UNITED STATES DEPARTYiENT OF THE INTERIOR

GEOLOGICAL SURVEY

EL CENTRO CALIFORNIA DIFFERENTIAL GROUND h1OTION AR:l Y

by G. Noel Bycroft

OPEN-FILE REPORT 80-919

This report is pre1iminary and has not been edited or reviewed for conformity

with Geological Survey stapdards and nomenclature

Henlo Park, California

1980

6() 46

~ ~

El Centro, California Differential Ground Hotion Array

by

G. N. BycroftU.S. Geological SurveyMenlo Par k, California

Abstract

Differential ground motions due to horizontally propagating seismic

surface waves are important in determining the stresses developed in extended

structures such as large mat foundations for. nuclear power stations, dams,

bridges, and pipelines. This report discusses the design of an array to

measure these differential ground motions and describes one such array,

recently installed at El Centro, Calif. The records from the October 15, 1979

Imperial Yalley earthquake are presented.

Introduction

Aseismic design has generally assumed'hat all points on the ground move

in unison with the freefield motion over a region that is larger than the

foundation of the structure. This assumption is based on the notion that

seismic wives are substantially propagated in high-wave-velocity basement rock

and, transmitted vertically to the region of interest .through lower velocity

layers. However, it is now realized that surface waves, propagating

horizontally through surface layers, may have wavelengths along, the surface of

the same order as the dimensions of a large structure, (Luco, 1969; Trifunac,

1972; Mong and Trifunac, 1974). The foundation of the structure would then

undergo differential motions that would cause additional strains to be

superimposed on those due to inertial loading. Thus, adjacent bridge piers

would move relative to each other and cause substantial stresses in the piers

and the bridge decking. Structures built on spread footings, dams, and

pipelines would be similarly affected. A large relatively rigid raftfoundation, such as may be used for a nuclear power station, would move less

1

're". '= Id t o.", (Bycroft, 19'.0) and so the input to s',"...cturr. ('n

s ' = : : .'a:i: n w,"uld be attenuated; the forcing roti;... or: such .: ",.;'.„ ~ s

would differ from the free-field motion.

Tc. s'.udy such motion, diffe"ent ial ("<'unC motions m Jst be meas.red anl

.-.:'.'.; .'c o, utilizing this ',rfor;.,ation in ~seismic Cesigr, r,:„st

The .""'.:p„", e,"--.r'. of frEe-f;eld gro::. d r.>', ion is relat'.'vcly stra..c".'.-'orward in

that nc spatial parameter is involved. For differential ground m tions,

hc:.ei=", s"r ace waves may propagate at wavelengths cc';..„-arable tc ~e s'" ('fthe foundation, and so a spatial array of instruments is needed. If expense„

were no consideration, a fully three-dimensional array comprising many

instr .-.,ents could be built. Initially, it would appear more adva. 20cous to

divi =- these instruments a-.ong sevei al simpler arrays in different suitaMe

regions that should, of ccurse, be nf hiqh seismicity both in amplitude and

occu: rcnce. In order tha'. surface ~ 'ves of signif icant a-.plitude be g:ncrated

it is necessary to chc ose regions of large contrast in wave velocity between n

the surface and underlying layers. The upper layer should be of as low a

velocity as possible, so that the wavelengths are as short as possible.

Furthermore, the selected region should be flat, homogeneous, secure power

should be readily availab'ie; and the regional velocity profile should be known.

The simplest array th- would give useful information appears to be one

comprising several instrurents alonq a straight line. This line should point

toward the epicenter of an itr.-.,inent larqc earthquake to measure the maximumI

a"'plitudes of the incident transverse surface waves. It is difficult,hc'wcver, to predict such an orientation, and so the best that can be done is

tc poirt the array toward a general region of recent activity. If enough

.instruments „=: c ai iilable, an additioral perprndicular art ay h

advantagco s.

The differential motion between any two points is a func Lien of wave)~ rgth

and of the m=gnitude of the ccmponent of that h.ivclc q'.)i in th: c c"n"

m ~ t >of> ~ <)'i" ci» c ~ ence in I )i ion be»'een pc iir>'s at v" yir" c's'.a 4< 5

apart r,:.s'. b:- ™-a'.ut'. If n instru.—.:ents are tn be used, t"c: e are n(n- 1)/2

pairs of pcin.s whose distances apart may be arrang=". to be dif,erent, The

ins'.r .-.=nls should be sc placed along the line of the array that these

distances increase reasonably uniformly from smallest to largest, assuming

that the region is uniform over an area somewhat larger than that of the

array. T).e lenc'.h ni th array shculd be determined largely hy the size of

the largest struc'.u: cs envisaged.

E 1 Centro di ferential ari ay

Such an array, un,";ed jointly by the Federal Hip':»ay ACministration and

the U.S. Geolngical Sv1vey, »'as I ecently installed at El Centro, Calif. in

time to record the carthqua) e of October 15, 1979. The Jrperial Valley

1rrigation Dist,r'.ct permitted the array to be placed in

near El Centro s',a--.; station No. 4 that is secured'y a

meets the other requirer.'. nts discussed above. F"igure 1

and table 1 the velocity profile, of the region; f ic.re

a large vacant area

cyclone fence and

sho»s the structure,

2 shows the location

of the array with respect to local faults. Because faults surround the area,

no particular nri>cntation appeared optimal, and so the array was laid south to

north along the insid< of the fence running along Dcg»'ood Road (fig. 3). This

area is re-..ote frcm the po»er plant, and so interacticnal effects should be

negligible. Tlute six instrur ents were placed at distanc s of 0, 60, lSO, 4?0,

700, and 1030 ft :nd nu,,"ered 1 through 6 respectively; these spacings give

cs bi t»'n any two instru:;:) nts nf 60, 120, i/A, 240 280 3nO

4?0, 5"0, 580, 640, 700, S20, 940,,.nd 1000 ft.The sensors are triaxial downhole force-balance accclercmeters

manufactured by the Terra Technology Corp., Seattle, Rash. The sensors w".) e

place in 5- in-die let cr holes 4 ft d( cp and t a)))))ed in»ith 2 to "r.:-,. ccarse

san>d, ard cc.n:..ected by wi~ing laid in conduit to DCA-"0>0 recorders in an

a il -c""d: ioned bui 1 ding at the south end of t he array. The at a I) c g s igna1

from the sersors is digitized at intei vals of 1/100 s and stored or r'.=qn.-tie

tape. he system continually remembers events as long as 1.5 s before

triggering, and thus permits a record of the events that precede triggering.

The six instrum>ents are triggered frcm an St';".-1 seismoreter and have a co-.—..on.

clock. The radio time signal RRVB is also recorded on the tape.

The installation of this array ha" h) on delayed by many fac'tors but,

>ortu'tously, was completed about t»o »'),eLs he.ore the Imperial Valley

eartt quake of October 15, 1979. There were several malfunctions. Instrument

6»as nut of tape owing to some unLno»n extraneous triggering before the

event. There is a certain amount of noise on some records that can be

removed. The instruments were not synch) onized in time'as planned,.and the

time sianal was not recorded owing to a receiver problem. Consequently,

cc;,.a n time»'as lost.

An-)og printouts of the tapes fr,m the five stations (figs. 4-6) are

similar in shape at the five stations. The left side of these figures is not

co;.-.an ti-.e. The maximum accele) ation of 0.67 g occurred at stations 1 and 2

in an unusual event early in the vertical cor.pnnent; station 5, however,

recor c " this event at a maximum of 0.3." q. The maximum accelerat ion in the

hori ontal direction is about 0.43 g. Although these acceleration

records ap„"-ar very si.".;ilar, 1 i L t le (an b( said about thi

to ause they ar e assc: ia'.: d with the 1(";:c r f rc „";:cr:y CC-;.

displace...ents can be obtained with suitable corrections by d...'c int c;"ation

of .he -ccele: ograms. Terra Technoloqy Compo! ation has trar«'..".. 'heir'.n a c.-;...'.er-ce.":;-'l.lc t;. i ai d h.. ~ e.t;eblis"=" c-- r t'-.c ': so '",c t P-'s'ave al rl'l fre,;. an a t('r. ~ hock 7,5 p.in af 'rtechriq"e should allcw the corputation- of the differenti

disp',ace,—..ents. P ma'.",e;.,al ical r.;fidel of the sur face-wave

al er:..'d

pr,~":-::.'=r. will be

constructed to deterrine the notion at points other than the -:as rinQ

stations.

References cited

Bycroft, G. N., 1980; Soil foundation interaction and differential groundMotions: Journal of Earthquake Engineering and Structural Dynamics [inpress].

Hansen, M. R., Meiss, R. 8., Idress, I. H., and Cluff, L . S., 1973,Geotechnical data compilation for selected strong-motion sites:Moodward-Lundgren Assoc. unpublished report for the National Oceanic andAtmospheric Administration.

Luco, J. E., 1969, Dynamic interaction of a shear wall with the soil: AmericanSociety of Civil Engineers Proceedings, Engineering Mechanics DivisionJournal, v. 95, no. EM2, p. 333-346.

Shannon 8 Wilson, Inc., and Agbabian Associates, 1976, Geotechnical andstrong-motion earthquake data from U.S. accelerograph stations: U.S.Nuclear Regulatory Comnission Report NUREG-0029, v. I.

Trifunac, M. D., 1972, Interaction of a shear wall with the soil for incidentplane SH waves: Seismological Society of America Bulletin, v. 62, no. 1,p. 62-83.

Mong, H. L. and Trifunac, M. D., 1974, Interaction of a shear wall with thesoil for incident plane SH waves: Elliptical rigid foundation:Seismological Society of America Bulletin, v. 64, no. 6, p. l825-1842.

Table 1.- Low strain, shear wave velocitiesE 1 Centro

Test depth interval(Ft) (V~ters)

Averages shear w;.ve v.'i<city(F t/sec) (l'./s.". )

0-1616-323?-72

72-116116-225225-271271-3443-;4-390

0 4.94.9-9.8

0 8 21 0

?1.9-35.4'35.4-68.66R.6-8?.6

82. 6-104. 9104 o 118 9

400550700850

1,0001, 1501,3201,450

122

2<'QP

3514024|'.2

Note: Shear wave velocities were obtained from field, dovnhole

geophysical measurements at strain levels on the order of 10

percent. Velocities were ver god for the indicated test intervals

(Shannon and Milsnn, 1nc., and Agbabian Associates, 1976).

' ~ ~ ~

Figure 1.

Figure Captions

Geologic structure at El Centro Diff(ici>t i;:1 G~c:;r<'!'..".'.n / "i:.y

(Hansen, M. R., and others, 1973).

Figure 2. Map showing location of El Centro Diff~i~ nt ial Ground t';.', inn

Array (Latitude 32. 796 degrees North; 1!.~(gi'.ude 11! ..'"": C ul i ps

Vest). Array runs south to north.

F igure 3.

Figure 4.

Figure 5.

Figure 6.

El Centro Differential Ground Motion Array boite. View l<~ <Ling

north. Recording house shown in right cc::ter of p!:."'.c;:; h. Black

box adjacent to house (to left) is Station Nc. l; ve~ain,lcr of array

extends north (into photo background).

Vertical accelerations at the El Centrn D "..fe .en:ial Grou,d V.;;ion

Array.

fast-west accelerations at the El Cen'rc Pifferential G~ c..~ d l'.ition

Array.

North-south accelerations at the El Centrn Different ial G"mund

Motion Array.

ri'guru I,'eologic structure at Centre Differential Ground Mpn Array( Ha n s en , M . R . , a nd 0 Me rs , 1 973 ) .

Dcf:si tvp7~~

SACS EO,+lg Skkf kn E C Di ffe rd'n~tfgl Arra~b.~g~'~r f. Rt!.'" > F-t™c S-4'mc Sift Ct 70 r-.e.cps m7sec

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CALIPATRIA0

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ill

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Figure 2. Map showing location of El Centro Differential Ground Motion Array(latitude 32.796 degrees North; longitude 115.535 degrees West),Array runs south to north.

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Figure 3. El Centro Differential Ground Notion Array site. View looking noi t.h.Recording house shown in right center of photograph. Black boxadjacent to house (to left) is Station No. 1; remainder of arrayextends north (into photo background).

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