection in terns of seismological parameters

Fig. 4. Overview of the options available for selecting accelerograms to be used in engineering analysis and design.

required7. All codes that discuss the application of acceleration time-histories allow the use of real records, with the exception of the Portuguese code that specifies only spectrum-compatible artificial motions. Some codes, amongst them UBC 1997 and IBC 2000, favour real records but allow the design engineer to supplement these with simulated motions when sufficient suitable real records cannot be found.

More than half of the design codes reviewed do not specify the critical issue of the number of records to be selected; amongst those that do, the most commonly encountered figure is three. This is the number specified both in UBC 1997 and IBC 2000, but both of these codes have the provision that if only three records are used in the analyses, the maximum structural response must be used, whereas if seven or more are used, the average response may be used; the same specification is made in EC8. Other variations exist, such as the stipulations presented in ISO/DIS 19901-2 [ISO, 20031 for the seismic design of offshore structures, which specifies that a minimum of four time-histories should be used "to capture the mndomness in a seismic event' and that the structure must be demonstrated to survive under four or half of the time-histories, whichever is the greater. The issue of the number of records to be used in dynamic analyses is chscussed further in Sec. 4.1.

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..An important implicit assumption underlying the recommendations made in this paper is that provided similar tectonic environments are considered, strong-motion records horn one country can be selected and applied in another. The three basic categories of tectonic regions that should be considered, for matching the design situation and the selected recordings, are subduction zones, active crustal regions and stable continental regions.

3.1. Selection in terms of strong-motion pammeters

Guidance given in seismic design codes on how to select appropriate real records is usually focused on compatibility with the response spectrum rather than seismolog- ical parameters, for the simple reason that the information on seismic source zones and activity rates that underlie zonation maps is not presented and only the uni- form hazard spectrum (UHS) is given. In current codes, earthquakes are effectively invisible and for this reason the engineer using the code will not easily be able to identify scenario earthquakes. This gives rise to the use of generally rather vague specifications such as that encountered in the 1995 Greek seismic code, which states that the selected records "must be representative of the ground motion at the site and must be recorded at a consistent sowce-site distance7'. An important exception to the general shortcomings vis-&vis seismic design codes is represented by the dashed arrow in Fig. 4, which corresponds to the unique situation in the United States. Since the zonation map and uniform hazard spectrum in IBC 2000 are closely based on the USGS hazard maps, users can obtain disaggregations - and even suites of hazard-consistent stochastically generated acceleration time-series - from the USGS website s t http://eqint.cr.usgs.gov/eq/htmI/deugginZ.

Where specific criteria for selecting records are provided in seismic codes, they are generally based on the ordinates of the elastic design spectrum, although some, notably the Spanish code, only specify a match with PGA. Some codes do not spec- ify the relationship between the selected records and the elastic design spectrum, but rather specify that the base shear obtained from dynamic analysis should not be lower than a certain proportion - usually between 0.7 and 0.9 - of that ob- tained using the equivalent lateral force method, which does not actually help the engineer in making the initial selection of records.

Most of the codes that give some guidance on the preparation of suites of accel- eration time-series to be used as input to dynamic analyses specify conditions that the records must meet with respect to the ordinates of the elastic design spectrum. These matching criteria are discussed in Sec. 4.1. In order to implement searches that will produce records likely to meet the spectral matching criteria, or at least to do so with a minimum of manipulation of the records, it is useful to have a tool that allows records to be searched on the basis of the spectral ordinates. Such a tool is included in the new European strong-motion data CD-ROM discussed previously [Ambraseys et al., 20041, which allows records to be searched by match- ing the spectral shape to the shape of the design spectrum. The search is based on .

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The Use of Real Eadhquake Accelemgrams 55

the average root-mean-square deviation of the observed spectrum from the target design spectrum:

S A , ( ~ . ) ) ~ - N PGA, 1

where N is the number of periods at which the spectral shape is specified, SA, (Ti) is the spectral acceleration from the record at period T,, SA,(Ti) is the target spectral acceleration at the same period; PGA, and PGA, are the peak ground acceleration of the record and the zero-period anchor point of the target spectrum, respectively. The smaller the value of D,,, the closer the match between the shape of the record and target spectrum; the value specified will depend on the extent of the databank being accessed and the number of records required. Smaller values of D,,, can be specified if the spectral matching is being done at short rather than longer spectral response periods. Making searches on a database of about 7000 accelerograms held in the Imperial College London strong-motion archive, it was found that to return less than about 30 accelerogams, values of D,,, o f the order of 0.15 were needed for matching ordinates in the period range of 0.4- 0.8 second, whereas values as low as 0.06-0.07 could be used for matching the spectral ordinates from 0.1 to 0.3 second [Bommer et ul., 2003al. By simultaneously specifying an acceptable match with the design PGA, the search then matches the record and target spectrum in the specified period range. This procedure is superior to matching on the basis of spectrum intensities (area below the response spectrum) in the specified period range, because a good match in that case could easily be obtained with the record having ordinates significantly above the target spectrum at one period and significantly below at another. The procedure proposed effectively limits the maximum deviation of individual peaks or troughs on the spectrum from the target ordinates.

The most serious limitation with any selection procedure based solely on the ordinates of the elastic spectrum is that the records obtained can have very different durations. If the starting point for the selection is a seismic design code, in which the earthquake actions are represented by an elastic response spectrum of acceleration, the duration of the design ground motions will generally not be specified. Amongst the 33 seismic design codes reviewed by Bommer and Ruggeri [2002], only six specify duration criteria, and only two of these - the codes of France and Turkey - actually specify how the duration is to be measured, an important issue given that there are more than 30 different definitions of strong-motion duration in the technical literature [Bommer and Martinez-Pereira, 1999, 20001. This problem is not easily overcome because no code currently includes a map of hazard in terms of duration of shaking and without knowledge of earthquake magnitude, it is very difficult to estimate this parameter. The absence of suitable criteria can also lead to unrealistic specifications. One code that provides a great deal of information about the criteria that the records should fulfil is the 1990 French code, although these are

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so demanding that it is almost impossible to obtain realistic ground motions that satisfy them; the artificial accelerograms in Fig. 1 were generated to satisfy these requirements, but no real accelerograms could be found- that also met the criteria. One criterion in the French code that makes it so difficult to obtain compatible real records is the specification of a minimum duration of 20 seconds for real records.

The 1988 Iranian seismic design code makes the selection of input accelero- grams for dynamic analysis very easy, specifying use of the Naghan record of the 1977 Ardal (Ms 6) earthquake and the Tabas record of the 1978 Tabas (fils 7.3) earthquake, both recorded within 5 km of the seismic source and with PGA values in excess of 0.6 g and 1.0 g respectively. Although very simplistic - since no con- sideration is given as to whether such severe motions could be generated at the site under consideration - this approach could usefully be adapted by suites of suitable records being specified in codes for different hazard zones and site categories, as discussed later.

3.2. Selection in terms of geophysical pammeters

If the engineer has at his or her disposal a site-specific seismic hazard assessment, then the possibilities for selecting suitable records are quite different. If a determin- istic seismic hazard assessment (DSHA) has been employed, the design earthquake scenario will be fully defined, at least in terms of the earthquake magnitude, the distance from the site to the fault rupture, and the nature of the surface geology at the site (Fig. 4). The search could then be performed directly in terms of these three parameters, as well as others such as style-of-faulting. If PSHA has been used, then the controlling earthquake scenarios need to be obtained by disaggregation, using one of several techniques that have been developed for this purpose [Chap man, 1995; McGuire, 1995; Harmsen et al., 1999; Bazzurro and Cornell, 19991. These techniques yield dominant scenarios contributing to the hazard at different parts of the response spectrum, defined by a magnitude, distance and number of logarithmic standard deviations above or below the logarithmic mean from the ground-motion prediction equation used in the analysis. If the vertical component of motion is considered important, it should be borne in mind that the control- ling bl-R (rnagnitude-distance) scenarios for the vertical component motion at the fundamental period of the structure may be different from those for the horizontal component, which creates an additional complication for performing the searches. Once the controlling earthquake scenarios have been identified, then the searches can be undertaken in the same way as would be the case had a DSHA been carried out to define the design spectrum for the site. The number of standard deviations above the median, often specified as E , will generally not be used as a search param- eter. The design spectrum will nearly always have ordinates above those predicted by the median values from attenuation equations using the scenario M-R pair, even if it is the result of a deterministic assessment since current practice in DSHA is generally to use the Wpercentile level of motion [e.g. Krinitzsky, 20021. As a result,

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The Use of Real Earthquake Accelerograms 57

the selected records will generally need to be scaled or adjusted to match the design spectrum, as discussed in the next section.

Clearly, if a search is carried out in terms of an exact match with the design scenario, for example a normal-faulting earthquake of magnitude M, 6 -4 recorded at 1 2 km on a site with a V.,J() of 470 m/s, it is very unlikely to yield any records. Therefore, the search must be performed with less restrictive criteria, and for this reason it is important to decide which parameters should be included in the search (apart fiom the tectonic criteria discussed earlier), and for each parameter how much tolerance should be allowed in the degree of matching between the record and the scenario.

3.2.1. Earthquake magnitude

Opinions differ about the importance of correctly matching parameters such as earthquake magnitude: Shome et al. [I9981 concluded that provided the records are scaled to match the elastic design spectrum a t the fundamental period of the structure, then matching the records for the magnitude-distance combination of the design earthquake scenario is not important. The core of the issue is the degree to which the duration of shaking influences structural demand, an issue of on-going debate and investigation. Most studies do specify that magnitude should be a search parameter, indeed even Shome et al. [I9981 recommend in their conclusions that the user should use "records from roughly the same magnitude". Others are more adamant, such as Stewart et al. [2001], who state that it is important to select records from events of appropriate magnitude because this parameter strongly in- fluences frequency content and duration of the motion, going on to recommend selecting records &om events within 0.25 units of the target magnitude. Since there is little doubt that earthquake magnitude exerts a very pronounced influence on du- ration (or number of cycles) and on the shape of the response spectrum (Fig. 5), we are of the opinion that it is an indispensable selection parameter, and furthermore that the match between the record and scenario magnitudes should be close, if pos- sible within 0.2 magnitude units either side of the target value; this is in agreement with the proposal of Stewart et al. [2001] but given that magnitude is generally expressed to the nearest decimal the value is rounded down rather than up. An objection that has been raised to using such a narrow window of magnitude is that the interval is comparable to the standard deviation associated with magnitude determinations, which is generally of the order of about 0.2. However, it has been pointed out that the standard deviation of the individual station determinations of magnitude is not really a measure of the uncertainty in the published magnitude values, and that a better measure would be the standard error of the mean [J. Dou- glas, personal communicatzon, 20031, obtained by dividing the standard deviation of the observations by the square root of the number of observations. The standard error of the mean magnitude estimate is generally an order of magnitude less than the standard deviation of the station determinations.

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Although techniques are available to adjust strong-motion records in a way that allows the spectral shape to be altered, as presented in Secs. 4.2 and 4.3, it is generally preferable to keep the degree of alteration to a minimum (although there may be cases, such as records containing strong resonance a t a particular frequency, in which it will be desirable to change the record before using it in analysis). For this reason, and given the pronounced effect of magnitude on the shape of the response spectrum, a close match between the scenario and record magnitudes should be sought.

3.2.2. Source- to-site distance

The second parameter that must be included in defining the search window is distance. Figure 6 shows normalised spectral shapes, using the same attenuation equations as in Fig. 5, constructed from median values predicted for rock sites located at 5, 20 and 50 km from a magnitude 7 earthquake. The spectral shape appears to be much less sensitive to distance than to magnitude; if the ratios of the ordinates for 5 and 50 km were calculated from each of the equations, the average ratio calculated from the four values would be almost invariant with period (Fig. 7).

Campbell (1 997)

I . . . . ! . . . . I . . . .

Period (seconds) Period (seconds)

Period (seconds) Period (seconds)

Fig. 5 . Response spectral shapes (norrnalised to the ordinate at 0.2 s) for rock sites at 10 km from earthquakes of magnitude 5.5, 6 and 7 using the median values obtained from the follow- ing attenuation equations (clockwise from top left): Ambraseys et al. [1996], Campbell [1997], Abrahamson and Silva (19971 and Boore et al. [1997].

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The Use of Real Earthquake Accelerogmms 59

Krinitzsky and Chang [19?7] proposed that if scaling factors of 4 or more needed to be applied to accelerograms, then the records should be rejected, although no justification was given for this assertion. Subsequently Vanmarcke [1979] proposed reduction of the limits on scaling to a factor of 2 for liquefaction analysis, although the limit of 4 was upheld for linear elastic systems. Vanmarcke [I9791 based his con- clusions on a study of inelastic spectra and of correlations amongst different strong- motion parameters, using a dat aset of 70 accelerograms. The dataset only included 12 accelerograms that had horizontal peak accelerations of at least 0.2 g and 41 of the accelerograms had PGA values below 0.1 g. Despite the limitations of the data and the analyses underlying the conclusions, the recommendations from these two studies are hequently used as a rule-of-thumb in practice: Malhotra (20031 finds a scaling factor of 5.84 is required for one record used in his study and concludes that this "is hzgher than the normally accepted upper limit of 4". Presumably, the rationale behind imposing limits on scaling is to avoid creating unrealistic ground motions, since this would undermine the inherent value in using real accelerograms in the first place. However, it is not clear that such severe restrictions on scaling values are justified, since over the distance ranges for which spectral shapes are depicted in Fig. 6, amplitudes of ground motion can vary significantly: from 5 to

Period (seconds) Period (seconds)

Abrahamson & Silva (1 997)

I Period (seconds) Period (seconds)

Fig. 6 . Response spectral shapes (normalised to the ordinate at 0.2 s) for rock sites at 5 , 20 and 50 km from an earthquake of magnitude 7 using the median values obtained from the fol- lowing attenuation equations ( clockwise from t o p left) : Ambraseys et al. [l996], Campbell [l997], Abrahamson and Silva [I9971 and Boore et al. [1997].

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- 0 0.5 1 1.5 2

Period (seconds) Fig. 7. Ratios of spectral ordinates for a magnitude 7 earthquake at 5 and 50 km from the earthquake source, calculated from the median spectra shown in Fig. 6 . The mean of the four ratios shows very little variation with period.

50 km, median values of spectral ordinates will reduce by about a factor of 7, as shown in Fig. 7; the very rapid decay of amplitudes with distance is often not appreciated because of the tendency to plot attenuation curves in log-space. The duration of the motion, if measured using the significant duration concept (the in- terval over which a specified proportion of the Arias intensity is accumulated), does increase with distance, due to different wave propagation velocities and scattering, but according to the equations of Abrahamson and Silva [I9961 the increase is only about 0.6 seconds for every 10 krn (Fig. 8). We propose, therefore, that in making, selections of real records, the search window should be as narrow as possible in terms of magnitude, and if it needs to be widened to capture the required number of-records, that the distance range be extended.

There are two important exceptions to this line of reasoning, the first being if records are selected from soft soil sites, since weak distant motion would not'scale linearly for sites closer to the source due to soil non-linearity. The second excep tion is if near-source rupture directivity effects are to be considered as part of the design scenario. The effect of forward directivity is to produce short-durat ion mo- tions with high-energy pulses that amplify the spectral ordinates at intermediate or long periods [Somerville e t al., 19971. Near-source directivity effects cannot easily be artificially introduced into real accelerograms hence if this is a design criterion the search will need to specifically identi& records obtained at short distances and in the forward directivity zone. The number of records available which correspond to such conditions is relatively small, but a start can be made using the database presented by Somerville e t al. (19971 and near-source recordings from recent earth- quakes including the 1999 events in Turkey and Taiwan.

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The Use of Real Eadhquake Accelerogmms 61

Closest Distance (km) Fig. 8. Predicted median values of significant duration ( 5 7 5 % of Arias intensity) at rock sites from the equation of Abrahamson and Silva (19961.

3.2.3. Site classzficatzon

The third parameter that is obviously desirable to include in the search is the site classification, since this also exerts a strong influence on the nature of the ground motion, affecting both the amplitude and shape of response spectra. However, spec- ifylng a close match for this parameter may not always be feasible since the geotech- nical profile has been determined with confidence for a relatively small number of strong-motion recording sites. Even if reliable site classifications are available, these will generally be based on, at best, the nature of the uppermost 30 m at the site, whereas the deeper structure can also exert an important influence [Boore, 20041. Within any site class - and especially within sites classified simply a s "rock" - there can be considerable variation in dynamic response characteristics. Adding site classification as a third search parameter will obviously reduce significantly the number of records returned for any given magnitude-distance window [Bommer and Scott, 20001. In light of these issues, there may be cases in which it would be advisable to relax the matching criteria for site classification in order not to restrict too severely the number of records obtained. Clearly, if the site of interest is characterised by hard rock, it would be advisable to exclude soft soil recordings from the suite of records compiled for dynamic analysis, but any greater restriction should be imposed only if there are sufficient records providing a reasonable match to the design scenario in terms of magnitude and distance. If the number of avail- able records matching the magnitude and distance criteria is small, we recommend that records be considered f i ~ m sites that are within one site class (e.g., NEHRP or

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62 J. J . Bomrner & A. B. Acevedo

EC8) either side of the classification of the site under consideration. This approach is far from ideal and it is not intended to discount the vital importance of site effects in ground-motion estimation, but it reflects a pragmatic attitude towards the data about site geology and site response generally available.

3.2.4. Additional selection criteria

In the case that a good number of records (- 10-20) can be obtained specifying the site class, and using a suitable window in magnitude-distance space, a further refine- ment could be to also consider the rupture mechanism, if this is determined as part of the design scenario. There is no definitive evidence for systematic and significant differences between the ground motions &om normal and strike-slip faulting earth- quakes, but there is general consensus that reversefaulting events produce larger amplitudes of motion. There is less agreement on the ratio of reverse to strike-slip motions' and the extent to which the ratio varies with response period [Douglas, 20031. The best estimate of this ratio given by Bommer et al. [2003b] implies a variation of about 12% in period range from 0.1 to 1.0 second, which would suggest that inclusion of style-of-faulting in the record selection is not vital.

A point that is not often stated, but which is worth bearing in mind, is that an additional criterion should also be added when setting up a small suite of real records: the records should not come predominantly horn one recording station. A possible exception to this condition would be in the case of the recording station being located very close to the site of interest. Another possible criterion is that any suite of records used in dynamic analyses should not be dominated by accelerograms £?om a single earthquake event.

4. Matching Selected Records to the Elastic Response Spectrum

Whether records are selected by performing searches in terms of response spectral ordinates or in terms of seismological and geophysical parameters, there will gen- erally be a requirement to ensure that the records conform to some specified level of agreement with the ordinates of the design response spectrum. Figure 9 provides an overview of the options available for adjusting the selected records, as well as the alternative of using artificial spectrum-compatible time-series, which obviate the need to apply subsequent adjustments to match the design spectrum.

4.1. Matching cr i ter ia

Spectral matching criteria specified in seismic design codes vary from being purely descriptive to being highly prescriptive. An example of the former is the 1992 New Zealand code in which the matching criterion is that "over the period range of in- terest for the structwe being analysed, the 5% damped spectrum of the earthquake record does not di f fer significantly from the design spectrum". In the 2002 version

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The Use of Real Earthquake Accelemgrams 63

Scale in amplitude G

1 L I

1 I

Scale in time and amplitude

D S W

s a b PGA T

Adjust by wavelets or by FFT +

Fig. 9. Overview of the options available for scaling selected accelerograms match the ordinates of the elastic response spectrum specified for design. The box marked 'Lselection" is expanded in Fig. 4.

t .pzzq* X records sa, &%A

T

T

of Part 1 of Eurocode 8, the specification is that no value of the mean 5% damped elastic spectrum calculated for all of the selected records is less than 90% of the cor- responding value of the 5% damped elastic response spectrum; the period range over which this criterion must be met is between 0.2T1 and T I , where TI is the natural period of the structure. In UBC 1997, the average ordinates of the individual spec- tra - calculated as the square root of the sum of the squares (SRSS) of the two horizontal components - should not be less than 1.4 times the design spectrum ordinates in the range from 0.2T to 1.5T, where T is the fundamental period of vibration of the building. The factor of 1.4 is simply to make the SRSS spectrum comparable to that From the code, and in effect the criterion is that the average spectrum of the records should not be below the design spectrum over the period range specified.

;* T -

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64 J . J . Bommer & A. 8. Aceuedo

One notable feature of the specifications in seismic design codes is that it is the average ordinates of the real spectra that have to match the target and not the individual spectra. If matching is required over a wide period range, there may be advantages in matching the average spectral ordinates of the scaled records to the elastic design spectrum, but this overlooks - and indeed can conceal - the equally important issue of the maximum exceedance of the target spectrum by the ordinates from any individual record. The derivation of the target spectrum should be borne in mind when using records scaled to match its ordinates: any design spectrum that is derived through the use of PSHA will include the influ- ence of the scatter in the ground-motion prediction equations, which represents the aleatory variability in ground-motion parameters for given combinations of magni- tude, distance and site conditions. The strong-motion parameters of the selected records will also display an aleatory variability, although'the standard deviation of these values about their mean may be slightly smaller than the standard devi- ations associated with ground-motion prediction equations [Bommer et al., 1998; Shome et al., l998]. Therefore, if the analysis is performed following the procedure presented in some design codes of selecting three accelerograms, scaling their aver- age ordinates to not fall below the design spectrum, and then using the maximum structural response as the basis for design decisions, the variability in the ground motion is effectively being double counted. This procedure should, therefore, not be used. A common misconception is to assert that the selected suite of accelero- grams should capture the variability in ground-motion amplitudes, whereas this variability is already fully accounted for in the derivation of the probabilistically defined response spectrum. For this reason, Stewart et al. (20011 propose that if only three records are used, they should be adjusted with one of the techniques presented in the next subsection, to remove their peaks and troughs so that "the results of stmctvral analyses are not unduly controlled by the particzilar time his- tories that are chosen". An alternative - and unorthodox - approach is that the hazard assessment could be performed using only median values from the ground- motion prediction equations and then using large suites of unscaled accelerograms to capture the aleatory variability in the ground shaking [Bommer et al., 19981. We do not recommend that this latter approach be adopted, but it does bring out the issue of taking explicit and measured account of the aleatory variability in ground motions.

From a seismological perspective, a preferable approach may be to use at least seven records and then use the average response obtained from the structural anal- ysis. However, we resist making idexible recommendations on this issue since in any design situation a balance must be found, generally driven by considerations of time and cost, between using realistic input for dynamic analyses and reducing the number of analyses that need to be performed. Many engineers will consider that using adjusted time-histories, which may not be entirely realistic, is an accept- able price to pay for being able to limit the number of complex dynamic structural analyses performed.

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The Use of R e d Earthquake Accelerognzm 65

The dispersion of the results of dynamic analyses has been shown to be inversely proportional to the square root of the number of records used. Shorne et al. [1998] demonstrated that seven is a suitable number to produce acceptably low dispersion in the results, although other studies have found that to obtain a stable mean in the results of the structural analyses at least 10 records are required [A. Pecker, personal communication, 2003]. As well as ensuring a stable mean of the results, consideration should also be given to the maximum spectral exceedance of any of the individual records [McGuire et al., 20011 since even if the average of the scaled record spectra match the target spectrum, there may be individual records imposing exceptionally high demands on the structure. Selection procedures that include criteria such as the D,,, residual, as described previously, will help to avoid these problems. As noted in the next subsection, spectral matching techniques can also remove pronounced peaks and troughs from the selected records.

A final issue to also be considered is the issue of the two horizontal components of . motion from each triaxial accelerogram. For any analysis requiring two orthogonal components of horizontal motion to be used, careful consideration mustbe given to the selection and scaling of the two acceleration time-histories, an issue addressed by Malhotra [2003]. The guidelines for seismic design of bridges (Part 2) in Eurocode 8 expressly, and quite correctly, forbids the use of the same acceleration time-history simultaneously in both horizontal directions. When the two components of one real accelerogram have been chosen, it is recommended that their average ordinates be used in deriving the scaling factor by comparison with the design spectrum and the factor then applied to the two components separately in order to conserve their differences, particularly for those cases where there is fault-normal and fault-parallel polarisation [Stewart et al., 20011. The definition of the horizontal components of motion used in deriving the design spectrum should also be kept in mind when deriving the scaling factors, since ground-motion predict ion equations use a variety of definitions, the most popular being the larger of the two horizontal components and their geometric mean [Douglas, 20031.

4.2. Selective manipulation of accelerogmms

Techniques are available that allow the user to manipulate real records not only to scale their spectral ordinates but also to change the spectral shape [Preumont, 1984). The techniques make no claim to have a geophysical basis and their expressed purpose is to obtain suites of records with low variability in order to reduce the number of structural analyses required to obtain stable results. Some of these tech- niques operate in the fkequency domain by adding harmonic components through- out the record; the most widely used of these techniques is that embedded in the program WES RASCAL [Silva and Lee, 1987) which has been widely employed and recommended [e.g. Idriss, 19931.

A time-domain method, based on earlier proposals [Kaul, 1978; Lilhanand and Tseng, 19881, with an improved capacity to preserve the non-stationary

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66 J . J. Bummer €Y A. B. Acevedo

0.1 1 o, Original - i I

Time (seconds)

- 1, RASCAL

Time (seconds)

3 1 , RSPMATCH Y I

Time (seconds)

Period (seconds)

Period (seconds)

RASCAL

RSPMATCH __---"----*-----I---- ----;

t . . . . I . . . . I . . . . , . . . . , . . . .

0 10 20 30 40 50 60

Time (seconds)

Fig. 10. Modification of the Corralitos record (top left) of the 1984 Morgan Hill earthquake using RASCAL (middle left) and RSPMATCH (bottom lefi), to match the EC8 acceleration spectrum (top right). The resulting velocity spectra (middle right) and Husid plots (bottom e g h t ) are also compared.

characteristics of the motion, has been developed by Abrahamson [1993] in the program RSPMATCH. This program adds wave packages to those parts of the time-series for those frequencies for which there is a mismatch between the record and target spectrum; the use of wavelets for this purpose is discussed by Iyama and Kuwamura (19991 and by Mukherjee and Gupta [2002]. Results obtained applying the two methods to a single record are compared in Fig. 10.

These techniques are convenient and can be used selectively to satisfy match- ing criteria for a suite of accelerograms by adjusting the ordinates of the records causing the most problematic deviations from the target. Stewart et a!. [2001] rec- ommend that in general it is preferable to use a large number of accelerograms without making adjustments to their spectral shapes, but again the final decision

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The Use of Real Earthquake Accelerograms 67

should be based on the acceptable balance between the realistic nature of the input motions and the number of analyses that can be performed. Surprisingly, despite being matched to the elastic design spectrum over a wide period range, adjusted records can sometimes produce lower structural demands than linearly scaled real accelerograms, especially if the latter have been matched to the target spectrum in log space and include significant exceedances of the target spectrum [N.A. Abra- hamson, personal communication, 20031.

4.3. Linear scaling in time and acceleration

Accelerograms can be scaled to achieve an improved match with the target spec- trum, and possibly other specified criteria, by applying scalar factors to the ac- celeration and/or time axes of the record. However, if there is a large mismatch between the duration of a selected accelerogram and the duration specified for the design scenario, there is no acceptable procedure to close the gap. Seed and Idriss [I9691 produced an artificial accelerogram for a large magnitude (M 8.25) earthquake by scaling and splicing records from smaller earthquakes, but this was done at a time when the global strong-motion databank was very sparse. Scaling the time axis of an accelerograrn can increase or decrease the significant duration, which might be acceptable to compensate for small changes associated with dis- tance (Fig. 8), but not to compensate for any mismatch in magnitude because for that it would also be necessary to change the number of cycles of motion. Scaling the time axis of a record changes not only the duration of the motion but also the frequency content of the record over the entire period range; Kramer (19961 suggests that this procedure should be used with caution, advice with which we strongly agree.

Procedures to obtain appropriate scaling factors, with the particular aim of reducing the scatter in the ordinates of the scaled spectra or in the results of inelastic analyses, have been discussed by Nau and Hall [1984], Matsumura [1992], Shome et al. [1998], and Kappos and Kyriakakis [2000]. Martinez-Rueda [I9981 performed a parametric study on the response of inelastic SDOF systems with the objective of identifying a suitable instrumental measure of ground-motion intensity for the scaling of natural accelerograms. The scaling procedures proposed by all of these researchers, with the exception of Shome et al. [1998], are based on the use of variations of spectrum intensity. This is convenient, since the ordinates of a design spectrum will invariably be available to the engineer.

Shome et al. [I9981 propose that records be scaled to match the median spectral acceleration, obtained from ground-motion prediction equations, at the fundamen- tal period of the structure. There are potential pitfalls in only considering the spectral ordinate at the fundamental period since there is generally uncertainty in the estimation of this parameter. Furthermore, as damage progresses there will generally be period elongation and if higher modes contribute to the response then shorter periods will also be of interest. For these reasons, in the current study the

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focus is on scaling to match the target spectrum across a period range around the fundamental period of the structure being analysed-

The present paper is considered to be complementary to these earlier studies rather than to supersede them or to contradict their conclusions, for the simple reason that the focus herein is largely on the selection of the records, which was not a key issue addressed in the previous papers. Nau and Hall [1984] used only 12 ground-level records obtained on various different sites from both crustal and sub- duction earthquakes with a large range of magnitudes. Nlatsurnura [1992] also used only 12 components from US and Japanese accelerograms, the magnitude, distance and site classifications not even being mentioned. Martinez-Rueda (19981 used both horizontal components &om a total of 50 accelerograms recorded at epicentral dis- tances of up to 400 km horn crustal and subduction earthquakes with magnitudes ranging from I1.1, 5.4 to 8.1; soft soil, stiff soil and rock site recordings were included. The importance of selecting records on a consistent basis is implicitly recognised, however, in the example application presented by Martinez-Rueda (19981, for which 10 Californian accelerograms obtained at distances of less than 30 km &om crustal earthquakes with magnitudes in the range 6.4-7.2; all but two of the records are obtained from stiff soil sites.

The problem of defining appropriate input to dynamic structural analyses ulti- mately involves aspects of both engineering seismology and structural dynamics; the studies cited above have generally placed greater emphasis on the latter, whereas the current study primarily addresses engineering seismological aspects. Kappos and Kyriakakis [2000] used 11 records horn 11 Greek earthquakes, and another 13 records from eight US earthquakes, both data sets being approximately divided between recordings from rock and soil sites; the authors claim that the "compz- lation of records permits consideration of the effect of soil conditions (inevitably in a rough way) as well as of the tectonic regime". The Greek records are filtered with a low-frequency cut-off at 1 Hz, whence the elastic and inelastic displacement spectral ordinates at longer periods, discussed at some length in the paper, should really have been neglected. Differences in the characteristics of the two data sets are instead attributed to other factors including "the deeper deposits in some CaG ifornian sites (e.g. the Bay area)", despite the fact that two of the three records obtained in the San Francisco Bay Area are from rock sites and the third (Parking Garage, Stanford) is not located on Bay mud. As in other studies, the dispersion of structural responses is measured by the coefficient of variation (COV), which is the standard deviation divided by the mean. Kappos and Kyriakakis (20001 find that for inelastic spectral responses the COV increases with increasing ductility factors, but this may simply be due to the large range of magnitudes - and hence durations - in their data sets. A major conclusion of their study is that the COV is higher for the rock data sets than for the alluvial (soil) data sets, which they attribute to the different site classifications. However, it is also possible that the difference was mainly due to the different ranges of magnitude in the data sets: for the US data, the maximum differences in magnitude amongst the soil records was

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The Use of Real Earthquake Accelerogmms 69

1.4 as opposed to 1.8 for the rock data, and for the Greek records the magnitude variation amongst the soil site recordings was just 1.3 compared with 2.1 for the rock site accelerograms.

Amongst the studies cited above, the one which did give carefil consideration to seismological and geophysical selection criteria was Shome et al. [1998], who used records selected in magnitude-distance bins from stiff soil sites, excluding accelerograms with near-source forward directivity pulses. As mentioned previously, however, the study concluded that if the records are then individually scaled to match the predicted median elastic spectral acceleration ordinate corresponding to the scenario at the centre of the bin, careful selection in terms of magnitude and distance becomes unnecessary. This critically important point is re-visited in the final section of this paper.

5 . An Illustrative Example ,

The possibilities for using real accelerograms as input for engineering analysis and design, and the issues involved, axe best illustrated by practical examples. In this section two approximately compatible data searches are defined, one based on the parameters of an earthquake scenario, the other on the ordinates of the elastic design spectrum. The suites of records obtained from both searches are examined and from each, using careful selection and adjustment of the records, appropriate input for dynamic analysis is prepared.

5.1. Selection criteria and recovered data sets *

In order tocompare and contrast the use of seismological and strong-motion param- eters as the basis for data selection, two design situations have been defined. The first is one that may be typical of engineering practice where the designer has ac- cess to very little information about the underlying hazard and is simply presented with an elastic design spectrum, in this case the Type 1 spectrum from Eurocode 8 [CEN, 20021. The design peak ground acceleration in bedrock is taken to be 0.3 g

and the chosen site class is B, corresponding to very dense sand or gravel, or very stiff clay, with a 30 rn shear wave veldcity in the range from 360 rn/s to 800 m/s. The 5% damped acceleration spectrum is anchored at 0.36 g (the product of the bedrock PGA and the soil factor of 1.2), with a constant acceleration plateau at 0.9 g between 0.15 and 0.5 seconds (Fig. 11). The search for direct matching to the EC8 spectrum was performed on the basis of the average D,,, residual (see Sec. 3.1) on the spectral shape being no greater than 0.09 in the period range from 0.1 to 0.4 seconds, and the PGA of the record lying in the range from 0.26 g to 0.46 g. The period range was chosen because between these limits there is close agreement between the ordinates of EC8 spectrum and the spectral ordinates of the design scenario described below (Fig. 11), and the maximum ordinates are the same in both cases. At longer periods there is considerable divergence between the

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- Design scenario (M6.4, 10 km, 0.6 sigmas)

! I

Period (seconds)

Fig. 11. Response spectra corresponding to criteria used for strong-motion data searches: EC8 class B spectrum anchored to 0.3 g in bedrock (thick line), and a scenario represented by M , 6.4, a stiff soil site at a distance of 10 km, and 0.6 standard deviations above the median motions (thin line).

ordinates of the code and scenario spectra, which is to be expected; the spectral shape in EC8 was calibrated to normalised spectra from European accelerograrns from earthquakes with magnitudes in the range from M, 6 to Ms 7, but with a strong bias towards the larger values [Rey et al., 20021. The search performed on the basis of matching, in an average sense, the ordinates of the EC8 spectrum in Fig. 11 yielded 40 strong-motion records from 22 earthquakes.

The second search is performed on the basis of an earthquake scenario chosen to be representative.of the results that might be yielded from a disaggregation of the 500-year hazard in the seismically active parts of Europe. The scenario is defined by a surface-wave magnitude Ms of 6.4, a source-to-site distance of 10 km, and an exceedance of the median values of spectral acceleration of about 0.6 standard deviations; this value of 0.6 for E corresponds to the 73-percentile ground motion. As for the Eurocode 8 spectrum, the site is characterised by stiff soil with a Vs,30 in the range from 360 to 800 m/s.

The spectral ordinates for this scenario, obtained using the prediction equations of Arnbraseys et ol. 119961 - after smoothing the coefficients with a 114-112-114 running average - are shown in Fig. 11.

As shown in Sec. 3.2, the mod important geophysical parameter for selecting records is earthquake magnitude, and therefore the search was designed to have a small window on magnitude and a larger window on distance. Table 1 shows the number of records recovered from different M-R search windows, including the effect of adding the site classification as a third search parameter, for searches performed

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The Use of Real Earthquake Accelerogrctms 71

Table 1, Numbers of records obtained using different search windows on magnitude and distance; numbers in parentheses are records from stiff soil sites.

Distance MS n/rg

Ranges 6.04.8 6.1-6.7 6.2-6.6 6.3-6.5 - - -

d:&sUkm 377(121) 315(106) . 213(76) 72(28) d: 0-30 km 229 (58) 186 (47) 114 (25) 40 (13) d: 0-20 knl 157 (37) 121 (30) 69 (15) 26 (10) d: 5-15 krn 94 (21) 72 (17) 37 (9) 12 (6)

on the Imperial College strong-motion data archive, which contains about 7000 records. The final search window, which yielded 55 ground-motion records from 16 earthquakes, was defined by the following limits: 6.2 5 h.l, 5 6.6, 0 5 Rjb 5 40 krn, and stiff soil.

An initially surprising result of the two searches is that there was not a single accelerogram common to the two sets. However, if the basis of the selection criteria are carefully considered, this result is perhaps less unexpected: unless an elastic design spectrum has been obtained fiom a OSHA using median values fiom ground- motion prediction equations, the scenario will always include an E term that will be responsible for an appreciable proportion of the spectral amplitudes. For PSHA in which no truncation is applied to the scatter in the ground-motion prediction equations, the contribution from E will grow with the return period [e.g. Restrepo- VBlez & Bommer, 20031. Only one earthquake was common to the results obtained from the two searches (the 15 October 1979 Imperial Valley, California, main shock) but each search picked up different records from this event. The M-R distributions, and the site classifications for the records obtained by spectral matching, are shown in Fig. 12.

The objective of the exercise is to produce an optimal suite of 10 accelerograms born each dataset, for which there is a good match with the elastic spectral ordinates specified for design, and for which there is low variability amongst the spectral ordinates of the scaled records; an additional, but less critical objective, is to obtain the suite with the least amount of scaling possible. The match is defined by the average ordinates of 10 scaled spectra not being below the target spectrum in the period range from 0.1 to 0.4 seconds. To begin with, there is as many as five times the' required number of records in each data set, so some preliminary "manual" pruning of the selections can be applied. This is done for each data set, and then the reduced data sets are examined in order to perform further selection and then to scale to the design spectrum; for both cases, the ordinates of the Eurocode 8 spectrum are taken as the target.

For simplicity, it is assumed that only a single horizontal component of motion is required for each dynamic analysis. An additional clarification is required at this point, related to the definition of the horizontal component of motion, as noted in Sec. 4.1. Spectral ordinates predicted by the equations of Ambraseys et al. (19961

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72 J . J . Bommer E4 A. B. Acevedo

unknown A soft soil V stiff soil O alluvium O rock

Distance to surface projection of rupture (km)

Distance to surface projection of rupture (km) '

Fig. 12. Magnitude-distance distributions of records recovered from searches by matching to ordinates of code spectrum (upper) and by use of a magnitudedistance window and specification of the site classification (lower).

correspond to the envelope of the two horizontal components and the calibration of the Eurocode 8 spectrum used a similar definition. For the records obtained from the search in terms of magnitude, distance and site classification, the larger component is chosen on the basis of the larger spectrum intensity; in most, but not all, cases this is also the component with the larger PGA. For the search performed

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The Use of Real Earthquake Accelerogmms 73

in terms of spectral ordinates, individual components are returned hence the issue of selecting components does not arise.

5.2 . Records selected by spectral ordinates

The search based on minimising the average SRSS residuals between the actual and target spectra yielded 42 single component records. A few of these can be elimi- nated on the basis of representing outlier cases or other reasoning. For example, one of the soft soil stations is actually classified as "very soft" (Vs,30 < 180 m/s), so this may be eliminated (although it is noted in passing that there also exists a possibility of some of the stations that are classed as "alluvium" or "unknown" being equally soft). Additionally, the recording obtained at a distance greater than 70 Ism can be dropped; this is from a site classified as alluvium and to produce such high accelerations (the PGA is 0.27 g) at such a distance it is likely that the soil deposits are very soft. There is a group of six recordings, all obtained at very short distances, born the A.l, 7.6 1999 Chi-Chi (Taiwan) earthquake; since the sce- nario under study is nominally for southern European conditions, this event may be considered excessively large and therefore these records can also be eliminated. The recordings from smaller magnitude earthquakes can also be eliminated since these are very unlikely to be of engineering significance by virtue of their relatively low energy content and limited number of cycles of motion; a threshold of A& 5.7 is applied for this criterion, only retaining records from larger events. The applica- tion of these criteria removes 13 of the records, leaving a total of 29 records horn 19 earthquakes.

A possible way forward is to simply select from'the remaining records those recorded on sites 'classified as stiff soil, thus matching one of the features of the specified design scenario. This would yield 11 records, which would be easily handled and make the optimum selection of 10 records relatively straightforward. However, as was mentioned earlier, site conditions may not be one of the most critical selection criteria and it is preferable to retain all 29 records while considering other criteria.

The records have been selected on the basis of a search that considered both an approximate match to the design PGA of 0.36 g and an approximate agreement with the spectral shape in the period range from 0.1 to 0.4 seconds. A very close match of both the PGA value and the spectral shape is likely to indicate a record that will require little adjustment. An additional indicator of the match between the record and target spectrum is the ratio of spectrum intensity in the period range 0.1-0.4 s; the value of the spectral intensity in this range for the EC8 spectrum is 10.45 cm. Figure 13 shows the normalised average D,,, residuals plotted against the ratios of record-to-target PGA and record-to-target spectrum intensity for the remaining 29 records.

The best fit to the target spectrum is defined by a very low value of the average residual combined with a value of both PGA and spectrum intensity ratios very close to unity. The data manipulation may be made easier if a few records with

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Record-to-Target PGA Ratio

*

Record-to-Target Spectrum Intensity Ratio

unknown A soft soil V stiff soil [7 alluvium 0 rock

Fig. 13. Measures of matching between the record and target spectra for the accelerograms selected on the basis of spectral matching.

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The Use of Real Earthquake Accelerogmms 75

relatively high average residuals and ratios significantly different from unity are eliminated. On this basis, three records with average residuals greater than 0.078 and PGA ratios smaller than 0.8 are removed from the suite. The cluster of three records with similarly high residuals but with PGA residuals above 1.2 are not removed for the following reason: the ultimate goal is a suite of 10 records whose average spectrum does not fall below the target EC8 spectrum in the period range from 0.1 to 0.4 seconds. Since in both plots of Fig. 13 there are far more points with ratios lower than unity than greater than one, it is decided to retain these three records with rather high ratios of PGA and spectrum intensity, since they may contribute to finding a good average match with the target spectrum without scaling. This is done herein in order to reflect current code procedures.

The reduced data set now consists of 26 component records from 17 earthquakes. The next step is to find the average spectral ordinates of groups of 10 records in order to identify if there are combinations that wiIl produce a mean spectrum whose ordinates in the range 0.1-0.4 s are always equal to or greater than those of the EC8 spectrum. It is found that in fact there are several sets of records whose mean ordinates, in the period range of interest, are always above the EC8 spectrum ordinates, hence no scaling is required a t all (in fact, scaling factors of less than unity could be applied to reduce the amount by which the ordinates of the design spectrum are exceeded). Figure 14 shows the mean ordinates of 10 of the records compared with the elastic design spectrum. All of the combinations that produce mean spectral ordinates above the target spectrum include no fewer than six records from a suite of 10 that from amongst the 26 that have particularly high

1 Li 1 - EC8spectmm 1 Mean of 10 records

I I

Period (seconds) Fig. 14. Mean ordinates of a suite of 10 accelerograms compared with the EC8 target spectrum; for this combinations of records, no scaling or adjustment is required.

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76 J . J . Bommer & A. B. Acevedo

amplitudes. All but one of these 10 records are from earthquakes of magnitude 6.6 or greater, with four coming from events of magnitude of 7.2 or above. The single record from a smaller magnitude ( M , 5.9) earthquake was recorded at 5 km on soft soil. In conclusion, this exercise reinforces the fact that selecting in terms of matching to elastic spectral ordinates only is unlikely to result in accelerograms that are consistent with the underlying design earthquake scenario (which, it is recalled, in this case is l\il, 6.4 at 10 km and a s t 8 soil site). This is the fundamental problem in the specification of acceleration time-series in current seismic design codes, as discussed previously.

5.3. Records selected by geophysical criter ia

The search for records horn stiff soil sites with magnitudes in the range fils 6.2- 6.6 and distances in the range from 0 to 40 krn yielded 55 accelerograms from 16 earthquakes; a point worth noting is that 20 of the component records are from a single event, the 1983 Coalinga (California) earthquake, all recorded in the distance range from 31 to 40 km.

There are several options for reducing the dataset of 55 records to a more man- ageable number, but care must be taken not to remove potentially useful records in the process.

The 10 records with distances closest to scenario distance of 10 km are from six earthquakes, with four of the records coming from a single event. As was mentioned earlier, it is advisable not to have any single earthquake event or recording station excessively represented, whence it would be advisable to drop at least two of the four records from the same earthquake, and choose others, albeit from greater dis- tances, in their place. This procedure will be re-visited after considering alternative strategies.

Another way to proceed with the search would be to now include the style-of- faulting as a fourth criterion, which would generally be feasible because in most regions of the world the dominant rupture mechanism for nearby seismic sources is usually known with some confidence. However, there are two arguments against doing this, as indicated previously, the first being that the style-of-faulting does not exert such a strong influence on the ground motion, and the second being that the inclusion of this parameter can be very restrictive in terms of the number of records that will be retained. There are six records from two normal faulting earthquakes, 13 records horn six strike-slip events, and 35 records from six reverse ruptures, plus a single record from an event classified as oblique.

An alternative way to proceed is now to search within the selected suite of 55 records using the spectral matching criteria that were used to search the suite of accelerograms examined in the preceding subsection. Figure 15 shows the same information as Fig. 13 but for the records selected on the basis of magnitude, distance and site classification. Since all the records are from the same site class, different symbols are used to represent the style-of-faulting of the earthquakes. The

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The Use of Real Earthquake Accelerograms 77

A normal oblique

V strike-slip 0 reverse

Record-to-Target PGA Ratio

Record-to-Target Spectrum Intensity Ratio

I ? . 5 Measures of matching between the record and target spectra for the accelerograms selected on the basis of geophysical parameters.

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larger component of each accelerogram is selected on the basis of the larger value of spectral intensity in the 0.1-0.4 s range, which in all but a few cases is also the component with the larger value of PGA; it is found that for those situations where this is not so, the two PGA values are generally quite similar. The ratios of the record PGA and SI to those of the target spectrum .are plotted against the normalised average residual, as was done previously for the other data set.

Comparison of Figs. 13 and 15 shows that the agreement with the target spec- trum for the records selected on the basis of the earthquake scenario is much poorer, with only two records in the residual-ratio space covered by the records selected

' by spectral matching. This result, however, is hardly surprising, since most of the selected records are from much longer distances than the target of 10 km (Fig. 12) and since the target spectrum corresponds to the 73-percentile motions, only one- in-four of the records obtained at the scenario distance would be expected to match the target spectrum. Therefore, it can be concluded at this point that it is very unlikely that a suite of records can be found whose mean ordinates will match the elastic design spectrum without some form of scaling being applied to the records.%

Record-to-Target Spectrum Intensity Ratio

Fig. 16. Measures of matching between the record and target spectra for the accelerograms with Ad9 in the range 6.2-6.6 and recorded at distances no greater than 25 km. Different symbols are used to identify records coming from either the same earthquake or the same station.

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The Use of Real Earthquake Accelemgmms 79

Figure 16 shows the spectrum intensity ratios and average norrnalised residuals, without distinction by style-of-faulting, for only those records recorded within 25 km of the source. Comparison of Figs. 15 and 16 shows that the use of a smaller window on distance mainly removes the very low amplitude records, although a few records with favourable characteristics have also been lost. Nonetheless, this reduced selection of 17 records is retained since it has been obtained in a fashion that is likely to be followed in routine data searches. The 17 records come from nine earthquakes, and two of them are recordings horn one station of two consecutive events separated by a few hours (each shown as an asterisk in the plot). Immediately there is the issue that in selecting the final 10 records it is desirable not to have any one event or station contributing excessively to the suite of accelerograrns, whence the different symbols used in Fig. 16. Application of a minimum average norrnalised residual of 0.18 - double that used in the spectral matching selection described previously - reduces the number of accelerograms to 13 records horn nine earthquakes, with one station represented twice and one earthquake represented four times.

The basic characteristics of the 13 selected records are presented in Table 2. One additional record is also brought into the selection at this stage, bringing the total number to 14. This record is from a distance of 37 km, and therefore was excluded by the limit of 25 km used to reduce the data set. However, as can be appreciated from Table 2, despite the distance, the record displays high values of PGA and SI (0.392 g and 9.88 cm, respectively); these values are close to the target values of 0.36 g and 10.45 cm. This record is included to illustrate the point made

Table 2. Records selected by magnitude (M, 6.2-6.6), distance less than 25 km except No. 14 (see text), and stiff soil sites.

No. Date Time ~ e c h l 1 Station R:, D,ms SI PGA F~

Matahina Dam

Lake Hughes 12 Pasadena J f L

Cerro Prieto

OTE, Aegion

Cerro Prieto

Bar - Skupstina Tivat - Aerodorm Kotor - Naselje Kotor - Zovod

Centerville Beach

Centerville Beach

Big Bear CC

Fortuna Fire St.

Notes: 1 - rupture mechanism: N - normal, R - reverse, SS - strike-slip. 2 - Joyner-Boore distance (horizontal distance from surface projection of rupture) in km. 3 - scaling factors applied to records (see Fig. 19).

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in Sec. 3.2 regarding the recommendation to impose strict limits of magnitude but not necessarily on distance. This record is assigned the identifier no. 14; application of the minimum D,,, value of 0.18, discussed above, would not have removed this record from the dataset.

The variation in spectral amplitudes amongst the selected records is very large. The largest ordinates correspond to the recording of the 1995 Aegion earthquake in Greece (No. 5), which is a clear case of forward rupture directivity [Lekidis et al., 19991, and the recording of the 1992 Big Bear earthquake in California (No. 13), which produced ground motions that were on average twice as high as expected for an earthquake of this magnitude in California [Cramer and Darragh, 19941. The lowest amplitudes correspond to the recordings of the 1979 Montenegro earthquake in Yugoslavia (Nos. 7-10), all of which were obtained in the backward directivity zone according to the orientation of the fault rupture plane [Boore et al., 1981].

As before with the records selected on the basis of spectral ordinates, groups of 10 accelerograms were selected from amongst the 14 candidate records in Table 2 and their mean ordinates compared with the ordinates of the design spectrum. For each grouping, the scaling factor required to bring the minimum ordinate of the average spectrum to the level of the EC8 spectrum was calculated; although strict limits on scaling are generally not warranted, as discussed previously, it would nonetheless be desirable to have a scaling factor as close to unity as possible. Fig- ure 17 shows the mean spectra of a suite resulting in a low scaling factor of just 1.158. Figure 18 shows a suite that includes record No. 14, which allows an even lower factor (L.138) to be applied. However, in both cases this is achieved through the inclusion of records Nos. 5 and 13, which exceed appreciably the target spec-. trum and whose ordinates will be raised even higher by the application of the scaling factors.

In general, discussions of the issue of scaling suites of record have focused on identifying the scaling factor required to ensure that the average ordinates do not fall below the target spectrum and then applying this factor to each of the records. An alternative is to find the optimum combination of records and individual scaling factors to simultaneously consider the very important criterion of minimal disper- sion amongst the spectral ordinates of the scaled records; an algorithm could be developed to at least partially automate this process but for illustrative purposes herein a suite of individually scaled records has been prepared manually. Several of the selected records in Table 2 have response spectra that are in good agreement with the target EC8 spectrum shown in Fig. 11, hence these are retained without scaling. Some of the stronger records are then added to the suite, and their ordi- nates reduced by applying scaling factors of less than unity. Finally, other records are added in, scaled up from their original amplitude, to achieve the required match with the target spectrum. Table 2 presents one such combination and the scaling factors applied; Fig. 19 compares their average spectral ordinates with the EC8 target spectrum. More optimal combinations of records and scaling factors could be identified (the search is time-consuming if not automated), but even the results

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The Use of Real Earthquake Accelerogmms 81

Average of 10 records

Period (seconds) Fig. 17. Combination of 10 records whose mean ordinates are close to the target spectrum, using only records from less than 25 km.

Period (seconds) Fig. 18. Combination of records whose mean ordinates are close to the target spectrum, including record No. 14 (see Table 2).

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82 J . J . Bommer & A. 8. Acevedo

1

1 I i I I

0.2 0.4 0.6- 0.8 1

Period (seconds) Fig. 19. Comparison of average spectral ordinates with EC8 target spectrum and the suite of 10 individually scaled records identified in Table 2.

s - 0

Records scaled as group .- 3-111 Scaled Individually

.- . Records selected by matching spectrum

Period (seconds) Fig. 20. Comparison of coefficients of variation, against period, for the suites of 10 records whose average ordinates are presented in Figs. 14, 18 and 19.

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The Use of Real Earthquake Accelemgmms 83

in Table 2 are encouraging: as many as four of the 10 records are retained at their natural scale, and, as noted below, a relatively low level of dispersion is achieved.

As has been mentioned previously, a key issue in compiling a suite of records to be used in dynamic analysis is the dispersion amongst the scaled records. In fact, the real issue is the dispersion in the inelastic structural responses, but this is dependent upon the specific structural model employed and is therefore beyond the scope of this study. Figure 20 shows the variation of COV (coefficient of variation) with response period for the suites of scaled records whose average spectral ordinates are shown in Figs. 14, 18 and 19. The loweit dispersibn is clearly that of records that were selected on the basis of matching spectral ordinates (Fig. 14). This does not, however, militate against the use of seismological criteria for the selection of records since the dispersion of the inelastic responses obtained using the latter records may well be higher due to the greater variation in the strong-motion durations. However, it would still be desirable to reduce the COV values for the records selected on the basis of seismological and geophysical parameters. Figure 20 clearly shows that the dispersion of the records that are individually scaled after selection by seismological criteria is not much larger than that associated with the records selected on the basis of matching the target spectrum, with the advantage of more consistent durations. Of course, even lower COV's could be obtained using the adjustment procedures described in Sec. 4.2, but at the expense of creating less realistic ground motions.

6. Discussion and Conclusions

6.1. Strong-motion data

Real earthquake accelerograms are clearly a viable option for providing input to dynamic analysis of structures, being more realistic than spectrum-compatible ar- tificial records and easier to obtain than synthetic accelerograms generated from seismological source models. Real accelerograms are increasingly available to engi- neers through CD-ROM collections and Internet sites, although the available search tools associated with many of these data sources could be improved. Depending upon the specific application, and particularly the sensitivity to displacements or to long-period motions, the engineer may need to pay particular attention to the signal processing to which the records have been subjected. In cases where displace- ments or long- period spectral ordinates are important, the preferred options may be either to access records from the PEER databank or to obtain uncorrected records and apply an appropriate correction technique for baseline errors and long-period noise.

This paper is not, however, intended to present a case against the use of syn- thetic accelerograrns but rather to address the issues related to the use of real accelerograms and to provide some guidelines on selection and scaling. We view synthetic records as a complement to real accelerograms and each will have its own merits for different applications. Certainly synthetics are an attractive choice for

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scenarios (combinations of magnitude, distance and site classilication) not covered by the existing empirical database, although by definition synthetic accelerograms have not been validated for these situations.

The geographical distribution of the global strongmotion databank is very un- even, and for projects located in regions of sparse or indeed no strong-motion data it is necessary to make the assumption that regional differences in ground-motion characteristics are sufliciently small to allow records obtained in one country to be used for design or analysis in another. Clearly distinction needs to be made between crustal and subduction earthquakes, and within the latter category, be- tween in-slab and interface events, and caution must be exercised if accelerograms horn seismically active areas are to be used in low seismicity stable continental re- gions. However, amongst seismicdy active regions, the implicit assumption made in this study is that regional differences amongst the ground motions horn crustal earthquakes in different regions are, for similar combinations of magnitude, depth, style-of-faulting, distance and site classification, sufficiently small to be ignored. This is a topic that is clearly worthy of further research, but there is currently no convincing evidence to invalidate the assumption.

6.2. Selection criteria

In order to make use of real accelerograms in dynamic analysis, selection criteria are required to obtain a suitable ensemble of records. These criteria will clearly de- pend on the information available to the engineer regarding the underlying seismic hazard, but it is preferable to base the search on a specific earthquake scenario. The authors recommend that the search be based on achieving a good match (to within 0.2 units) with the design earthquake magnitude, but that, if necessary, reasonably large mismatches with the target distance can be tolerated, since these can be compensated for by application of linear scaling factors, which do not nec- essarily need to be limited to the often cited ranges that are part of the "folklore" surrounding this subject. A match with the site classification of the project is also highly desirable but not necessarily essential and this criterion can be relaxed if few records with reliable site classifications are available; an alternative formulation of

. the problem is to exclude records horn sites with very different classification from that of the project site. The inclusion of style-of-faulting as a fourth search param- eter, with the inevitable reduction of available records, is not recommended at this time, unless the search in terms of magnitude, distance and site classification has yielded a sufficiently lacge number of records.

There are studies that suggest that close matching between the earthquake magnitude of the design scenario and the records is not necessary, most notably the award-winning study by Shome et ol. [1998]. Our opinion is that it has yet to be shown conclusively that earthquake magnitude - and therefore by impli- cation strong-motion duration - has a negligible influence on inelastic structural response beyond controlling the spectral acceleration a t the fundamental period of

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The Use of Real Earthquake Accelemgrums 85

the structure. Such a conclusion will depend on large numbers of studies considering a wide range of realistic structural models. Martinez-Rueda [I9981 urges caution in extrapolating his conclusions, which are based on inelastic analysis of SDOF models, to MDOF structures. Kappos and Kyriakakis [2000] examine more realistic MDOF structural models, but as has been pointed out in this paper, their analysis has some shortcomings in terms of the characterisation of the input. The conclusions made by Shome et al. [I9981 are also based on analysis of MDOF structural models, but only one model is used, representing a steel structure - which can be expected to be less affected by duration than a reinforced concrete structure [e.g. Jeong and Iwan, 19881 - of 5 storeys, and for the ,damage metric based on dissipated energy their conclusion of duration exerting a negligible influence did not hold. Bornmer et al. [ZOO41 studied the inelastic response of a series of masonry structures to a large suite of strong-motion accelerograms and correlated the damage, measured in terms of the loss of initial strength, with the average ordinate of the elastic acceleration spectrum from initial period of the structure to a period about three times greater. The study showed that some of the scatter in this correlation could be explained by differences in the strong-motion duration of the records. However, that study was focused on the assessment of existing vulnerable building stock rather than the earthquake resistant-design of new constructions.

6.3. Matching records to the target spectrum

Once an initial search in terms of magnitude, distance and site classification has been performed, depending on the number of recordsretrieved, further pruning then needs to be carried out to acquire the number of records deemed necessary to obtain stable results from the inelastic dynamic analyses. If there are far more records than actually needed, the obvious choice would be to apply a second sweep of the search using more restrictive criteria, such as a smaller distance range or insisting on a close match with the site classification. The authors recommend that as an alternative the user should consider performing a search within the results of the first sweep through the database in terms of matching spectral shapes and perhaps, but less importantly, spectral amplitudes. At this stage it is also advisable to pay attention to the numbers of records coming from individual recording stations, and ensuring that none is over-represented in the final suite of acceleration time-series.

The appropriate number of real accelerograms required to obtain stable mea- sures of inelastic response is a subject on ongoing research and debate, but estimates generally fall in the range horn seven to ten. The practice of using small numbers of records and then taking the maximum inelastic response should be abandoned; if only a small number of records is to be used, these should be adjusted using programs such as RASCAL or RSPMATCH in order to obtain a closer match with the target spectrum. For larger number of records, the current practice of scaling records so that their mean ordinate matches or exceeds the target elastic spectrum is a reasonable approach, but it has been shown that careful manual selection of

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86 J. J . Binnmer El A. B. Acevedo

subsets of records, and the application of individual scaling factors, can result in a good match with the target spectrum and reduced dispersion amongst the scaled records, as well as allowing many of the records to be used either at natural scale or with only limited adjustment of their original amplitudes. It must be pointed out, however, that this will not hold for situations with very high amplitudes of design motions that may arise for the low annual frequencies of exceedance specified for critical projects.

Executing fully dynamic non- linear structural analyses is t ime-consuming (and therefore costly) in engineering practice and the use of spectrum-compatible records, which allow fewer rum to be made, will often be preferred by design engineers. Our recommendation is that in such cases use should be made of "intelligent artificial" records - obtained by adjusting real accelerograms (see Sec. 4.2) - rather than artificial time-series generated from white noise. In such cases, the guidelines pre- sented in this paper are still applicable for selecting the seed accelerograms from which the intelligent artificial records will be produced.

6.4. Code applicutions

In many cases, searching records in terms of earthquake scenarios will not be pos- sible because the engineer will only have access to the design response spectrum without any knowledge of the underlying hazard calculations. Matching PGA and the spectral shape has been shown to be a superior approach to matching spectral intensities, but regardless of the search parameters there is likely to be little control on the duration - especially if the matching is focused on short-period spectral ordinates - and hence there may be a large dispersion in the results of inelastic analyses.

In nearly all current seismic design codes the earthquake actions are represented by an approximation to a uniform hazard spectrum (UHS) obtained from a PSHA, hence if accelerograms are to be selected in terms of magnitude and distance the codes will need to present, in some form, the controlling earthquake scenarios to the users. The disaggregated hazard can be displayed as supplementary maps showing controlling scenarios for spectral ordinates at different periods [e-g. Harmsen et a[., 19991; if the code drafters were prepared to surrender the UHS representation, then an alternative would be to actually replace current zonation maps in codes with maps showing contours of hazard-consistent magnitude and hazard-consistent distance, from which the spectral ordinates can then be calculated [Bommer, 2000). In view of the degree of approximation in the specification of earthquake actions in seismic design codes, it would probably be feasible to simply present, in tabular form, the magnitude-distance pairs defining the controlling earthquake scenarios for each seismic zone. A useful extension to this would be for the code to provide a list of suitable records identified to match each of these scenarios; the records themselves, perhaps pre-scaled to ensure matchmg with the design elastic spectrum, could be made available either on a CD- RObI distributed with the code or else accessed

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The Use of Real Earthquake Accelemgmms 87

through an Internet site maintained by the code authority or another approved agency. The complementary relationship between current US codes and the USGC disaggregation web site is a good model for such a solution.

Further work is needed to establish definitive criteria for selecting and scaling real accelerograms, systematically exploring the influence of different selection crite- ria and scaling factors on the inelastic response of realistic multi-degree of heedom structures. In the meantime, engineers will need to continue to make judgements regarding the degree to which the acceleration time-histories used in structural analyses should reflect the characteristics of recorded motions, and to balance this with the time and cost constraints on the number of inelastic dynamic analyses that can be performed for an engineering project.

The authors firstly wish to express their gratitude to Dr John Douglas for his interest in the work and for providing us with the current statistics of the European strong-motion database. Additional thanks are due to Dr Douglas for carrying out the data searches using the beta-version of his CD-ROM based search tool; the opportunity to employ this facility before its general release is greatly appreciated.

We also express our thanks to Dr Rui Pinho and Dr Alain Pecker who read and commented constructively on an early version of the manuscript. The second version of the paper was further improved by very helpful comments from Edmund Booth and Jonathan Hancock; particular thanks are due to Juliet Bird and Luis Fernando Restrepo-V61ez7 who both provided useful reviews of two different versions of the manuscript. Very thorough reviews by Dr David Boore, Dr Norm Abrahamson and an anonymous reviewer, all of which significantly improved the paper, are also noted with special gratitude. The first author also wishes to acknowledge the insights obtained from discussions of the issues addressed herein with Dr Paul Sornerville.

We are also grateful to Dr Walt Silva, for providing us with the RASCAL computer code, and to Dr Norm Abrahamson, for providing the RSPMATCH code. We also extend our thanks to Melinda Squibb for providing us with information about the COSMOS database.

References

Abrahamson, N. A. [I9931 "Spatial variation of multiple support inputs ," Proceedings of the First US Symposium on Seismic Evaluation and Retrofit of Steel Bridges, Univer- sity of California at Berkeley, 18 October.

Abrahamson, N. A. and Shedlock, K. M. (19971 "Overview," Seismological Research Letters 68(1), S 2 3 .

Abrahamson, N. A. and Silva, W. J. [I9961 "Empirical ground motion models," Report to Brookhaven National Laboratory.

Abrahamson, N . A. and Silva, W. J. [1997] "Empirical response spectral attenuatidn relations for shallow crustal earthquakes," Seismological Research Letters 68(1), 94-127.

Dow

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ded

by [

New

Yor

k U

nive

rsity

] at

11:

52 1

7 O

ctob

er 2

014

88 J. J. Bmrner & A. B. Acevedo

Ambraseys, N. N and Bommer, J. J. [I9901 "Database of European strongmotion records," European Earthquake Engineering 5 (2), 18-37.

Ambraseys, N. N and Bommer, J. J. I19911 "Uniform magnitude re-evaluation for the strong-motion database of Europe and adjacent areas," European Earthquake En@- neering 4(2), 3-16.

Ambraseys, N. N., Douglas, J., Rinaldis, D., Berge-Thierry, C., Suhadolc, P., Costa, G., Sigb jomsson, R. and Smit , P. [ZOO41 "Dissemination of European strong-mot ion data, Vol. 2,". CD-ROM Collection, Engineering and Physical Sciences Research Council, United Kingdom.

Ambraseys, N. N., Simpson, K. A. and Bommer, J. J. [1996] "The prediction of horizontal response spectra in Europe," Earthquake Engineering & Structuml Dynamics 25, 371400.

issemina- Ambraseys, N. N., Smit, P., Berardi, D., Cotton, F. and Berge, C. [2000] "D' tion of European strong-rnotion data," CD-ROM Collection, European Commission, Directorate-General XII, Environmental and Climate Programme, ENVPCT97-0397, Brussels, Belgium.

Ambraseys, N. N., Smit, P., Douglas, J., Margaris, B., Sigbjljmsson, R., ~lafsson, O., Suhadolc, P. and Costa, G . [2003] "Internet site for European strong-motion data," Bolletino di Geofisim Twrica ed Applicata, in press.

ATC (19991 "Proceedings: Workshop on improved characterization of strong ground shak- ing for seismic design," ATC-35, Applied Technology Council, Redwood City, Cali- fornia, 70 pp.

Bazzurro, P. and Cornell, C. A. (1 9991 "Disaggregation of seismic hazard," Bulletin of the Seismological Society of America 89, 501-520.

Beresnev, I. A. and Atkinson, G. M. (19981 "FINSIM - A FORTRAN program for simu- lating stochastic acceleration time histories from finite faults," Seismological Research Letters 69(l), 27-32.

Bommer, J. J. [2000] uSeismic zonation for comprehensive definition of earthquake ac- tions," Pmceedings of the Sixth lnternational Conference on Seismic Zonation, Palm Springs, California.

Bommer, J. J. and Ambraseys, N. N. [I9921 "An earthquake strong-motion databank and database," Proceedings of the Tenth World Conference on Earthquake Engineering, Madrid 1, 207-210.

Bommer, J. J. and Martinez-Pereira, A. [I9991 T h e effective duration of earthquake strong motion," Journal of Earthquake Engineering 3 (2), 127-172.

Bommer, J. J. and Martinez-Pereira, A. [2000] "Strong-motion parameters: Definition, usefulness and predictability," Proceedings of the Twelfih World Conference on Earth- quake Engineer6ng, Auckland, Paper No. 206.

Bommer, J. J. and Ruggeri, C. (20021 T h e specification of acceleration time-histories in seismic design codes," Eumpean Earthwake Engineering 16 (I), 3-1 7.

Bommer, J. J. and Scott, S. G. [2000] "The feasibility of using real accelerograms for seismic design," In: Implications of Recent Earthquakes on Sezsnazc Risk, eds. A.S. Elnashai and S. Antoniou, Imperial College Press, pp. l l b l 2 6 .

Bommer, J. J. Acevedo, A. B. and Douglas, J. [2003a] 'The selection and scaling of real earthquake accelerograms for use in seismic design and assessment ," Pmceedzngs of ACI International Conference on Seismic Bridge Design and R e t d t , American Concrete Institute.

Bommer, J. J., Douglas, J. and Strasser, F. 0. [2003b] "S tyle-of-faulting in pound-motion prediction equations," Bulletin of Earthquake Enginee7ing 1(2), 171-203.

Dow

nloa

ded

by [

New

Yor

k U

nive

rsity

] at

11:

52 1

7 O

ctob

er 2

014

The Use of Real Earthquake Accelerograms 89

Bommer, J. J., Magenes, G., Hancock, J. and Penazzo, P. [2004] "Influence of strong- motion duration on the seismic response of masonry structures,;' Bulletin of Earth- quake Engineering 2(1).

Bommer, J. J., Scott, S. G. and Sarma, S. K. [I9981 "Time-history representation of seismic hazard ," Proceedings of the El eventh European Conference on Earthquake Engineering, Paris.

Bornmer, J. J., Scott, S. G. and Sarma, S. K. [2000] 'LHazard-consistent earthquake sce- narios," Soil Dynamics & Earthquake Engineehg 19, 219-231.

Boore, D. M. [2003] "Simulation of ground motion using the stochastic method," Pure and Applied Geophysics 160, 635-676.

Boore, D. M. [ZOO41 "Can site response be predicted?" thls volume. Boore, D. M., Stephens, C. D. and Joyner, W. B. (20021 "Comments on baseline correc-

tion of digital strong-motion data: Examples from t he 1999 Hector Mine, California, earthquake," Bulletin of the Seismological Society of America 92(4), 1543-1560.

Boore, D. M., W. B. Joyner and Fumal, T. E. (19971 "Equations for estimating horizontal response spectra and peak acceleration from western North American earthquakes: A summary of recent work," Seismological Research Letters 68(1), 128-153.

Boore, D. M., Sims, J. D., Kanamori, H. and Harding, S. (1981) "The Montenegro, Yu- goslavia earthquake of April 15, 1979: Source orientation and strength," Physics of the Earth and Planetary Interiors 27, 133-142.

Campbell, K. W. [I9971 "Empirical near-source attenuation relationships for horizon- tal and vertical components of peak ground acceleration, peak ground velocity, and pseudo-absolute acceleration response spectra," ~eismolo~z&l Reseuxh Letters 68(1), 154-179.

CEN [ZOO21 Eurocode 8: Design of structures for earthquake resistance. Part 1: Gen- eral rules, seismic actions and rule for buildings. Draft No. 5, May 2002, Document CEN/TC250/SC8/N317, Cornit6 Europkn de Normalisation, Brussels.

Chapman, M. C . [I9951 "A probabilistic approach to ground-motion selection for engi- neering design," Bulletin of the Seismological Society of America 85, 937-942.

Cramer, C. H. and Darragh, R. B. [I9941 "Peak accelerations from the 1992 Landers and Big Bear, California, earthquakes," Bulletin of the Seismological Society of America 84(3) 589-595.

Douglas, J. (20031 "Earthquake ground motion estimation using strong motion records: A review of equations for the estimation of peak ground acceleration and response spectral ordinates," Earth Science Revlews 6 1, 43-104.

Gasparini, D. A. and Vanmarcke, E. H. [1979] "Simulated earthquake motions compatible with prescribed response spectra," Evalzlation of Seismic Safety of Buildings Report No. 2, Department of Civil Engineering, MIT, Cambridge, Massachusetts, 99 pp.

Harmsen, S. D., Perkins, D. and Frankel, A. [I9991 "Deaggregation of probabilistic ground motions in the Central and Eastern United States," Bulletin of the Seismological Society of Arnericu 89, 1-13.

Hudson, D. E. [I9791 "Reading and interpreting strong motion accelerograms," EERI Monograph, Earthquake Engineering Research Inst it ut e, Oakland, California.

Idriss, I. M. [I9931 "Procedures for selecting earthquake ground motions at rock sites," NIST GCR 93-625, National Institute of Standards and Technology, Gaithersburg, MD, 35 pp.

IS0 [2003] Petroleum and natural gas industries -.Specific requirements for offshore structures - Part 2: Seismic design procedures and criteria. ISQ/DIS 19901-2, In- ternational Organization for Standardization.

Iyama, J. and Kuwamura, H. (19991 "Application of wavelets to analysis and simulation of earthquake motions," Earthquake Engineering & Structural Dynamics 28, t255-277.

Dow

nloa

ded

by [

New

Yor

k U

nive

rsity

] at

11:

52 1

7 O

ctob

er 2

014

Jeong, G. D. and Iwan, W. D. (19881 T h e effect of earthquake duration on the damage of structures," Earthquake Engineering & Stmctvral Dynamics 16, 1201-121 1.

Kappos, A. J. and Kyriakakis, P. [2000] "A reevaluation of scaling techniques for natural records," Soil Dynamics & Earthpalce Engineering 20, 111-123.

Kaul, Evl. K. [I9781 "Spectrum-consistent time-history generation," ASCE Journal of the Engznee7ang Mechanics DiPrlsh 104(ME4), 781-788.

Kramer, S. L. [1996] Geotechnid Earthquake Engineering, PrenticeHall. Krinitzsky, E. L. [2002] "How to obtain earthquake ground motions for engineering design,"

Engineering Geology 65, 1-16. Krinitzsky, E. L. and Chang, F. K. (19771 "Specifying peak motions for design earth-

quakes," State-of-the-Art for Assessing Earthquake Hazards in the United States, Re- port 7, Miscellaneous Paper S-73-1. US Army Corps of Engineers, Vicksburg, blissis sippi.

Lee, W. H. K., Shin, T. C., Kuo, K. W., Chan, K. C. and Wu, C. F. [2001] "CWB freefield strong-motion data from the 21 September Chi-Chi, Taiwan, earthquake," Bulletin of the Seismological society of America 9 1 (5) , 137&1376.

Leeds, D. J. [I9921 "Recommended accelerograms for earthquake ground motions," State- of-the-Art for Assessing Earthquake Hazards in the United States, Report 28, Miscel- laneous Papers 5- 73-1, US A m y Corps of Engineering, Vicksburg.

Lekidis, V. A., Karakostas, C. Z., Dimitriu, P. P., Pl/largaris, B. N., Kalogeras, I. and Theodulidis, N. (19991 "The Aigio (Greece) seismic sequence of June 1995: Seisme logical, strong motion data and effects of the earthquakes on structures," Journal of Earthquake Engineering 3(3), 34S380.

Lilhanand, K. and Tseng, W. S. [I9881 "Development and application of realistic earth- quake time histories' compatible with multiple-damping design spectra," Proceedings of the Ninth World Confemnce on Earthquake Engzneering, Tokyo-Kyoto 2, 819-824.

Malhotra, P. K. [2003] "Strong-motion records for site-specific analysis," Earthquake Spec- tm 19(3), 557-578.

Martinez-Rueda, J. E. [I9981 "Scaling procedure for natural accelerograms based on a system of spectrum intensity scales," Earthquake Spectra 14(1), 135-152.

Matsurnura, K. [I9921 "On the intensity measure of strong motions related to structural failures," Proceedings of the Tenth World Conference on Earthquake Engineering, Madrid 1, 375-380.

McGuire, R. K. [1995] "Probabilistic seismic hazard analysis and design earthquakes: Closing the loop,", Bulletin of the Seismological Society of America 85, 1275-1284.

McGuire, R. K. , Silva, W. J. and Constantino, C. J. [2001] ''Technical basis for revision of regulatory guidance on design ground motions: Hazard- and risk-consistent ground motion spectra guidelines," NUR EG/CR-6728, US Nuclear Regulatory Commission, Washington D . C .

Mukherjee, S. and Gupta, V. K. [ZOO21 "Wavelet-based generation of spectrum-compatible time-histories," Soil Dynamics & Earthquake Engineefing 22, 799-804.

Naeim, F. and Anderson, J. C. 119961 "Design classification of horizontal and verti- cal earthquake ground motion (1933-1994)," A Report to the USGS, JAMA Report No. 7738.68-96, John A. Martin & Associates, Inc., Los Angeles.

Naeim, F. and Lew, M. 119951 "On the use of design spectrum compatible time histories," Earthquake Spectm 11(1), 111-127.

Nau, J. M. and Hall, W. J. [I9841 L'Scaling methods for earthquake response spectra," ASCE Journal of Structuml Engineering llO(7), 1533-1548.

Preumont, A. [I9841 "The generation of spectrum compatible accelerograms for the de- sign of nuclear power plants," Earthquake Engineering 0 Structuml Dynamics 12(4), 481-497.

Dow

nloa

ded

by [

New

Yor

k U

nive

rsity

] at

11:

52 1

7 O

ctob

er 2

014

The Use of Real Earthquake Acceierogram 91

Reiter, L. [1990] Earthquake Huzard Analysis: Issues and Insights. Columbia University Press.

Restrepo-VBlez, L. F. and Bommer, J . J. [2003] "An exploration of the nature of the scatter in ground-motion prediction equations and the implications for seismic hazard assessment ," Journal of Earthquake Engineering 7(Special Issue 1) , 171-1 99.

Rey, J., Faccioli, E., and Bommer, J. J. [2002] "Derivation of design soil coefficients (S) and response spectral shapes for Eurocode 8 using the European strong-motion database," Journal of Seismology 6 , 547-555.

Row, L. W. [I9961 "An earthquake strong-motion data catalog for personal computers - S MCAT," User ilianual (version 2. U), National Geophysical Data Center, Colorado.

Seed, H. B. and Idriss, I. M. [I9691 "Rock motion accelerograms for high magnitude earth- quakes," EERC Report No. 69-7, Earthquake Engineering Research Center, University of California at Berkeley.

Seekins, L. C., Brady, A. G., Carpenter, C. and Brown, N. [I9921 YXgitized strong-motion accelerograms from North and Central America through 1986," US Geological Survey Digital Data Series DDS- 7.

Shome, N., Cornell, C. A., Bazzurro, P. and Carballo, J. E. [I9981 cLEarthquakes, records and nonlinear responses," Earthquake Spectra 14(3), 46!3-500.

Silva, W. J. and Lee, K. (19871 "WES RASCAL code for synthesizing earthquake ground motions," State-of-the-Art for Assessing Earthquake Hazards in the United States, Report 24, Miscellaneous Paper $73-1. US Army Corps of Engineers, Vicksburg, Mississippi.

Somerville, P. G., Smith, N. F., Graves, R. W. and Abrahamson, N. A. [I9971 lLModification of empirical strong ground motion attenuation relations to include the amplitude and directivity effects of rupture directivity," Seismological Research Letters 68(1), 199-222.

Stepp, C. J. [ZOO01 "Coordination of strong-motion programs and strong-motion data dissemination," P m d i n g s of the Twelfth World Conference on Earthquake Engi- neering, AuckIand, Paper No. 2600.

Stewart, J . P., Chiou, S.-J., Bray, J. D., Graves, R. W., Sornerville, P. G. and Abrahamson, N. A. [2001] "Ground motion evaluation procedures for performance-based design," PEER Report 2001/09, Pacific Earthquake Engineering Research Center, University of California, Berkeley.

Sunder, S. S. and Connor, J. J. [1982] LLA new procedure for processing earthquake strong- motion signals," Bulletin of the Seismological Society of America 72 (2), 643-661.

Trifunac, M. D., Udwadia, F. E. and Brady, A. G. [I9791 'LAnalysis of errors in digitized strong-motion accelerograms," Bulletin of the Seismologicd Society of America 63 ( I ) , 157-187.

Vanmarcke, E. H. [I9791 "Representation of earthquake ground motion: Scaled accelere grams and equivalent response spectra," State-of-the-Art for Assessing Earthquake Hazards in the United States, Report 14, hdiscellaneous Paper S-73-1. US Army Corps of Engineers, Vicksburg, hlississippi.

Wald, D. (19971 "Surfing the Internet for strong-motion data," Seismological Research Letters 68(5), 766-769.

Zeng, Y., Anderson, J. G. and Yu, G. [I9941 "A composite source model for comput- ing realistic synthetic strong ground mot ions," Geophysical Research Letters 2 1(8), 725-728.

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