We recovered 52 timed analog accelerograms from 25 aftershocks of

the 1979 Imperial Valley earthquake, between 3:33p.m. P.d.t. October

16 and 5:43 a.m. October 31. The largest aftershock that we

recorded (M_L =4.9) occurred at 4:16p.m. October 16. This aftershock

triggered eight accelerographs; preliminary estimates of epicentral

distance range from 7 to 35 km. The data from this aftershock may be

useful for study of both source and wave-propagation phenomena in

the Imperial Valley

Anderson, J. G.

Heaton, T. H.

Caltech Authors - Main

AFTERSHOCK ACCELEROGRAMS RECORDED ON A TEMPORARY ARRAY 
By J. G. ANDERSON, 
ScRIPPS INSTITUTION OF OcEANOGRAPHY; 
and 
T. H. HEATON, 
CALIFORNIA INSTITUTE OF TECHNOLOGY 
CONTENTS 
Page 
1\bstract -------------------------------------------------- 443 
Instrumentation ------------------------------------------ 443 
Recovereddata -------------------------------------------- 443 
1\cknowledgrnents ___ _____ _____ _____ ___________ _ __________ 445 
References cited _ ____ _ ___ ___ ___ _____ _______ ______ _____ ___ _ 445 
ABSTRACT 
We recovered 52 timed analog accelerograms from 25 aftershocks of 
the 1979 Imperial Valley earthquake, between 3:33p.m. P.d.t. Octo-
ber 16 and 5:43 a.m. October 31. The largest aftershock that we 
recorded (ML =4.9) occurred at 4:16p.m. October 16. This aftershock 
triggered eight accelerographs; preliminary estimates of epicentral 
distance range from 7 to 35 km. The data from this aftershock may be 
useful for study of both source and wave-propagation phenomena in 
the Imperial Valley. 
INSTRUMENTATION 
In the evening hours after the October 15, 1979, 
earthquake, a field crew was organized. Beginning 
early the next morning, they installed 10 accelerog-
raphs along the Imperial fault from north of Brawley, 
Calif., to the United States-Mexican border, with the 
object of recording aftershocks. The accelerographs 
(Kinemetrics model SMA-1) record three components 
of acceleration on 70-mm film, in the amplitude range of 
less than 0.01 to more than 1 g and in the frequency 
band 0 to about 25Hz. These instruments were slated to 
be installed in the Los Angeles area as part of a network 
being established there by the University of Southern 
California and the California Division of Mines and 
Geology. 
Each instrument has an internal clock (Kinemetrics 
TCG-1) that generates a time code to be recorded on the 
film. The clocks have a temperature-compensated crys-
tal oscillator with a frequency stability of ±3x 10- 7 in 
I 
the temperature range 0°-50°C; this stability implies a 
daily drift of less than 26 ms, provided the crystal fre-
quency is set properly. The clocks tended to drift much 
less than this limit. Clocks were corrected in the labora-
tory before instrument deployment, at the time of in-
stallation on October 16, at most stations on October 17, 
and at the time the instruments were recovered on 
November 1 or 2. There is no reason to suspect an uncer-
tainty of greater than 10 ms in the clock corrections 
interpolated to the occurrence times of aftershocks. 
Table 48lists the locations, elevations, and orientations 
of the accelerograph installations (see fig. 296), as well 
as the sensor characteristics for each accelerometer. 
RECOVERED DATA 
One or more of the accelerographs were triggered by 
at least 25 aftershocks with preliminary local mag-
nitudes estimated at from less than 3.0 to 4. 9. Because of 
clock problems, the times of 5 of the 52 total triggerings 
are uncertain. Triggering times and S-wave arrival 
times for the remaining 4 7 records (table 49) were read 
directly from the film accelerograms. In addition to 
these data, table 49 lists the stations that recorded the 
aftershocks and the peak accelerations recorded for 
each aftershock. These peak accelerations, obtained 
from the maximum trace amplitudes, have not been 
corrected for the response of the accelerometers. Figure 
297 illustrates the spatial relation between the pre-
liminary aftershock locations and the accelerographs 
that recorded them. About 75 percent of the triggerings 
occurred on the three northernmost accelerographs 
(stations 3689, 3692, 3698, fig. 296); the two south-
ernmost stations (3695, 3696) did not record any after-
shocks. This pattern is consistent with the distribution 
of the aftershock sequence, in which most of the later 
aftershocks occurred north of Brawley, Calif. (Johnson 
and Hutton, this volume). 
443 
444 THE IMPERIAL VALLEY, CALIFORNIA, EARTHQUAKE OF OCTOBER 15, 1979 
TABLE 48.-Temporary accelerograph sites 
[Location is within ±0.003°. Sensor orientation is direction of ground motion that causes positive trace motion on the film; upward groun~ acceleration causes positive trace motion on the 
vertical component. Accelerograph characteristics were supplied by Kinemetrics; damping is 60 percent of critical on all instruments 
Location 
Site Station 
Latitude N. Longitude W. 
Del Rio Country Club ______________ 3698 33.0095° 115.5207° 
Brawley Airport 
------------------
3689 32.9914° 115.5167° 
Sam Etchegaray 
Livestock Co. ____________________ 3692 32.9816° 115.4743° 
Doyle McDuffy home ______________ 3681 32.9133° 115.4920° 
Memory Gardens Cemetery ________ 3693 32.8919° 115.5676° 
Sharon Fox home __________________ 3678 32.8437° 115.4996° 
University of 
California Field 
Station 
------------------------
3697 32.8044° 115.4467° 
McGrew Farm ____________________ 3688 32.7769° 115.4448° 
Jasper Road 
----------------------
3695 32.7090° 115.4244° 
Tuttle Ranch ______________________ 3696 32.6940° 115.3745° 
Station 3698 did not record any events between 4:16 
p.m. P.d.t. October 16 and 1:33 p.m. the next day be-
cause the film jammed. The vertical component of the 
station 3692 instrument was knocked badly out of 
alinement on the trip to the Imperial Valley or during 
installation and, therefore, did not record any data. 
Stations 3688, 3693, 3695, and 3696 were not on battery 
chargers, and the batteries were drained when those 
instruments were recovered on November 2. 
The largest aftershock recorded was an M L = 4. 9 event 
that occurred at 4:16p.m. P.d.t. October 16; this after-
shock triggered the eight northernmost accelerographs. 
The preliminary epicentral location is 7¥2 km north-
northwest of station 3698, at a depth of 5 km. This event 
is characterized by strong relatively long period accel-
erations on the horizontal components of the three 
northernmost stations, and by high-frequency shaking 
on the vertical components. At stations 3689 and 3698, 
Sensor orientation Accelerograph characteristics 
Elevation (azimuth) Natural (ft) Axis Sensitivity fre(}l=}cy Longitudinal Transverse (mm/g) 
-145 081° 351° Long. 18.9 25.8 
Vert. 18.8 25.6 
Trans. 18.3 25.7 
-130 250" 160° Long. 18.7 25.0 
Vert. 18.6 24.8 
Trans. 19.0 24.9 
-135 338° 24SO Long. 18.5 24.5 
Vert. 18.5 25.5 
Trans. 17.4 26.4 
-132 334° 244° Long. 19.7 24.2 
Vert. 18.2 25.7 
Trans. 17.5 26.3 
-80 095° 005° Long. 19.6 24.2 
Vert. 18.0 25.6 
Trans. 19.3 24.7 
-100 357° 267° Long. 18.1 26.1 
Vert. 19.4 25.6 
Trans. 18.3 25.8 
-55 000" 270° Long. 18.9 24.9 
Vert. 19.4 25.2 
Trans. 17.6 26.0 
-25 35~ 266° Long. 19.5 24.3 
Vert. 17.6 25.8 
Trans. 17.0 27.2 
15 180° 090° Long. 18.1 26.3 
Vert. 18.7 25.3 
Trans. 19.3 25.2 
25 100" 010° Long. 17.8 26.2 
Vert. 18.2 25.4 
Trans. 17.1 26.9 
near-field effects are clearly visible on the horizontal 
components before the arrival of the S wave (fig. 298). 
These records may be useful for studies of wave propa-
gation and attenuation along the fault zone. 
Several accelerograms in figure 298 have been digi-
tized on the automatic digitizer at the University of 
Southern California and integrated according to the 
accelerogram-processing techniques of Trifunac and 
Lee (1973). Figure 299 shows the results for the three 
northernmost stations. This processing is not optimal 
for all these records because long-period ringing from 
the filter may be present on some processed displace-
ment traces, and nearfield permanent displacements 
are removed by the baseline correction. The peak accel-
erations on these digitized and processed records are 
smaller than those listed in table 49 for all stations, and 
for the vertical component at station 3689 the reduction 
is large-from 0.63 to 0.26g. This effect, which has been 
AFTERSHOCK ACCELEROGRAMS RECORDED ON A TEMPORARY ARRAY 445 
noticed before (California Institute of Technology, 
1976), results from sampling the data at too large a time 
interval to define the high-frequency peaks adequately. 
The solitary large peak on the station 3689 record (fig. 
115°45' 
33"00' 
32"45' 
115"30' 
~3698 
3689 ~ _},3692 0 Brawley 
)> 3681 
UNITEDSTATES- -------
--------MEXICO 
0 5 10 15KILOMETERS 
FIGURE 296.-Locations of temporary strong-motion accelerometer 
stations (see table 48). Arrows show direction of ground acceleration 
that causes positive amplitudes of recorded motion on longitudinal 
component of accelerometer; lines without arrows denote direction 
of positive motion on transverse component. Location of Imperial 
fault after Strand {1962). 
298) has a duration of 0.03 s between adjacent zero 
crossings and was sampled at 0.02-s intervals, and so it 
is not surprising that the digitized peak is greatly re-
duced. 
Horizontal velocities and displacements on these rec-
ords have relatively simpleS-wave pulses that probably 
are dominantly controlled by the physics of the source 
for this particular aftershock. 
ACKNOWLEDGMENTS 
Several persons and institutions readily cooperated to 
help install the accelerographs quickly after the earth-
quake. These included A. Amini and B. Westermo of the 
University of Southern California, D. Helmbergerofthe 
California Institute of Technology, D. Halverson of 
Kinemetrics, Inc., and D. Hadley of Sierra Geophysics. 
Without their prompt response, this project would not 
have been possible. We also thank the following persons 
and institutes for allowing us to install an accelero-
graph on their property: Del Rio Country Club, Brawley 
Airport, Sam Etchegaray Livestock Co., the University 
of California Field Station, D. McDuffy, S. Fox, E. 
McGrew, and the California Institute of Technology. 
V. W. Lee, M.D. Trifunac, and M. Dravinski helped to 
digitize and process the accelerograms. The Los Angeles 
accelerograph network is supported by National Sci-
ence Foundation Grant PFR 78-06926. 
REFERENCES CITED 
California Institute of Technology, 1976, Strong-motion earthquake 
accelerograms index volume: Pasadena, California Institute of 
Technology, Earthquake Engineering Research Laboratory Re-
port EERL 76-02, 42 p. 
Kinemetrics, Inc., 1977, Operating instructions for the time code 
generator, TCG-1: Pasadena, Calif., 14 p. 
Strand, R. G., compiler, 1962, San Diego-El Centro sheet of Geologic 
map of California: California Division of Mines and Geology, 
scale 1:250,000. 
Trifunac, M. D., and Lee, V. W., 1973, Routine computer processing of 
strong-motion accelerograms: Pasadena, California Institute of 
Technology, Earthquake Engineering Research Laboratory Re-
port EERL 73-03, 360 p. 
446 THE IMPERIAL VALLEY, CALIFORNIA, EARTHQUAKE OF OCTOBER 15, 1979 
+ + + 
io !. 
'· 
io i. 
0 0 • 0 
0 
0 0 • 0 
0 
0 0 • 0 
0 
0 0 • 0 
0 
0 0 • 0 
0 
289 22 33 00 289 23 12 0 289 23 16 0 0 289 23 17 0 0 290 00 06 0 0 0 
ML =3.1 ML =3.4 ML =4.9 ML=3.3 
+ + 
io i. i. ~0 io 
0 0 0 0 0 
0 0 0 0 0 
0 0 0 0 0 
0 0 0 • 0 0 0 0 0 0 
290 00 34 oo 290 02 28 0 290 06 14 0 0 290 10 13 oo 290 13 17 0 0 0 
ML =3.5 ML =3.6 
+ io + io + i. io '· 
0 0 • 0 • 0 0 • 0 0 
0 0 0 0 0 
• 0 0 • 0 0 0 0 0 0 
+ 290 13 53 0 0 290 16 17 0 0 290 19 14 0 0 290 22 19 0 0 290 22 45 0 0 
Mt.=3.3 ML =4.2 ML =4.6 
'· '· 
!+ 
0 t 0 to 
0 0 0 0 0 
0 0 0 0 0 
0 0 0 0 0 
0 0 0 0 0 
0 0 0 0 0 
290 22 45 0 290 22 47 oo 290 22 54 0 290 23 27 Oo 291 02 29 0 0 0 0 
ML =3.4 ML =3.2 
+ i. •• i • 
t 0 ~0 
0 0 ~ 0 0 0 0 0 0 0 
0 0 0 0 0 
0 0 0 0 • 0 0 0 0 0 
291 12 01 0 0 291 12 42 0 0 291 13 20 Oo 291 14 56 0 0 304 11 43 ~0 
ML =3.5 ML =3.2 ML=3.3 ML =3.4 
FIGURE 297 .-Summary of strong-motion data recorded at temporary accelerometer stations (see table 49). Each frame is a 
simplified map representing one aftershock. Stations (see fig. 296 for locations) are shown as either circles (no record) or 
dots (record), and preliminary epicentrallocations, where available, as crosses. Julian day, hour, and minute (G.m. t.) of 
aftershock is given in lower left corner, and preliminary local-magnitude estimates are given for events of ML;;;;.3. 
AFTERSHOCK ACCELEROGRAMS RECORDED ON A TEMPORARY ARRAY 447 
TABLE 49.-Summary of aftershock accelerograms 
[Time of event is Julian day (Jan 1= 1, Oct. 15=289) and Greenwich mean (G.m.t.); for Pacific time, subtract 8 hours (P.s.t.) or 7 hours (P.d.t.). Locations were determined without use ofS-wave 
arrival times listed for these acce~erometers. Depth measurements are particularly unreliable (C. E. Johnson, oral commun., 1979). See table 48 for azimuthal directions of horizontal 
components of peak accelerations 
Event Preliminary Peak accelerations {g) 
Second Location Depth Magnitude Station ea:~:~~l 
Day Hr Min (lat,long) (km) Trigger S arrival (km) Longitudinal Vertical Transverse 
289 22 33 59.53 61.50 3.1 3689 0.02 0.03 0.02 
289 23 12 38.36 39.19 33"4.32' N., 1.8 3.4 3698 7.0 .05 .03 .02 
38.46 40.75 115°31.94' w. 3692 11.5 .03 .06 
289 23 16 33.16 34.46 33"4.48' N., 5.0 4.9 3698 7.8 .30 .26 .16 
34.21 34.94 115°33.26' w. 3689 10.0 .29 .63 .22 
33.71 36.22 3692 12.8 .41 .29 
35.68 38.14 3681 19.0 .06 .04 .05 
36.07 39.32 3693 20.3 .10 .03 .12 
36.92 40.63 3678 26.0 .09 .04 .07 
40.84 42.47 3697 32.0 .08 .01 .05 
44.74 (') 3688 35.0 .04 .02 .02 
289 23 17 29.81 31.49 3689 .09 .05 .02 
290 00 06 23.70 25.44 33°3.80' N., 6.3 3.3 3689 8.8 .04 .06 .03 
24.26 26.56 1W32.75' W. 3692 11.3 .07 .09 
290 00 34 6.41 8.46 3689 .02 .04 .02 
290 02 28 20.44 22.24 33°4.77' N., 5.0 3.5 3689 10.3 .05 .07 .08 
20.92 23.35 115°33.16' w. 3692 13.0 .05 .03 
290 06 14 3.87 5.60 33°4.31' N., 5.6 3.6 3689 9.8 .06 .05 .05 
4.31 6.61 115°33.04' w. 3692 12.8 .08 .07 
290 10 13 19.27 12.37 3697 .11 .06 .11 
290 13 17 20.56 22.68 3689 .03 .01 .02 
290 13 53 47.31 50.04 3697 .02 .02 .02 
290 16 17 39.02 41.07 33"1.13' N., 6.2 3.3 3689 5.8 .02 .01 .02 
115°34.26' w. 
290 19 14 41.41 44.08 33°58.32' N., 4.7 4.2 3689 8.5 .10 .02 .05 
41.62 44.91 115°36.36' w. 3692 12.5 .05 .05 
40.91 43.65 3681 12.5 .06 .05 .06 
40.09 42.03 3693 9.8 .25 .08 .39 
290 22 19 47.60 50.67 3~44.47' N., 17.8 3697 7.5 .02 .02 .02 
115°24.83' w. 
290 22 45 35.14 36.37 33"2.73' N., 3.9 4.6 3698 5.0 .11 .15 .09 
35.41 37.18 115°29.40' w. 3689 6.8 .21 .22 .13 
35.66 38.06 3692 7.3 .16 .18 
39.71 40.41 3681 14.8 .03 .02 .03 
290 22 45 56.44 57.60 3698 .05 .03 .05 
56.46 58.22 3689 .12 .15 .10 
57.03 59.23 3692 .10 .09 
290 22 47 7.86 9.16 3698 .03 .04 .03 
8.50 9.95 3689 .05 .04 .04 
8.88 10.66 3692 .04 .05 
290 22 54 21.80 23.13 33°2.58' N., 4.8 3.4 3698 4.5 .01 .02 .02 
1W29.39' W. 
290 23 27 32.82 33.80 33°2.52' N., 7.1 3.2 3698 4.0 .03 .02 .02 
1W30.19' W. 
291 02 29 14.45 15.72 33°1.21' N., 5.0 3698 1.3 .04 .04 .04 
115°31.08' w. 
291 12 01 15.17 16.33 3~58.25' N., 15.3 3.5 3692 13.5 .02 .01 
115°36.99' w. 
291 12 42 24.78 25.90 33°0.40' N., 5.1 3698 2.5 .02 .03 .01 
25.22 27.34 115°29.75' w. 3692 3.5 .09 .13 
291 13 20 29.81 32.89 3~54.17' N., 5.0 3.2 3692 9.0 .02 .02 
115°29.48' w. 
291 14 56 21.04 22.37 33"2.19' N., 5.1 3.3 3698 4.3 .03 .03 .03 
115°29.59' w. 
304 11 43 49.66 52.78 3~43.35' N., 17.4 3.4 3697 9.5 .03 .04 .06 
115°24.80' w. 
For the following events, the clocks did not work properly: 
.05 291 02 29? ---- ---- ---- 3689 .09 .04 
or 12 01? 
291 12 42? 3689 .09 .05 .07 
291 13 20? 3689 .03 .03 .02 
or 14 56? 
? 3678 .04 .02 .04 ? 3678 .03 .03 .04 
'Triggered during S-wave arrival. 
448 THE IMPERIAL VALLEY, CALIFORNIA, EARTHQUAKE OF OCTOBER 15, 1979 
3698 
-·---- . ------ --- . ------- - ·- -------
... ---· ... -·. --·. -· ... --· -· ..... 
Clock correction: -0.09 s 
3689 
A 
t Up 
t 351° 
------.-.. -_-:-.-.-~--7.-.-.---.-.-..:..-.-.---.-.-.-.-_-_:-_-.-.-.-.-.-.-.-_:__:-.. -.. -.-.-.. -.-_-_:-_-_-.-.---.-.-_-_.._-_-_-.-.. -.---.-.. -~-.-.---.. -.-.-_-_-_:-_-.-.-.-.-.. -.-.--·-·.-.-.-.-.---.. 
Clock correction: + 0.96 s 
---........._.. ~ /\, (\ A t 25oo -~~~~~·~~ ~~-------------------------------------
3692 
·.-.. -_-.-.. -----.-.-_:_-.-.. -.. -.-.. ---.-.-~--.-.-_-.. -.-.-.. ---_·.---.. -.. -.. -.-.-.-.. --·-· .. - .. -.-.-.. -.-.. ---_-.-----_-_-----.-.--·----.. -.. -.-.. ---.. -.. -_-_ -.. -.-----.. -.-.. -_-_·_---.-.. -.. -.. -.. - .. -.. -_·..: ... -.. -.-.. -.. -_-
Clock correction: + 1.31 s 
t 338" 
t Up 
t 248° 
3681 
-- - . --- ------ -- -------- . --------- --- --- ---- . - ----- --- - --- ------ -- -------- -· 
-- - - - - . - . - -- - .. - .... --- ...... - - -- -- .. - - - .. -- - - - . - - - - -- - . - - . - - - -- - - . - - . - .. 
Clockcorrection: +0.06s 
t 334° 
------~~~~~~---------------------------------------------------~ t Up 
t 244° 
0 2 3 4 5 6 7 8 9 10 11 
TIME, IN SECONDS 
FIGURE 298.-0riginal accelerograms from ML =4.9 aftershock of October 16, 1979. Records are shown from north (top) to south (bottom) in order of 
increasing epicentral distance. Time trace at tip of each record gives a code with 5 (downward) pulses per second; code is interpreted as in 
Kinemetrics, Inc. (1977). Clock corrections at time of aftershock are also shown. Numbers to right on each trace indicate longitudinal, vertical, 
and transverse azimuths from top to bottom, respectively. A, Northern four records. B, Southern four records. 
AFTERSHOCK ACCELEROGRAMS RECORDED ON A TEMPORARY ARRAY 
B 
3693 
------ -- ------- ·---
·- -- ----·--.-~-..... -.-_-_-_-_-_._-_-_-_-_-_-_-_-_-_· __ -_-_-.-~---_-_-_._-_-_-_-_-_-_-_-_-_·_-_-_-_-_-_-_-_-_-_·_-_-_-_ ... - --- -- ~ ........ . 
Clock correction: + 1.15 s 
t Up 
3678 
Clock correction: -1.02 s 
t Up 
3697 
Clock correction: -0.10 s 
t Up 
3688 
Clock correction: + 1.24 s 
2 3 4 5 6 7 8 9 10 
TIME, IN SECONDS 
FIGURE 298.-Continued 
t 356" 
t Up 
11 12 13 14 15 16 
449 
17 18 
450 THE IMPERIAL VALLEY, CALIFORNIA, EARTHQUAKE OF OCTOBER 15, 1979 
v max= 58.3 cm/s 
Bmax= -133.1 cm/s2 
umax=3.46cm 
TIME, IN SECONDS 
A 
vmax= 10.5crn/s 
t Up 
Vmax=16.3cm/s 
t 351° 
amax =261.8crnlr 
a max=- 285.4 cm/s2 
Bmax= 186.8cm/s2 
Umax=-4.54cm 
10 
v max= 37.3 crnls 
vmax=-10.6cm/s 
--1 
Vmax= 15.2 cm/s 
B 
FIGURE 299.-Digitized, baseline-corrected, and integrated accelerograms from three stations closest to epicenter of ML=4.9 aftershock of 
October 16, 1979: 3698 (A), 3689 (B), and 3692 (C). Only first 10 s of digitized record is shown; numbers to right on each graph indicate 
longitudinal, vertical, and transverse azimuths from top to bottom, respectively. amax• maximum acceleration; Umax. maximum displace-
ment; Vmax. maximum velocity. 
AFI'ERSHOCK ACCELEROGRAMS RECORDED ON A TEMPORARY ARRAY 451 
Bmax = -377.1 cm!s2 v max= 45.5 cm/s 
Umax=11.0 em 
Bmax= 267.6cm/s2 Vmax =-28.6 cm/s 
Umax=-5.69cm 
5 10 
TIME, IN SECONDS 
c 
FIGURE 299.-Continued 


Aftershock Accelerograms Recorded on a Temporary Array

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Aftershock Accelerograms Recorded on a Temporary Array

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