CALIFORNIA INSTITUTE OF TECHNOLOGY

EARTHQUAKE ENGINEERING RESEARCH LABORATORY

STRONG-MOTION EARTHQUAKE MEASUREMENT

USING A DIGITAL ACCELEROGRAPH

BY

WILFRED D. IWAN, MICHAEL A. MOSER

AND CHIA-YEN PENG

REPORT No. EERL 84-02

A Report on Research Conducted Undr aGrant from the National Science Foundation

PASADENA, CALIFORNIA

APRIL 1984

Strong-Motion Earthquake Measurement Usinga Digital Accelerograph

By Wilfred D. Iwan, Michael A. Moser, and Chia-Yen Peng

ABSTRACT

This paper presents results of a study of some of the characteristics of the

Kinemetrics PDR-1 digital strong-motion accelerograph. The paper gives the results of

laboratory tests of the background noise level of the instrument and compares these

results with previously reported observations for optical instruments. The determina-

tion of displacement from acceleration data is discussed and results of laboratory tests

are presented. Certain instrument anomalies are identified, data correction algorithms

proposed, and examples given. The paper also presents the results of a comparison of

earthquake records obtained from side-by-side digital and optical analog instruments.

Finally, some results obtained from a recent Chinese earthquake are discussed.

INTRODUCTION

The development of reliable, digital strong-motion accelerographs is having a very

significant impact on the measurement of strong ground motion. After analog amplifi-

cation, and conversion to digital format, the digital instruments record the data on

magnetic tape or a core memory in a computer-readable form, thus bypassing many of

the steps of the analog-to-digital conversion process associated with optical analog

accelerographs. Timing is generally quite accurate in these digital recorders, being of

the order of one part in 107. In addition, digital instruments commonly have a pre-

event memory so that the entire history of the motion is recorded along with the initial

state of the instrument. The resolution of the digital instruments is typically 66 dB (with

12 bits of digital data), and the dynamic range is increased by 36 dB through the use of

an autoranging analog amplifier.

Because of their inherent advantages, the use of digital instruments should become

very widespread throughout the world in the years to come. However, since these

instruments have only recently been put into service, their characteristics are not well

known. The purpose of this paper is to present results of a study of the recording char­

acteristics of a typical strong-motion recorder / transducer instrument. The background

noise level of the recorded data is compared with that of optical recorders. Integration

of the recorded acceleration time history to obtain displacement is discussed and the

results of laboratory tests presented. Certain instrument anomalies are identified and

correction algorithms are proposed. Also presented is a comparison of side-by-side

recorded results obtained from analog and digital instruments.

NOISE LEVEL OF DIGITAL INSTRUMENT

Considerable laboratory and field experience has been gained with optical strong-

motion recorders. The limitations of these instruments are fairly well understood and

substantial effort has been expended to maximize the data return from such instru­

ments using various filtering and correction algorithms (Hudson, 1979). Noise is intro­

duced into the optical recording from a number of sources including transducer drift,

recording medium lateral drift and speed variation, trace density variations and the

digitization process. This noise limits the range of ground motion for which useful data

can be obtained. Generally speaking, the optical instruments will have difficulty pro-

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viding useful data for earthquakes of M=4 or smaller even in the epicentral region and

for earthquakes of magnitude M=6.5 if further than 200 km from the source.

Digital instruments have inherent advantages so far is noise is concerned. Noise

associated with media drift, transport speed variation and trace density variation are

essentially eliminated with the digital format. Furthermore, digitization error is greatly

reduced by eliminating the intermediate step of optical recording.

In order to obtain quantitative data on the background noise of a typical digital

strong-motion recorder, a Kinemetrics PDR-1/FBA-13 instrument was tested. The

PDR-1 is a gain ranging recorder with 12 bit resolution and a 102 dB dynamic range.

The FBA-13 is a three-component force balance accelerometer unit. The accelerometers

nominally have a 50 Hz natural frequency and 70<Yo damping and are capable of dc

measurement. The transducers tested (Ser. Nos. 16345 and 16347) had a range of ±2g.

The FBA-13 transducer was mounted on an air suspension optical table in a quiet

laboratory environment. Samples of background noise with a duration of 20 seconds

were recorded using the PDR-l "run" mode, and subsequently processed to obtain the

time history of the noise and the pseudo-velocity response spectrum. Care was taken in

balancing the transducer so that maximum system gain was obtained. No data was

obtained at lower gains. The input low-pass filters of the recorder were set at their

maximum value of 50 Hz and the sample rate was 200 sps. No correction was intro­

duced into the processing except for a uniform baseline shift to eliminate the dc compo­

nent of the accelerogram.

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Figure 1 shows the time history of the background noise acceleration for four differ­

ent test runs. It is seen that the noise is generally associated with fluctuations in the last

bit of the digital data. Some low frequency drift is evident. Figure 2 shows the Fourier

Amplitude Spectrum of one of the noise time histories of Figure 1. This spectrum was

obtained using a Kaiser-Bessel filtered version of the data. The result shown is typical.

Figure 3 shows the superimposed zero-damped response spectra for the four differ­

ent test runs. Also shown in Figure 3 is the band of average digitization noise plus one

standard deviation for a typical hand digitized optical record as reported in the litera­

ture (Hudson, 1977; Hudson, 1979). In addition, the figure shows the 20% damped

average digitization noise spectrum which is claimed for the automatic digitization

system employed by the California Division of Mines and Geology.

It is seen from Figure 3 that there is a very striking difference in noise level between

the analog and digital systems. The difference varies from more than one order of

magnitude at short periods (high frequency) to about two orders of magnitude at long

periods (low frequency). It is clear from the figure that the digital instrument is capable

of accurately measuring the acceleration from much smaller events than the analog

instrument. It is also clear that the digital instrument should give superior results for

integrals of the acceleration; i.e., velocity and displacement. The digitization noise in

the displacement at a period of ten seconds is of the order of two inches for the optical

instrument but is closer to 0.02 inches for the digital instrument. It would therefore be

anticipated that the digital instrument could be used to extract significantly more

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information about displacement than the analog instrument. This possibility is exam­

ined in the next section.

COMPUTATION OF DISPLACEMENT FROM ACCELERATION

In principle, velocity and displacement time histories could be obtained directly

from an acceleration time history by numerical integration. However, for optically

recorded data, this usually results in errors which are unacceptably large. This is due to

the fact that digitization and other errors associated with optical recording increase

with period and therefore become amplified when the time history is integrated. In

addition, an initial portion of the acceleration time history is lost in the analog instru­

ment due to the finite time required to trigger this instrument. The digital instrument

has a much lower inherent digitization noise. Furthermore, the pre-event memory

incorporated into most instruments eliminates the ambiguity in the initial conditions of

the record.

In order to examine the nature of the errors associated with integration of accelera­

tion time histories obtained from digital instruments, a series of laboratory tests were

conducted using the PDR-l/FBA-13 instrument. The transducer was moved horizon­

tally in a straight line on a very flat, level surface through a known displacement. The

recorder was triggered from the output of the accelerometer oriented in the direction of

motion. The acceleration data was then integrated to obtain the time history of velocity

and displacement. A typical result for a test with a relatively smooth displacement is

shown in Figure 4.

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For the test results shown in Figure 4, the actual permanent displacement of the

transducer as 10±0.1 inches (25.4±0.3 em). It is seen that the integrated acceleration

time history gives an accurate measure of this permanent displacement. The only cor­

rection applied to the data was to adjust the zero offset of the accelerogram by the

average value of the pre-event data (the first 1.0 seconds of the record were used for this

average) and apply an instrument correction to remove transducer distortion. The

input low-pass filter of the PDR-l was set at 50 Hz and the sample rate was 200 sps.

The PDR-l does not have a high-pass filter, so the dc output from the accelerometers is

retained. This is a necessary feature of the instrument if permanent displacement

information is desired.

It would not be possible to generate a displacement time history like that shown in

Figure 4 from an optical recording instrument due to absence of pre-event data and the

presence of sibrnificant long-period noise. In an attempt to minimize the effect of these

factors, optically recorded data is typically subjected to a number of corrections. These

include baseline corrections and low- and high-pass filtering. In order to obtain a better

understanding of the effect of these corrections, the acceleration time history of Figure 4

was processed using the standard correction algorithms employed in the report series

entitled "Corrected Accelerograms" published by the California Institute of Technology

(Trifunac and Lee, 1973). The results are shown in Figure 5. It is seen that the effect of

the correction algorithm is to "level out" the displacement time history thus eliminating

any permanent displacement and simultaneously reducing the magnitude of the

dynamic displacement.

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ANOMALIES IN THE DIGITAL INSTRUMENT

Unfortunately, the excellent results of double integration of the acceleration indi-

cated in Figure 4 are not altogether typical. Rather extensive laboratory testing and

field use of the FBA-13 transducer has revealed an anomaly which has a significant

effect on data analysis. Figure 6 shows the result of direct numerical integration of a

permanent displacement test in which the transducer was moved with a rather rough

motion having associated high acceleration peaks. In this case, a very noticeable drift is

observed in the displacement time history which completely overshadows the actual

test permanent displacement which was again 25.4 cm.

Through further testing, it was determined that the baseline output of the FBA-13

could be observed to shift suddenly if a sufficiently large input acceleration pulse were

applied. This shift may be associated with minute slippage in the flexure support of the

transducer, or hysteretic behavior of the fine wires which attach to the moving trans­

ducer element. The phenomenon is not fully understood and is currently under further

investigation by the manufacturer. However, the anomaly has a very strong effect on

the acceleration data when it is double integrated.

An attempt has been made to develop a correction algorithm which can compensate

for the observed anomaly without having an adverse effect on the ability of the instru­

ment to predict permanent displacement. The proposed correction algorithm is quite

simple and based on the perceived mechanism of the anomaly. The acceleration data is

first corrected for the dc offset observed in the pre-event data. Typically, this is

accomplished by performing a time average on only the first one-half of the pre-event

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data in order to eliminate the possibility of including any actual earthquake data. Next,

the final offset of the acceleration is determined. If there were no instrument anomaly,

this offset should now be zero since the pre-event and final offsets should be equal.

However, due to slippage or other effects, these offsets will not, in general, be the same.

If the dynamic or oscillatory component of the acceleration at the end of the record is

smaller than the final offset of the record, as in Figure 4, the acceleration record may be

used directly to determine the final offset. However, it has been found that it is usually

more accurate to estimate the final acceleration offset from the final slope of the velocity

record since it has a relatively smaller oscillatory component. Normally, the last 15

seconds of the time history are adequate to determine the acceleration offset.

Finally, the cumulative effect of the transducer anomaly during the time of strong

shaking is estimated from the offset in the final velocity. This is accomplished by

assuming that the intermediate baseline shifts can, on the average, be replaced by a rec­

tangular acceleration pulse which occurs during the strongest portion of the

acceleration time history. The overall correction for the instrument anomaly is shown

schematically in Figure 7.

The time t1 and tz may be selected in a number of different ways and it has been

observed that the final results are generally fairly insensitive to this selection. Experi­

ments have shown that very little base line offset of the transducer occurs for

accelerations less than 50 cm/secz. Therefore, one possible approach is to select tl and

tz such that a(t) is less than or equal to 50 cm/ sec2 for all t on the interval (tl' t2)' This

approach (Option One) works well when there is known to be a real final net displace-

-8-

ment. Another approach would be to select tl as the time of the first significant accel-

eration pulse and then select t2 so as to minimize the computed final displacement. This

approach (Option Two) is useful when the existence of a final displacement is not

known a priori. If there is a real final displacement, Option Two will usually yield a

value of t2 which is less than tI, signifying that Option One is more reasonable.

The correction for the final offset is normally determined by a least-squares fit of the

final portion of the velocity data of the form

The intermediate range correction is determined from the relation

The result of applying the correction algorithm to the acceleration time history of

Figure 6 is shown in Figure 8. Option One was used in the selection of t l and t2' The

corrected displacement results show good agreement with the actual permanent dis-

placement of 25.4 em. Very little low frequency (long period) information seems to

have been lost from the data by the correction algorithm. Indeed, this was one of the

goals in the development of the algorithm.

Additional examples of the results of prescribed displacement tests using the pro-

posed correction algorithm are given in Figures 9-12. The FHA transducer was initially

adjacent to a ruler and the recorder set in the "triggered" mode. The transducer was

then moved slightly away from the ruler and translated parallel to the ruler. Finally,

when the desired displacement was reached, the transducer was moved back against

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the ruler. The test displacement in each case was 25.4±0.3 em. No special fixtures were

used to maintain a fixed orientation of the transducer while it was translated. Hence,

angular rotation of the transducer may account for some of the differences between the

actual and computed displacement. However, it is more likely that the single correction

algorithm used is not adequate when large amounts of transducer baseline shift take

place over a long duration.

Figure 9 shows the corrected version of the test shown in Figure 4. Figure 10 shows

a similar test with a fairly smooth displacement. Figure 11 shows a test in which the

motion was caused by a series of discrete impacts to the transducer. This test clearly

illustrates both the nature of the hypothesized instrument anomaly and the limitations

of the proposed simple correction algorithm. From the velocity time history, it is evi­

dent that there is a different transducer offset associated with each impulse. Averaging

the effect of these individual shifts over the approximately 10 seconds of strong accel­

eration to obtain a single effective shift am as prescribed by Option One is not adequate

for such a case. When the acceleration pulses are distinct, as in Figure II, it is possible

to apply an offset correction to each individual pulse in a manner analogous to that

used in the simple correction. Figure 12 shows the results of using such an approach on

the data of the test shown in Figure 11. The improvement in the computed time histo­

ries of the velocity and displacement is apparent.

Table 1 gives a summary of the values of the correction parameters and the final

displacements obtained from double integration of the acceleration for the tests pre­

sented in Figures 8-12. It is seen that the numerical values of the corrections af and am

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are generally quite small. The displacement results have been observed to be fairly

insensitive to the selection of the threshold acceleration level which defines tl and t2'

It is believed that the simplified correction algorithms will be adequate for most

short-duration earthquake-like time histories of motion. In that case, the time duration

over which the central correction is applied will generally be smaller than for the high

level acceleration tests presented herein and the permanent ground displacement will

also take place over a shorter time interval. This will be illustrated by examples which

follow.

It should be emphasized that the suggested correction algorithm has been intro­

duced only to eliminate an observed anomaly in the instrument. It is anticipated that

the source of this anomalous behavior will soon be eliminated so that correction of the

digital accelerogram will no longer be necessary.

COMPARISON OF ANALOG AND DIGITAL RECORDS

Following the Coalinga earthquake of May 2/ 1983/ a SMA-1 optical analog record-

ing accelerograph and a PDR-1/FBA-13 digital accelerograph were installed side-by­

side in a residential garage near the center of the city to measure aftershocks. On

May 8/ 1983/ an aftershock of ML=5.5 with an epicenter approximately 11 km to the

north of the city was recorded simultaneously by both instruments. The results

obtained represent a unique opportunity for a direct comparison between the optical

analog and digital recording instruments.

The time histories of the ground acceleration, velocity and displacement for the digi­

tal instrument are shown in Figures 13-15. The time histories shown were corrected

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using the algorithm described herein with tl and t2 selected according to Option Two.

The sampling rate was 100 sps.

The time histories obtained from the analog instrument are presented in Figures 16­

18. These results were obtained from accelerograms which were digitized and cor­

rected by the California Division of Mines and Geology. This data was band-pass

filtered with a low frequency cut-off of 0.2-0.4 Hz and a high frequency cut-off of 23-25

Hz. The maximum acceleration, velocity and displacement obtained from the two dif­

ferent instruments are compared in Table II.

In general, there is rather close agreement between the results obtained from the

analog and digital instruments. The greatest difference is, as expected, observed in the

displacement. The results obtained from the digital instrument show the presence of a

low frequency pulse with a duration of about 7 seconds while this pulse is filtered out

of the results from the optical analog instrument. The potential of the digital instru­

ment to provide long period displacement information is clearly indicated.

In order to obtain a more complete understanding of the difference between the

analog and digital records, a correlation analysis was performed. The cross-correlation

coefficient between each pair of records was computed and the time difference for

maximum correlation determined. The analog record was shifted by this time differ­

ence and then subtracted from the digital record to obtain the time difference accelera­

tion. The results are shown in Figures 19-21.

The difference acceleration shown in Figures 19-21 is somewhat larger than might

have been anticipated from the relatively high correlation of the analog and digital

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records. The most satisfactory explanation for this seems to be that there is a slowly

varying phase shift with time between the two records. Whether this is associated with

speed variations in the analog instrument, the digitization process or the instrument

correction is uncertain.

In Table 3 the maximum correlation time difference for each pair of records is com­

pared to the time difference between the largest acceleration peak of the records. In

general, there is fairly close agreement between the two measures of time difference.

This agreement holds as well for the difference between the largest velocity peak but

not for the peak displacement.

The Fourier Amplitude Spectra of the digital, analog and difference acceleration

time histories are shown in Figure 22-24. It is noteworthy that the Fourier Amplitude

Spectrum associated with the difference acceleration is generally significantly larger

than the difference between the Fourier Amplitude spectra of the digital and analog

time histories. This is another indication of the presence of phase differences in the

records that are not accounted for by a time shift corresponding to the maximum corre­

lation time. Although not shown herein, a significant phase variation was also

observed in the cross-spectrum of the digital and analog records even after the records

were shifted.

The zero-damped response spectra for the three components of this aftershock are

shown in Figures 25-27. The solid lines denote the spectra obtained from the digital

instrument and the dashed lines denote the spectra obtained from the analog instru­

ment. Two spectra are shown for each analog record. The higher response spectrum

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values were obtained using corrected accelerograms which were high-pass filtered with

a corner frequency of 0.07 Hz (14.3 sec period). The lower values were obtained using

the accelerograms filtered with a corner frequency of 0.4 Hz (2.5 sec period) which are

reproduced in Figures 16-18. It is understood that the corner frequency of 0.4 Hz was

selected by the California Division of Mines and Geology to correspond approximately

to the intersection of the 20% damped earthquake response spectrum and the nominal

noise response spectrum (see Figure 3). There is very close agreement between the

optical and digital response spectra for periods less than one second. However, there

are some very significant differences in the range of periods greater than one second.

The difference can be as large as an order of magnitude depending on the filtering used

and the period considered. In this particular case, the optical instrument results are

conservative compared to those of the digital instrument but this cannot be assured in

general.

The spectral differences observed are believed to be caused by the presence of low

frequency noise in the analog records which is not eliminated by the baseline and filter­

ing corrections applied to this data. This conclusion is supported by the background

noise level results presented in Figure 3. The absence of pre-event data along cannot

account for the observed differences. The fact that the accelerograms filtered at 0.4 Hz

have nearly the correct displacement asymptote appears to be coincidental.

From an engineering point of view, the comparison of the response spectra suggests

that for an earthquake with this level of shaking, the optical instrument is capable of

providing quite adequate estimates of the response of structures with periods less than

-14-

one second. However, the optical instrument data may lead to significant errors in

estimates of the response of structures with longer periods even when the data is fil-

tered according to accepted techniques. Additional data from side-by-side comparisons

would be useful in extending these observations.

RESULTS FROM A RECENT CHINESE EARTHQUAKE

A number of PDR-1/FBA-13 instruments have been installed in China as part of a

joint US/PRe cooperative project in strong ground motion measurement. This project

is sponsored by the U.S. National Science Foundation and the Chinese State Seismologi-

cal Bureau through the Instate of Engineering Mechanics in Harbin. A number of

instruments have been installed in arrays in the aftershock region of the 1976 Tangshan

*earthquake. On October 19, 1982 a Ms=5.3 event occurred near the city of Lulong

northeast of Tangshan. The location of the epicenter of this aftershock and the locations

of instruments recording the event are shown in Figure 28. The epicentral location

shown was determined by analysis of the strong-motion array data. The focal depth

was estimated at approximately 10 km and the closest instrument (TS-12) was 4.2 km

from the postulated epicenter. A comprehensive study of the aftershocks of this event

has been made by Wu (1984).

The accelerograms obtained from station TS-12 are shown in Figures 29-31. The

results shown are quite unique in that one channel of the horizontal data was not

recorded at all during the earthquake due to an instrument malfunction. This data was

. This event was originally assigned a magnitude of ML=6.2 by the Chinese Institute of Geophysics buthas recently been downgraded.

-15-

reconstructed later using the parity information recorded by the PDR-1. Time histories

of the velocity and displacement are also shown in Figures 29-31. All of the records

were processed using the correction algorithm described herein. However, due to the

presence of some obviously incorrect data (and associated parity errors) early in the

pre-event data, this data was averaged from 1.0 to 2.5 seconds. Application of Option

Two for the selection of t] and t2 yielded t2 < tl' Therefore, a form of Option One was

used. The final result was insensitive to the precise values of tl and t2 selected.

The results shown in Figures 29-31 predict a net horizontal absolute permanent dis­

placement in the direction S30E of 4.0 em and a net downward vertical permanent dis­

placement of 1.0 em. The rise time associated with the permanent displacement is of the

order of 1.5 sec. All of these results appear to be consistent with the known fault behav­

ior in the region and are not inconsistent with the predictions of theoretical source

models for an event of this magnitude. To the authors' knowledge, this is the first time

that such absolute ground displacement has been recovered from recorded data.

Results of this type present a unique opportunity for the calibration of seismic source

models.

CONCLUSIONS

Based on the observations reported herein, it is believed that the digital accel-

erograph will eventually become the standard instrument for strong-motion measure­

ment. Even allowing for anomalies which exist in some current instruments, it appears

that the digital instrument is capable of providing accurate data over a much wider

range of amplitude and frequency than was previously thought possible. Expanded use

-16-

of digital instruments could have a beneficial impact on source modeling and wave

propagation studies as well as structural response studies.

REFERENCES

Hudson, D.E. (1979). Reading and Interpreting Strong .Motion Accelerograms, EarthquakeEngineering Research Institute, Berkeley.

Hudson, D.E. (1977). "Reliability of Records," Panel 1, Earthquakes, Froc. Sixth WorldConf on Earthquake Engr., New Delhi, India.

Trifunac, M.D. and V. Lee (1973). Routine Computer Processing of Strong-MotionAccelerograms, Earthquake Engineering Research Laboratory Report No. 73-03, Calif.Inst. of Tech., Pasadena.

Wang, P., L. Song and F.T. Wu 91984). Near Field Seismic Observation with a MobileLittle Aperture Network, Proc. of International Workshop on Earthquake Engineering,Shanghai, China, March 27-31,1984.

ACKNOWLEDGMENT

The authors wish to acknowledge Mr. Raul RelIes who installed the instruments in

the Coalinga aftershock region. The authors also express appreciation to Mr. Tony

Shakal of the California Division of Mines and Geology for processing the analog film

records obtained from the Coalinga aftershock. The Lulong records were obtained

under a cooperative project between the United States and the People's Republic of

China. The project is directed by Profs. Liu Huixian, Xie LiLi and Peng Keshong of the

Institute of Engineering Mechanics, Harbin, and Dr. D.A. Boore and Profs. W.D. Iwan

and T.-L. Teng in the U.s. The second author was supported under the IBM Resident

Study Program. This project was sponsored in part by the National Science Founda-

tion. The opinions expressed are those of the authors and do not necessarily reflect

those of the sponsoring agency.

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TABLE 1. RESULTS OF DISPLACEMENT TEST

Testaf (em / see2 ) am(em/see2 ) tl (see) t2 (see) Compo Final

(Fig. No.) Displ. (em)8 -6.52 -7.52 2.03 7.48 23.79 -0.081 -0.08 2.13 5.78 25.8

10 -0.09 -0.18 1.98 3.92 25.811 - - - - 15.512 -1.10 - - - 25.9

TABLE 2 COMPARISON OF OPTICAL AND DIGITAL VALUES OF PEAKACCELERATION, VELOCITY AND DISPLACEMENT ­COALINGA AFTERSHOCK, MAY 8, 1984

COMPONENT DIGITAL REC. OPTICAL REC.

Max. Acee!. N35E 99.5 95.7

em/sec2 Up 73.3 -72.7S55E 117.3 114.6

Max. Vel. N35E 7.37 6.75

(em/sec) Up 2.17 2.20S55E 4.46 4.11

Max. Displ. N35E 0.97 -0.39

(em) Up 0.4 -0.17S55E 0.63 0.36

TABLE 3. COMPARISON OF TIME DIFFERENCE BETWEENANALOG AND DIGITAL

Time Difference (sec, ±0.01 sec)Component From Max. Correlation From Max. Accel. Peak

35 Deg. 4.06 4.05Up 4.06 4.01

125 Deg. 4.05 4.04

-18-

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CO. 0 2.0 4.0 6.0 8.0 10.0TIME (SEC)

0

0

Cl~

(J)\).

II\)

0nO

3:

I

"'"gJ.O 2.0 4.0 6.0 8.0 10.0TIME (SEC)

Figure 6 Displacement test showing effects of instru­ment anomaly.

Figure 7 Simple correction algorithm for instrumentanomaly.

n

~ P.+----...,.A-tJ.-++.+++++.J-A.iI-++lrl+if4:-HrIIH-lrf-hd!ArfYo-,-----~o V \lJ ,~rn"­(J)

rnnJ,~__----.--------r-----r------.------r~§J.O 2.0 4.0 6.0 8.0 10.0

TIME (SEC)

....o

<rnr

nP~---_-A-,~H+-I-t-f+-A-.f+.4t-A-A-M~fJTp'-------­3:0"­(J)

rnn

J,.-1-------.--------r-----r------.------rco. a 2.0 4.0 6.0 8.0 10.0

TIME (SEC)

0'..__------.,.------------------o

(J)

"lJ.I

I\.lo

J,.4------.------.-----r------.------,co. a 2.0 4.0 6.0 8.0 10.0

TIME (SEC)

Figure 8 Displacement test of Figure 6 with proposedcorrection algorithm

:ONnono·n3:......... !='(1)0rnn.........(I)

rn01~I\)

gJ.O 2.0 4.0 6.0 8.0 10.0TI.ME (SEC)

....<0

rnr·n~~o.........Virnn

I....co. 0 2.0 4.0 6. a 8.0 10.0

TIME (SEC)

....0

0.:....(I)

\J· ~0

n3:

I....co. a 2.0 4.0 6. a 8.0 10. a

TIME (SEC)

Figure 9 Displacement test of Figure 4 with proposedcorrection algorithm.

:D conon·n~,?Wornn,(J)rnnlco~ CO. 0 2.0 4.0 6.0 8.0 10.0

TI·ME - (SEC)

....0

<rnr·n?~o,Wrnn

I....co. 0 2.0 4.0 6.0 8.0 10.0TIME (SEC)

0

0

Q'......~.."• I

l\.l0

n~

I....co. 0 2.0 4.0 6.0 8.0 10.0

TIME (SEC)

Figure 10 Displacement test with proposed correctionalgorithm.

:DCl)

nono·n3:--...!=lU)orr1n--...U)

rr1nl

Cl)

~gJ. 0 3.0 6.0 9.0 12.0 15.0TIME (SEC)

I\)

0<rr1r·n!=l3:0--...U)

rr1n

II\)

cu. 0 3.0 6.0 9.0 12.0 15.0TIME (SEC)

I\)

0

CJ......U)

""'U· 0

0

n3:

II\)

cu. 0 3.0 6.0 9.0 12.0 15.0TIME (SEC)

Figure 11 Displacement test with proposed simple cor­rection algorithm.

:::D ronono·n3:,p(J)ornn,(J)rnnl

ro~gJ. 0 3.0 6.0 9.0 12.0 15,0

TIME (SEC)

I'.J0

-<rnr·nP3:0,(J)rnn

1I'.J

co. 0 3.0 6.0 9.0 12.0 15.0TIME (SEC)

"'"0

0~

(J)\)

· 0

0

n3:

I

"'"co. 0 3.0 6.0 9.0 12.0 15.0TIME (SEC)

Figure 12 Displacement test of Figure 11 with more ex­tensive correction.

::Dn-ono COALINGA AFTERSHOCK MAY 8, 1983

DIGITAL RECORD 35DEGn3:,?(J)oITtn,(J)ITtnl-§P.O 5.0 10.0 15.0 20.0 25.0 30.0

TIME (SEC)

CX)

<0ITtr.

n?3:0,(J)ITtn

ICX)

&.0 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

....0

0.-(J)-u.

?0

n3:

I-dl· O 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

Figure 13 Digital accelerogram with integrated velocity anddisplacement. Coalinga aftershock of May 8, 1983;35 degrees.

Figure 14 Digital accelerogram with integrated velocity anddisplacement. Coalinga aftershock of May 8, 1983;up.

:Dn rv

0nO COALINGA AFTERSHOCK MAY 8,1983

DIGITAL RECCR> 125 DEGn::3:

"P(f)ornn

"(f)rnnl~rv

gJ.O 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

:n<0rnr.nP::3:0

"(f)rnn

IUl

Jl.O 5. 0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

0

~~l--0

P0

n::3:

I0

~.O 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

Figure 15 Digital accelerogram with integrated velocity anddisplacement. Coalinga aftershock of May 8, 1983;125 degrees.

:D_nono exw...tQ Af1'ERSI«)Q( MAY 8..983·

MW.DG ft!CQR) 35 lEGn3:........ P(1)0rnn........(1)rnnl-~gJ. 0 5.0 10.0 15. O. 20.0 25.0 30.0

TIME (SEC)

?l<0rnr·nP3:0........(1)rnn

IcodJ·O 5.0 10.0 15.0 20.0 25.0 30.0

TIME (SEC)

-0

0......(1)""1J· 0

0

n3:

I-dJ·O 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

Figure 16 Analog acce1erogram with integrated velocity and dis­placement. Coalinga aftershock of May 8, 1983;35 degrees.

:DO)non 00AI....ItGA Afl'ERSHC)Q( MAY &.1983·

NW..DG f£0(R) lPn3:......... ?(J)ornn.........(J)rnnl~O)

5.0 10.0 15.0 20.0 25.0 30.0CO. aTIME (SEC)

~<0rnr·n?3:0.........(J)rnn

I

""dJ. a 5.0 10.0 15.0 20.0. 25.0 30.0TIME (SEC)

?""0

.......(J)\)

· 0

0

n3:

I0;"'0. a 5. a 10. a 15. a 20.0 25.0 30.0

TIME (SEC)

Figure 17 Analog accelerogram with integrated velocity and dis­placement. Coalinga aftershock of May 8, 1983; up.

:DI\)

nono OOAL.JGA Af1'ERH)Q( aMY 8.1983·

ANALOG REOQR) 125 lEGn3:,P(1)0rnn,(I)

rnnl~I\)

gJ.o 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

U1

<0rn,r·nP3:0,(I)

rnn

IU1

dJ·O 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

0

0)

CJ.......(I)

-0· 0

0

n3:

I0

~.o 5.0 10.0 15.0 20.0 25.0' 30.0TIME (SEC)

Figure 18 Analog acce1erogram with integrated velocity and dis­placement. Coalinga aftershock of May 8 t 1983;125 degrees.

COALINGA AFTERSHOCK HAY 8. 198335 DEGREES

:e_n o"-,-----:----.,.----..,._---__,_----_r__---_,_-------,no

COALINGA AFTERSHOCK HAY 8, 1983ANALOG 35 DEGREES

n3:,~

UlofTIn,UlfTI

nl+----~-----r------.,.----....._---__,_------i'-'~. 0 5.0 10.0 15.0 20. 0 25. 0 30. 0

TIME (SEC):::D ~.n o..,.----~-~---_r_--"-~----....._---__r---___...,

noCOALINGA AFTERSHOCK MAY 8, 1983DIGITAL 35 DEGREES

n3:

'~+-------"'.(1)0rTln........(1)

rTln ~+---___r----_r_--____:___r---~._--,-.-----;

'-'§}.o 5.0 10.0, 15.0 20.0 25.0 30.0TIME (SEC)

- ...,.----___r----......------.,.----.----__,_---___...,nOo::0::0

~+_-----:.,.----..,._---__,_----_r__---_,_---___i

~o. 0 2. 0 4.0 6.0 8.0TIME DELAY (SEC)

10.0 12.0

o...-U'! --.,. ..,._---__,_----.----_,_---___...,.,0•.,

COALINGA AFTERSHOCK MAr 8, 198335 DEGREES

n3: 0 "------"'1,. ""T

(1)0

rTln,(j)

rTll--'-- --.,.----..,._---___r----._---_,_-----tn (J1'

'-'C{). 0 5.0 10.0. 15.0 20.0 25.0 30.0TIME (SEC)

Figure 19 Correlation coefficient and difference between digital and analogaccelerograms. Coalinga aftershock, May 8, 1983; 35 degrees.

COALINGA AFTERSHOCK HAY 8.1983DIGITAL UP

:0 conO.,------,,..----....----.-------r-----.------,n

COALINGA AFTERSHOCK HAY 8. 1983ANALOG UP

n3:,~

(./)0

n1n,(./)

n1

~~+--------,r-------r----......------r------r-------lco. 0 5.0 10.0 15.0 20.0 25.0 30.0

TIME (SEC)

:0 eo.no..,.-------,r--------.----......------.-----.-----,n

n3:,~+---­(./)0

n1n,(./)

n1~ ~-+-----r-------r-----r--------r----,-----r-------l

co. 0 5.0 10.0 15.0 20.0 25.0 30.0TIME (SEC)

nOo::0::0

U'JALINGA AFTERSHCJCK HAY 8.1983UP

2..+_------,____,------.----.--------.-----.------\~.o 2.0 4.0 6.0

TIME DELAY8. 0

(S EC)10.0 12.0

o........ C'O ____,------.----.-------r-----,-----,.,0•.,

rClQU t\GA RFTF.RS;-JOCf"i MAY b. ~ 98:3UP

n3: 0 -L. _, ..(1)0

f"Tln,(J)

f"Tl,noo+-------,____,------.----.-------r----......-----\~C().O 5.0 10.0 15.0 20.0 25.0 30.0

TIME (SEC)

Figure 20 Correlation coefficient and difference between digital and analogaccelerograms. Coalinga aftershock, May 8, 1983; up.

:01'.:;rc..,...-------,----..,----.....,------r-----,------,no

CORLINGR RFTERSHOCK MRY 8.1983ANRLOG 125 DEGREES

n3:........ ~"""'''-/f#IM(..f)crnn........(..f)

rnr,0+------r----..,--------r-----r-----,-----j~§J.O 5.0 10.0 15.0 20.0 25.0 30.0

TIME (SEC)

:D1\.lno..,.------r----..,-----,-----..-------,------,no

COALINGR RFTERSHOCK MRY e.1983DIGITAL 125 DEGREES

n3:........~~----~ ...~(..f)ornn........(..f)

rnn,0+------r----..,--------r-----r-----,-----j~ gJ. 0 5. 0 10.0 15. 0 20. 0 25. 0 30. 0

TIME tSECJ

........,.----------,----.,.-----,------r-------,------,nOa:::0:::0

°n'0°rnIfIf

MRY 8, 1983

10.04.0 6.0 8.0TI ME DELRY (SEC)

2.0dJ·OJ..-I----"""'T""----r------r------,---~-.------j

12.0

CJ":-'CDIfo..,.------r----..,-----.....,------r------,------,If

COALI NGR RFTERSHOCK MAY -8 •. 1983125 DEGREES

n3: 0 L---..._14l1N......... .,.(..f)o

rnn........(..f)

rn lnCD+------r----..,------,------r-----,-----j~q).0 5.0 10.0 15.C 20.C 25.0 3C.C

TIME (SEC)

Figure 21 Correlation coefficient' and difference between digital and analogaccelerograms. Coalinga aftershock, May 8, 1983; 125 degrees.

"GlC:~-r-----,.----..,.--------r----r----..,:::0.........

COALINGA AFTERSHOCK MAY 8.1983ANALOG 35 DEGREES

+-<-fc:ornl\.)

o

n ~S 'Irn 0 ~....L--....!,-_----:-..l._-r--l-"":"':":!.......:'~'..iIl'~"""'~-~-----ln'_C::O.O 5.0 10.0 15.0 20.0 25.0

FREQUENCY (HZ.

COALINGA AFTERSHOCK MAY 8.1983DIGITAL 35 DEGREES

"Glenc: o,.....-----,.----..,.--------r----r-----,:::D.......rn::D

.......-fc:ornl\.)

o

n3::'­U)

rn°-lll~_---.:..~_......:....:........:_..,....!.....L.L...!~Uf~_=..::Oo::~¥-......_.._,"'9n'_C::O.O 5.0 10.0 15.0 20.0 25.0

FREQUENCY (HZ.)t:J

....... en-.-------..----..,.---------r----r-----"TI 0

"TI.C~ALINGA AFTERSHOCK MAY 8. 1983

n3:'­(J)

f'T1 0 ...~~U~:.:....::!..lI!..!:.-=,.:f.-..IT:Ya~~IIWdIlool!!~~-_-~n· -f- CP. 0 5. 0 10. 0 15.0 20. 0 25. O'

FREQUENCY (HZ)

Figure 22 Fourier amplitude spectra of analog and digital acce1erogramsand difference. Coalinga aftershock, May 8, 1983; 35 degrees.

11Qw­CO,------"T----,------,----,------,:::D

~ I:Dl\..ll3: 0"'lJr.....--l .Corn-o

CORLINGR RFTERSHOCK MRT 8. 1983RNRLOG UP

5.0 10.0 15.0FREQUENC'( (HZ.)

20.0 25.0

CORLINGR RFTERSHOCK MRT 8. 1983OIGITRL UP

11QO)co,------r----,---------r----...-------,:::D.....rn:::D

n3:.......(J)

rn 0 ~~~--.:.--.:..J..,......:...---J:.L..:-.:+_-----!~~-.!...:..!.....!....!.::rl~!!laQWtJti!.'::Cl:::l~n'..... CU.O 5.0 10.0 15.0 20.0 25.0

FREQUENC'( (HZ.)o..... 0)Ilo,------r----,..------r-~--,-------,

11

CORLI NGR RFTERSHOCK MRY 8. 1983

:D3:"'lJr_• 0

n3:.......(J)

rn 0 ~~tt!.:.~~~~T~~~--!J!.!..J..Q._..:.---.:~-~~~~~~n·-¥-..... CU.O 5.0 10.0 15.0 20.0 25.0

FREQUENC'( (HZ)

Figure 23 Fourier amplitude spectra of analog and digital acce1erograms anddifference.. Coalinga aftershock, May 8, 1983; up.

"GlCT.I ----.. -r- --,- ,-- -,co­::D.....rn::0

COALINGA AFTERSHOCK MAY 8. 1983ANALOG 125 DEGREES

.....

n::;::........(.f)

~ P,4::--!..--~:-'-...L.--+.::.::.:t~~~!"""""---""'!"""-0-----i250~q). 0 5.0 10.0 15.0 20. .

FREQUENCY (HZ. I"GlC g;.~------r-----r------r-.---._----__,::0.....

COALINGA AFTERSHOCK MAY 8. 1983DIGITAL 125 DEGREES

.....-iCornl'lo

n::;::........(.f)

rno~~--.:..~--!L:.-__l~'W:!~~..-.~~.......eT"'_...o.-"""__1n'~ q). 0 5. 0 10. 0 15. 0 20. 0 25. 0

FREQUENCY (HZ.)o..... CT.I,,0".

COALINGA AFTERSHOCK MAY 8. 1983

"Gc ....::0 0.

n::;::

"CJ)

rn 0 11!ll~~!!!:~::....Jl!!._~~~~t.;.'-dIll!lllll'o!!6wo~!I!!!!IIo...!!Ioofooj_...1n· -.'~q).0 5.0 10.0 15.0 20.0 25.0

FREQUENCY (HZ)Figure 24 Fourier amplitude spectra of analog and digital accelerograms and

difference. Coalinga aftershock, May 8,1983; 125 degrees.

.....a

C>

.....a-

COALINGA AFTERSHOCK MAY 8,1983

35DEG

.....r---r--r,...,.",rn--r-r-T'TT"rTTT--r-"""T"""T'"T"T"TT'rr----r-..,.-r-M'TTTl O....

....... .......

DIGITAL

---------- ANALOG 0.07 HZ (UPPER)

------- ANALOG 0.40 HZ (LOWER)....... .....0 a~ ....... ....... ....... ~.~

0 0 0 0 0I .,!.. C> - ....

N

PERreD (SEC)

tJ(j)

<

.- .......z~'-(j)

rnn

Figure 25 Zero-damped response spectra of analog anddigital acce1erograms. Coalinga aftershock,May 8, 1983; 35 degrees.

.- .-O.-----r--r~,..,..,.TTT'-_r__r_,...,..,~rr____,___,__rT'"T"lrTTT-_,..._,...'T""r........,.,...

COALINGA AFTERSHOCK MAY 8.1983

lP

-

..-

.-0.:.

DIGITAL

-------- ANALOG 0.07 HZ (UPPER)

------- ANALOG 0.40 HZ (LOWER)..... .-a ar .- ..- .- ~'~

a a 0 0 aI .:. t;> ... .....

PEAleo (SEC)

..-o

IJ I

(.J)

<

Figure 26 Zero-damped response spectra of analog anddigital acce1erograms. Coalinga aftershock,May 8, 1983; up.

........O~--'---'-""T"1rTTnT"-'-"'T'""'1I'"T'1!"TT1rr---'--'''''''''T"T"1'TTT-''''''''''''T'TTTTn...

COALINGA AFTERSHOCK MAY 8,1983

125DEG.... .....

---DIGITAL

---------- ANALOG 0.07 HZ <UPPER)

-- - - -- - ANALOG 0.40 HZ (LOWER)

.....OL---L--L...L.L-I-L.LI.L_...J.-...J.-L.L-Ll..LLL---l--l'-L..&...l.IL.U.L_-I...-I.....I-L.J...LI......

-'1; ;; ;; ;; ~.:.~ .~ -0 N

....o

,!.

PERI-eJD (SEC)

Figure 27 Zero-damped response spectra of analog anddigital acce1erograms. Coalinga aftershock,May 8, 1983; 125 degrees.

1180

119°

... '..

.... . .

...

Cha

ngll

., . ..

.......'

oo o

o

:...

..0

..,

..- ....

..

.'....

.....,

". .'.

.....

..o

'•

.....

....

....

....

. .'.o

ML

2.9

-3

.9o

4.0

-4

.9

a6

.29

~1,2

....

'.. --\

\V

/1

39

°30

'I

0~

....

f/0

>------

1..---

-f=~

I

Fig

ure

28T

ang

shan

aft

ers

ho

ck

arr

ay

show

ing

locati

on

of

Lu1

0ng

even

tan

dS

tati

on

11

.

Figure 29 Accelerogram with integrated velocity and displace­ment. Station 12. Lulong earthquake, October 19, 1982;North.

::DI\,)

nono w..c:JG EAR11iQUAI(E OCT 19.1982·

LPn3:,f'(J)orT1n,(J)rT1nl

I\,)

~2P.O 2.5 5.0 7.5 10.0 12.5 15.0TIME (SEC)

~<0rT1I·nf'3:0,(J)rT1n

II\,)

dJ·O 2.5 5. a 7.5 10.0 12.5 15.0TIME (SEC)

I\,)

0

0.-(J)-0· f'

0n3:

II\,)

dl· O 2.5 5.0 7.5 10.0 12.5 15.0TIME (SEC)

Figure 30 Accelerogram with integrated velocity and displace-ment. Station 12. Lu10ng earthquake. October 19. 1982;up.

:ONnono LU..ONG EARTHQUAKE OCT 19.1982.

EASTn3:'-.?(J)ornn'-.(J)rnn~ i I i I i~gJ. 0 2.5 S.O 7.5 10.0 12.5 15.0

TIME (SEC)

co<0f"TIr

n?~o'-.(J)f"TIn

1codJ·O 2. 5 5.0 7.5 10.0 12.5 15.0

TIME (SEC)

'!'"0

0.......th"""U.

?0

n~

I~

dJ·O 2.5 5.0 7.5 10.0 12.5 15.0TIME (SEC)

Figure 31 Accelerogram with integrated velocity and displace­ment. Station 12. Lulong earthquake, October 19, 1982;East.