What is a Poor Quality Strong-Motion Record?

Bulletin of Earthquake Engineering 1: 141–156, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

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What is a Poor Quality Strong-Motion Record?

J. DOUGLASDepartment of Civil and Environmental Engineering, Imperial College of Science, Technology andMedicine, London, SW7 2BU, United Kingdom (E-mail: [email protected])

Received 5 September 2002; accepted 15 January 2003

Abstract. Some accelerograms are affected by non-standard recording and digitization problemsthat mean they are often not used in strong-motion studies. These non-standard problems cannotbe corrected by the standard processing techniques that remove low and high-frequency noise fromthe time-history. Records from analogue instruments are more prone to these problems but evenrecords from digital instruments, which are becoming increasingly common, can be affected bysuch errors. Since all strong-motion data is valuable it is important to know whether any usefulinformation can be obtained from accelerograms that are affected by such problems. This articleexamines whether strong-motion records from analogue instruments that are missing their initial partdue to late triggering of the instrument and also strong-motion records from digital instruments withlow A/D converter resolution can be used for response spectral studies. It is found, by simulating sucherrors on high-quality strong-motion records, that good response spectral ordinates can be obtainedfrom such ‘poor-quality’ records within the period range of most engineering interest.

Key words: accelerograph design, low digitizer resolution, response spectra, S-wave trigger, simu-lated errors, strong-motion records

Abbreviations: PGA – peak ground acceleration; PGV – peak ground velocity; ISESD – internetSite for European Strong-Motion Data; A/D – analogue to digital

1. Introduction

Some accelerograms are affected by non-standard recording and digitization prob-lems that mean they are often not used in strong-motion studies. These non-standardproblems cannot be corrected by the standard processing techniques that removelow and high-frequency noise from the time-history. Records from analogue in-struments are more prone to these problems but even records from digital instru-ments, which are becoming increasingly common, can be affected by such errors.Since all strong-motion data is valuable it is important to know whether any usefulinformation can be obtained from accelerograms that are affected by such prob-lems. Hudson (1979) estimated that the cost of deploying and maintaining suitablestrong-motion networks results in a direct cost for each important strong-motionrecord of about $10,000. Therefore it is vital that the most use is made of allstrong-motion records even if they are of poor quality.

Currently the Internet Site for European Strong-Motion Data (ISESD) (Am-braseys et al., 2002) contains some records affected by such errors and there arealso many records contained within the databanks of the partners of this project that

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are felt to be too poor to be included on the Internet site. Improved knowledge aboutthe importance of errors on commonly used engineering parameters derived fromstrong-motion records would enable a more informed choice as to which recordsshould be placed on ISESD and used in strong-motion studies.

The number of records that are affected by errors contained within the strong-motion databank at Imperial College and elsewhere is likely to be an underestimateof the total number of records worldwide affected by these problems. This is be-cause analogue accelerograms that are affected by recording problems (such asS-wave trigger or termination early in the coda) are less likely to be digitized thangood quality accelerograms, since digitization is a costly procedure. Even if a poorquality record was digitized its dissemination to outside organisations would beless likely and the outside organisations would be less inclined to include poorquality records in their databanks.

2. Types of Errors that Occur

The types of problems that can occur in strong-motion records that cannot be cor-rected by standard processing techniques, are given in Table I. The number andpercentage of records affected by such problems within the Imperial College Eu-ropean strong-motion databank, which currently (as of 7th August 2002) contains3431 records, are also given in Table I.

Cambell (1981), Boore et al. (1993) and Spudich et al. (1999) exclude recordsfrom instruments that triggered on the S-wave; Chapman (1999) excludes recordsof short duration that terminated early during the coda; Sabzetta and Pugliese(1987) use only the first shock of a record if it is a well separated multiple shockand the magnitude and focal parameters refer to the first shock; and Simpson (1996)does not use poor quality records in his study to derive equations for the predictionof strong ground motions. It is likely that other authors have not included certainrecords in their analyses because of their poor quality but have not stated this intheir papers.

This study will concentrate on the effect of the instrument triggering on the S-wave, and hence not recording the initial part of the earthquake and low digitizerresolution. Simpson (1996) gives a description of how some errors (spikes and mul-tiple baseline shifts) can be approximately corrected through careful pre-processingand hence these errors are not considered here. The cause of early termination ofrecords is difficult to determinate and therefore it is difficult to simulate such errorsin a way consistent with the cause of the problem. Therefore it is not consideredhere.

3. Method used

If a record is of poor quality because of recording problems, such as S-wave trigger,then obviously it is impossible to know what the correct record would be and hence

WHAT IS A POOR QUALITY STRONG-MOTION RECORD? 143

Table I. Types of non-standard errors in strong-motion records and the number of records(and the percentage of total databank) affected by such errors in the Imperial CollegeEuropean strong-motion databank.

Problem Number % Comments

Insufficientdigitizerresolution

947 27.6 This is a problem with some records from digital in-struments that have a low bit range so although lowamplitude motions are recorded the accelerations onlyhave a few levels. There are 11 additional records (0.3%of total) from instruments that probably malfunctioned.

S-wave trigger 388 11.3 This is a problem with some records from analogueinstruments because they only start recording oncetriggered by acceleration above a trigger level (often0.005 g to 0.02 g) and they have no pre-event memory.Also it can be a problem with records from digital in-struments if the instrument’s pre-event memory is tooshort. This triggering, and because it takes time forthe instrument to start recording, means that the initialpart of the ground motion is not recorded. The part notrecorded usually contains mainly P waves, which areusually of low amplitudes, but can contain S waves,which could be of high amplitudes.

Insufficientsampling rate

215 6.3 This includes those records with missing sections ofdata points. There are 20 additional records (0.6% oftotal) that are affected by other digitization errors.

Multiplebaselines

129 3.8 This is a problem with some records from analogueinstruments that were digitized in sections and theseparate pieces were not spliced together well.

Spikes 117 3.4 These can often be corrected by either removing thesuspect points or reducing the amplitude of the spikeafter examining the original paper or film record.

Multiple shock 66 1.9 This is of course not an error but care needs to be takenif records are used which contain two well separatedshocks, especially for duration studies.

Earlyterminationduring coda

16 0.5 This problem can occur for records from both ana-logue and digital instruments for a variety of reasons,for example lack of film or memory or instrumentmalfunction.

Clipped 0 0 No records in the databank are thought to be affected bythis problem.

Total 1723 50.2 This is less than the sum of the above totals becausesome records are affected by two or more problems.

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know the exact effect of the error. If the digitization of a record was the cause ofthe errors, and the original film or paper was available, then they could be re-digitized. However, this is time-consuming and is not possible if the original filmis unavailable.

The method used in this study is to select some high-quality strong-motionrecords that are unaffected by the aforementioned problems and then simulaterecords containing predefined errors by modifying the high-quality records. Strong-motion parameters from the records with simulated errors can then be comparedwith the parameters from the corresponding error-free record to estimate the effectof the errors.

A single value in the time domain, such as peak ground acceleration (PGA)and velocity (PGV), is likely to be more greatly affected by errors in the time-history than system-domain quantities depending on the whole time-history, suchas response spectral ordinates.

4. Data used

Figure 1 shows the distribution of good quality records in the Imperial CollegeEuropean strong-motion databank with respect to magnitude and distance.

Figure 1 shows that the most important records from the engineering view-point, i.e., those from short and intermediate distances (d � 50 km) and moderatemagnitudes (M � 5), are less affected by non-standard recording and digitizationproblems. This means that the majority of the most important time-histories forengineering studies in the Imperial College database are of high quality.

High-quality records were chosen from digital instruments, which are assumedto measure the true ground motion and are used as reference time-histories. In total,421 triaxial records were chosen from earthquakes worldwide. The chosen recordscame from A700, A800, ETNA, GSR-12, GSR-16, SSA-1 and SSA-2 instruments.

5. S-wave Trigger

The PGA on a record from an instrument that triggered on the S-wave is a lowerbound to the true PGA that occurred provided that other types of errors are notaffecting the recording. Analogue instruments obviously cannot record horizontalaccelerations whose corresponding vertical accelerations are lower than the triggerlevel.

The specifications for a self-contained three-component mechanical-optical ac-celerograph from the Seismic Engineering Branch of the U.S. Geological Surveyof May 1977 prescribe an adjustable vertical trigger acceleration level of between0.005 g and 0.05 g and an operational rise time of not more than 0.1 s (Hudson,1979). These specifications were followed to select the combinations of triggerlevels and rise times used for this study, see Table II.


Figure 1. Distribution of good quality records in the Imperial College European strong-motiondatabank. Magnitude is Ms unless unavailable when mb, ML or Mw are used instead. Distanceis distance to the surface projection of fault (df ) unless unavailable when epicentral distance(de) is used instead.

Table II. Combination of vertical acceleration trigger leveland operational rise times investigated in this study.

ID Vertical trigger level Operational rise time ( s)

1 Reference record

2 0.005 g 0

3 0.005 g 0.1

4 0.01 g 0

5 0.01 g 0.1

6 0.02 g 0

7 0.02 g 0.1

Each of the selected records were used to simulate what an analogue instrumentwith the trigger levels and operational rise times given in Table II would recordsubjected to the same ground motions as the digital instrument which actuallyrecorded the earthquake. An example of the actual recorded ground motion as well

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Figure 2. Three component time-histories of waveform number 000878 (from Adra-Refugiode la Plaza (Spain), recorded on an SSA-2 instrument, of a Ms = 4.8 earthquake at anepicentral distance of 25 km on 4/1/1994). Top graph shows vertical time-history and thethree acceleration trigger levels considered in this study (see Table II). Middle and bottomgraphs show the two horizontal components with numbers marking the simulated start timesif this ground motion was recorded on an analogue instrument with different trigger levels (seeTable II).

as the start times if the ground motion was recorded on an analogue instrument isshown in Figure 2.

The original record and the simulated time-histories, modelling the recordedground motions obtained by a simulated analogue instrument with a vertical trig-gering mechanism, for the different trigger levels and instrument rise times werethen filtered using cut-offs of 0.05 and 50 Hz. The response spectra for 5% ofcritical damping for each of the recorded and simulated time-histories were thencomputed. An example of the comparison of response spectra for the differentstart times is shown in Figure 3. Figure 3 shows that although the initial partof the time-histories from instruments with no pre-event memory is missing theresponse spectra of the records within the period range of engineering interest(0.1 � T � 2 s) closely match those from instruments that recorded the completeground motion.

To enable an objective choice to be made regarding whether to use or rejecta strong-motion record affected by late triggering, the selected 421 high-qualitytime-histories were analysed in the same way as in the above example.


Figure 3. Comparison of response spectra of waveform number 000878 computed usingdifferent start times (see Table II) for the two horizontal components.

The ability of the time-history from an instrument that triggered during the S-wave of the ground motions to yield a response spectrum that matches the referenceresponse spectrum, as measured by the instrument that recorded the entire groundmotion, is related to the duration of ground motion that was recorded by the simu-lated instrument in proportion to the true duration of the ground motion. However,for actual records that do not include the initial part of the ground motion it isimpossible to know what the true ground motion was. Therefore, in this study the

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Figure 4. Ratio of response spectral acceleration for 5% damping and natural periodsT = 0.2, 0.5, 1.0 and 2.0 s, for time-histories from a simulated instrument with a verticaltrigger acceleration of 0.005 g and a zero operational rise time to the response spectral accel-erations calculated from the original time-histories, against the bracketed duration (thresholdacceleration of 0.005 g) of the simulated vertical acceleration time-history. Squares are forhorizontal components and triangles are for vertical components.

duration of the simulated ground motions was used. Since the instruments simu-lated in this study were triggered by a vertical acceleration above a threshold level(0.005 g, 0.01 g or 0.02 g) it was decided to define the duration of the ground mo-tion as the interval between the first and last times this threshold level was exceededin the simulated vertical acceleration time-history. This measure of strong-motionduration is known as bracketed duration (Bolt, 1973). In fact, because the simulatedinstruments were triggered based on the same threshold acceleration level used forthe calculation of the bracketed duration, the bracketed duration of the originalvertical time-histories and the simulated vertical time-histories are the same, exceptfor the simulated time-histories with a non-zero operational rise time for which thebracketed duration of the simulated time-history is 0.1 s less than the real bracketedduration.

Figure 4 shows the ratio between the response spectral acceleration computedusing the time-histories simulated to come from analogue instruments with a verti-cal trigger acceleration of 0.005 g and a zero operational rise time and the referenceresponse spectral acceleration for the same time-histories plotted against the brack-eted duration (for a threshold acceleration of 0.005 g) of the simulated vertical


acceleration time-history. It shows that good estimates of response spectral accel-eration for the main structural period range of interest (0.2 ≤ T ≤ 2 s) can beobtained even for records from such triggered instruments if the bracketed durationis sufficiently long, about 10 s. This is because only a small number of cycles(about ten) of harmonic motion are required for a linear elastic single-degree-of-freedom system with viscous damping to reach its steady state response (Cloughand Penzien, 1993, pp. 43–44) and the frequency content of the shear-wave portionof strong-motion records (as approximately measured by the bracketed duration)does not change significantly with time for moderate source distances.

Similar graphs to those shown in Figure 4 for the other combinations of triggerlevel and operational rise time listed in Table II are similar to Figure 4, i.e. if thebracket duration is sufficiently long, about 10 s, then good estimates of responsespectral accelerations within the period range 0.2 ≤ T ≤ 2 s can be obtained.Note that because the threshold acceleration used for these other simulated time-histories is much different than that used for the illustrated example (0.005 g), thelimit in bracketed durations found for the different example instrument types arenot comparable.

Bracketed durations (using a threshold acceleration of 0.02 g) of 10 s are onlylikely to be recorded for earthquakes with magnitudes greater than about Ms = 6.5,using log D = −1.26 + 0.35 Ms , where D is bracketed duration, from Papazachoset al. (1992) who assume that bracketed duration is independent of source distance.A similar calculation for bracketed duration based on 0.005 g cannot be madebecause there are no equations in the literature for the prediction of bracketedduration using a threshold acceleration of 0.005 g. However, it is likely that evensmall magnitude earthquakes will have a bracketed duration of more than 10 susing a threshold acceleration of 0.005 g. Therefore for records from instrumentswith a vertical trigger acceleration of 0.005 g that are missing their initial part ofearthquakes with a magnitude of engineering significance (Ms � 4) are likely togive good estimates of response spectral acceleration for the period range of mostengineering interest.

If an instrument employed a horizontal trigger mechanism, as did some olderinstruments, rather than a vertical trigger, the situation is likely to be worse thanshown above because the amplitude of the initial part of vertical ground motionsis usually higher than the initial part of horizontal ground motions. Therefore aninstrument with a horizontal trigger mechanism would usually trigger later than aninstrument with a vertical trigger mechanism and hence would record less of theground motion than would be recorded on a vertical triggering instrument.

6. Low Digitizer Resolution

For digital instruments with digitizers that do not have a dynamic range, recordsof weak motion can have a step-like appearance because the resolution is not greatenough to record details of the motion. This will occur if the A/D converter bit

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Table III. Combination of A/D converter bit ranges and amplitude rangesinvestigated in this study.

ID A/D converter Amplitude range, Af Resolution, r (ms−2)

bit range, n (bits)

1 Reference record

2 8 0.5 g 0.0383

3 8 1 g 0.0766

4 10 0.5 g 0.00958

5 10 1 g 0.0192

6 12 0.5 g 0.00239

7 12 1 g 0.00479

range is small, the amplitude range (full-scale amplitude) is too high and the groundmotion is of small amplitude. Therefore only weak ground motions will be affectedhence it is not usually too important for engineering purposes.

The resolution of the instrument can be calculated from:

r = 2Af

2n,

where Af is the instrument’s amplitude range (usually between 0.5 g and 2 g) andn is the A/D converter bit range (usually between 10 and 24 bits).

The PGA read from records from instruments with low digitizer resolution iswithin ±r/2 of the true PGA.

Therefore for an instrument such as the SMACH SM1, which has an amplituderange of 0.5 g and an A/D converter of 10 bits, the resolution is only 9.577 ×10−3 ms−2. Therefore if it recorded ground motion with PGA of 0.1 ms−2 [0.01 g],i.e., close to the trigger level of an analogue instrument, then only 20 amplitude lev-els would be used to record the ground motion. This causes the step-like appearanceof records from such instruments.

To simulate such problems the high-quality records mentioned above werepassed through a numeric A/D converter that rounds the acceleration from the refer-ence time-history to the nearest level. The simulated records from such instrumentswere then filtered with cut-off frequencies of 0.05 and 50 Hz and their responsespectra calculated. Different A/D converter bit ranges and amplitude ranges wereinvestigated, see Table III. For bit ranges and amplitude ranges different than thosegiven in Table III the table should be consulted for the value of r of interest. Forexample, for an instrument with a bit range of 12 bits and 2 g amplitude range,Table III should be consulted for r = 0.00958 ms−2, i.e., an instrument with 10bits and 0.5 g amplitude range.

Figure 5 shows the uncorrected acceleration time-histories that would have beenrecorded on digital instruments with amplitude ranges and A/D converter bits given


Figure 5. Recorded acceleration time-history of waveform 001213 (NS component) (Top),acceleration time-history of waveform 001213 (NS component) from a simulated instrumentwith an 8 bits A/D converter and amplitude ranges of 0.5 and 1 g (second row, left and right),acceleration time-history of waveform 001213 (NS component) a simulated instrument witha 10 bits A/D converter and amplitude ranges of 0.5 and 1 g (third row, left and right) andacceleration time-history of waveform 001213 (NS component) a simulated instrument witha 12 bits A/D converter and amplitude ranges of 0.5 and 1 g (bottom row, left and right).001213 is the record from Aydin (Turkey), recorded on a GSR-16 instrument, of the Kocaeliearthquake (17/8/1999, Ms = 7.8, Mw = 7.6) at a distance to the surface projection of thefault of 359 km.

in Table 3. These figures show the dramatic effect low digitizer resolution has onlow amplitude acceleration time-histories. The velocities and displacements ob-tained by filtering and integrating the low-resolution time-histories are much differ-ent from the velocities and displacements obtained from the original time-histories.If the digitizer resolution is too low then the ground motion is not recorded.

The Fourier amplitude spectrum of the original time-history of waveform001213 (NS component) is compared with the Fourier amplitude spectrum of thetime-history as recorded by a simulated instrument with an 8 bits bit range and anamplitude range of 1 g and a simulated instrument with a 12 bits bit range and anamplitude range of 0.5 g in Figure 6. The effect of the low digitizer resolution isto significantly alter the recorded amplitudes in the low-frequency (long-period)range and in the high-frequency (short-period) range. For the instrument with 8bits bit range and 1 g amplitude range the interval of frequencies where the Fourieramplitudes are similar to the reference Fourier amplitudes is only from about 0.2to 1 Hz. For the instrument with 12 bits bit range and 0.5 g amplitude range the

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Figure 6. Fourier amplitude spectra of reference ground motion and ground motion asrecorded by two instruments with A/D converters with different bit ranges and amplituderanges and the ratio of the two spectra. The left-hand figures shows the Fourier amplitudespectrum of the reference ground acceleration as recorded, the middle figures shows theFourier amplitude spectrum of the ground acceleration from the simulated instrument andthe right-hand figures show the ratio.

interval of frequencies where the instrument is recording the true ground motion isfrom about 0.05 to 2 Hz. The lack of sufficient bit range introduces high-frequencyand low-frequency noise into the recorded ground motions. This indicates a certainfrequency range where the spectral composition of the recorded signal is unaffectedby the A/D converter’s resolution. In this frequency range it is expected that deriv-atives, such as the response spectrum of the time-history, should also be unaffectedby the digitisation resolution.

Figure 7 compares the 5% response spectra of 001213 for different simulatedinstruments (see Table III). It shows that although the acceleration time-historiesfrom instruments with low resolution look poor the response spectra of the recordswithin the period range of engineering interest (0.1 � T � 2 s) match thosefrom higher resolution instruments closely. For short periods and long periodshowever the response spectra from low resolution instruments diverge from thetrue response spectra.

To enable an objective choice to be made regarding whether to use or reject astrong-motion record affected by low digitization resolution the selected 421 high-quality time-histories were analysed in the same way as in the above example.


Figure 7. Comparison of response spectra of waveform number 001213 (NS componentcomputed using different amplitude ranges and number of A/D converter bits (see Table III).

The ability of the time-history from an instrument with low digitizer resolutionto yield a response spectrum that matches the reference response spectrum, asmeasured by an instrument with a large bit range, is dependent upon the numberof acceleration levels present in the digitized time-history. This is proportional tothe PGA of the real ground motion. For example, the PGA of the time-historyin Figure 5 is 0.0585 ms−2, therefore for an instrument with an A/D converterwith 10 bits and an amplitude range of 1 g, giving a resolution r = 0.0192 ms−2,there are int(2 × 0.0585/0.0192) = 6 acceleration levels present in the simulatedtime-history (see Figure 5).

Figure 8 shows the ratio between the response spectral acceleration computedusing the time-histories simulated to come from instruments with bit range 8 bitsand an amplitude range of 1 g and the reference response spectral accelerationfor the same time-histories plotted against PGA. It shows that good estimatesof response spectral acceleration for the main structural period range of interest(0.2 ≤ T ≤ 1 s) can be obtained even for such low-resolution instruments ifthe PGA is sufficiently high, about 0.5 ms−2 [0.05 g], corresponding to about10 acceleration levels being present in the time-history from the low-resolutioninstrument. For longer periods, e.g. T = 2 s, the number of acceleration levelspresent in the time-history needs to be much higher, about 50 levels, to yield goodestimates of response spectral acceleration. Note that instruments with 8 bits andamplitude ranges of 1 g cannot record ground motions with PGA < 0.0383 ms−2

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Figure 8. Ratio of response spectral acceleration for 5% damping and natural periodsT = 0.2, 0.5, 1.0 and 2.0 s, for time-histories from a simulated instrument with 8 bits and1.0 g amplitude range to the response spectral accelerations calculated from the originaltime-histories, against the peak ground acceleration of the original time-history. Also givenon the figures are the number of acceleration levels present in the time-histories from thesimulated instruments. Squares are for horizontal components and triangles are for verticalcomponents.

corresponding to half a single acceleration level (the acceleration is rounded to thenearest acceleration level).

Similar graphs to those shown in Figure 8 for the other combinations of bitand amplitude ranges listed in Table III are similar to Figure 8, i.e., if there are asufficiently many (about 20 to 50) acceleration levels present in the time-historyfrom the low-resolution instrument then good estimates of response spectral accel-erations within the period range 0.2 ≤ T ≤ 2 s can be obtained.

Therefore if the number of acceleration levels present in the time-historiesrecorded by a digital instrument is greater than between about ten and 50, thenthe response spectral acceleration estimates obtained from the time-history will beaccurate for the period range of engineering interest even if the time-history itselfis of poor quality.


7. Discussion and Conclusions

This study has examined the effect of S-wave trigger and low digitizer resolutionon response spectra of strong-motion records. At present a significant proportionof the strong-motion records within the Imperial College European strong-motiondatabank are affected by such problems and they are usually not included in strong-motion studies because they are thought to be of too poor quality. This study,however, has shown that accurate response spectral ordinates within the range ofnormal engineering interest (0.2 � T � 2 s) can often be obtained from recordsaffected by such problems. Therefore the strong-motion records that are usuallydiscarded from sets of records used for estimating the ground motions that occurredduring earthquakes or are expected to occur during future earthquakes, can be usedas long as they conform to certain amplitude or duration limits. For instrumentswith vertical triggering mechanisms if the bracketed duration (using a thresholdacceleration equal to the triggering acceleration) of the record is greater than about10 s then response spectral accelerations obtained for periods between 0.2 and 2 swill be accurate. For digital instruments with A/D converters with low bit rangesif the number of acceleration levels present in the recorded accelerations is greaterthan about ten to 50 levels then response spectral accelerations obtained for periodsbetween 0.2 and 2 s will be accurate. This will obviously significantly increase thesize of the datasets that can be confidently used in future studies.

One consequence of this finding is that relatively cheap low specification instru-ments with 8 or 10 bits ranges can be used to record ground motions of engineeringsignificance if only response spectral ordinates are of interest. This is important forparts of the world where strong-motion networks are being established. If, how-ever, the actual time-histories of low amplitude ground motions are of interest thendigital instruments with 12 bits or more should be used.

As a consequence of this study additional records, which have been rejecteddue to their poor quality, will be placed on the Internet Site for European Strong-Motion Data (Ambraseys et al., 2002) because they are of sufficient quality to yieldgood response spectra within a limited period range.

Acknowledgements

An early version of this study was presented at XXVIII General Assembly of theEuropean Seismological Commission (SCF-1) in Genoa, Italy, on 5th September2002. I thank Profs N. N. Ambraseys and R. Sigbjörnsson, Drs S. K. Sarma andP. M. Smit for suggestions and advice during this study, Dr L. Gardner for proofreading this paper, Dr J. J. Bommer for suggesting that I submit this paper to BEEand two anonymous reviewers for their valuable comments. This work was fundedby grants from the European Commission (EVR1-CT-1999-40008) and the Engi-neering and Physical Sciences Research Council (GR/R52114/01); I thank bothfunding agencies for their support.

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What is a Poor Quality Strong-Motion Record?