ANALYSIS OF THE PACOIMA DAM ACCELEROGRAM--SAN FERNANDO, CALIFORNIA, EARTHQUAKE OF 1971 B~ 5/I. D. TRIFUNAC AND D. E. HUDSON

Transcription

1 Bulletin f the Seismlgical Sciety f America. Vl. 61, N. 5, pp Octber, 1971 ANALYSIS OF THE PACOIMA DAM ACCELEROGRAM--SAN FERNANDO, CALIFORNIA, EARTHQUAKE OF 1971 B~ 5/I. D. TRIFUNAC AND D. E. HUDSON ABSTRACT Integrated grund velcities and displacements calculated frm the accelergram recrded at the Pacima dam site indicate that the strng grund mtin was predminately in the vertical and NS directin, in general agreement with the mechanism f faulting as inferred frm aftershck studies, and with fault displacements bserved in the field. High-frequency peak acceleratins f 1.25 g were recrded in tw hrizntal directins, these being the highest grund acceleratins s far recrded fr earthquakes. Respnse spectrum curves calculated frm the accelergrams d nt shw unusual features, and the numerical values are cnsistent with past experience. The high-frequency, hlgh-amplitude impulsive grund mtin assciated with the highest peak acceleratins did nt cntribute significantly t the ver-all respnse spectrum values. The presence f the high-frequency mtins in the recrded accelergrams is presumably the cnsequence f the prximity f the recrding site t the fault dislcatin. INTRODUCTION The strng earthquake grund mtin f the San Fernand, Califrnia, earthquake f February 9, 1971 was recrded n ver 2 accelergraphs f the Suthern Califrnia strng-mtin netwrk. These accelergraphs were lcated at varius grund sites, buildings and dams. By number and quality f the recrds, the instrumental cverage f this earthquake is, by far, the mst extensive and cmplete in the histry f strngmtin seismlgy. Althugh this 6.6 magnitude earthquake is nt large frm the seismlgical pint f view, it was assciated with very severe grund mtins and must be ranked as a majr event frm the standpint f damage and general engineering implicatins. The instrumentally-determined fcus f the main shck, at a depth f abut 13 kin, represented the regin in which faulting was initiated. The fracture then prpagated up and t the suth, past and under the Pacima Dam accelergraph site, and intercepted the surface in the Sylmar-San Fernand area (Figure 1). The Veterans Hspital which cllapsed killing 44 peple, and the Olive View Hspital which was severely damaged with 3 deaths, were lcated abut 3 km t the nrth f the surface faulting (Figure 1). The lcal severity f shaking in the Sylmar-San Fernand area appears t have been as strng as wuld be expected fr the largest shcks in Califrnia, althugh lnger fault breaks wuld result in greater duratins f grund shaking. The idealized empirical relatin between magnitude and fault length (Husner, 197) wuld predict the fault length fr this earthquake t be abut 15 kin, which is in excellent agreement with the bserved surface faulting (Figure 1) and the ver-all dislcatin size utlined by the distributin f aftershcks (Figure 2). Fr an earthquake f magnitude 8 r greater, the surface faulting might extend fr several hundreds f kilmeters and as a result the strng grund mtin wuld last several times lnger than the mtin recrded during the San Fernand earthquake. 1393

2 1394 BULLETIN OF THE SEIS1V[OLOGIC2~L SOCIETY OF A_M]~RICA THE ACCELEROGRAP~ SITE Because f the imprtance f the Pacima Dam recrd, which was btained virtually at the center f the event, and because f the rather special nature f the site, it is believed that a relatively detailed descriptin f the site and f the instrument installatin is justified. Figure 3 shws an blique aerial phtgraph lking suth ver the Pacima Dam site with the San Fernand Valley in the distance. The lcatin f the AR-24 aecelergraph at Pacima Dam is shwn, and the lcatins f the tw heavily-damaged hspitals are indicated in the backgrund. Figure 4 shws a clse-up view f the dam, with the lcatin f the aecelergraph n a rcky spine adjacent t the dam abutment :.,,..... >, :,":... ~. 4~;,: "~." "... ~ ' :.". "~'~:I".. PACOIMA ~ =~ ( ~ap_,)an I ' MOTION ACCELEROGRAPH I ' I.:,',.' -..,...,.... ~... :': :':~... :-v..].:. ' :,,.::.;%.:;.:,":,:' =.]].i.: i,..]..,:i,:.; ~:::... D A M t--~... STRONG { :::! i:::,:! ~':~'~,,:~';):;]:I~:~;~',S~,... ". OLI(/E VIEW VETERANS- u# 2"617 i,,... HOSe HOSe...)~-.i.~( L., ::."..d~ "... &,~:,! <m~... ~ l. ~ l li lii Ill' l~ SURFACE FAULTING ~2"... ~[- ]:;" ;: } DISPLACEMENTS " " :~ L. "5 "~<" "' "...-,I.~" GIVEN IN METERS SYLMAR ~!" ] "2:':~":~;~'~:;:%" ~"r.% ~\,~/.8" ~.,:_~m, ~.~.,,'. '.,. ~,, '"UP~.~.;~..:"...,'?-,... \ ~>. ~.;..-~. ~ - "-..t...-:::q...-): ~ %~ / X~,,~.~?'uJZ,ng e, ~'i i'iul'~:l'-"~.. SAN :' i? % FERNANDO P /~O//... "'":" ~:~c.ag~.:".... a~. ~ e....--l. TUJUh ~ " 25 ~ 24 ~ 25 ~ 22 ~ 21 ~ 2 ~ Fr. 1. Surface faulting in the Sylmar-San Fernand-Tujunga area (reprduced by permissin frm the Divisin f Gelgical and Planetary Sciences, Califrnia Institute f Technlgy, and Kamb et al., 1971), and its relatin t the recrded strng grund mtin at the Paeima Dam. indicated. Figure 5 shws a view f the circular instrument huse seen frm belw n the dam, and Figure 6 is a view f the dam and the instrument huse frm abve lking nrth. In bth f these figures, extensive cracking f the gneissic granite-dirite rck will be nted. Many f the cracks penetrate thrugh the smth gunite cating int the rck belw. It is nt knwn, hwever, t what extent the surface fractures f the gunite cating reflect the cnditins f the majr rck mass belw. Relatively small cracks f the dimensins f the gunite fractures wuld be assciated with higher frequencies than thse invlved in the apprximately 1 eps mtins bserved n the recrd (Figures 11, 12 and 13). The extent t which presently unknwn details f the ridge structure may influence the recrded mtins is fr the time being a matter fr speculatin nly. As can be seen in Figure 5, abut 5 m t the west f the aeeelergraph, a small rck slide ccurred (abut 5-1 m s) during the earthquake, which frtunately was nt large

3 ANALYSIS OF THE PACOIMA DA~i ACCELEROGRAM 1395 enugh t disturb the aecelergraph fundatin. As will be seen in Figure 7, ne f the cracks penetrated int the fundatin f the instrument huse, althugh as will be nted, the instrument pier itself, which was separated by an inch r s frm the fundatin f the circular huse, was nt cracked. As f tw mnths after the earthquake, changes in the cnfiguratin f these fmldatin cracks indicate that lng-term mtin f sme kind is cntinuing. After the earthquake, the instrument munting pier was still slidly attached t the fundatin rck, and the munting blts attaching the aeeelergraph t the pier were tight and undisturbed. The nly sign f disturbance was I 1 N -~ ~ - ~ ' : I km \~ - = II "~ c~tic.. / ' ~ - " ~ I e--main Shck ~ AcP~tn ~Sugs ~ _ ] -. " Pacima Dam I//.'.'_ ~ ~ Accelergraph. ; \, I v a"~,, / "%. _ ~,~j u.. \.... h \4 -J \ "-K G,~a,~ ) FIG. 2. Map f the epicenters f the main shck and representative aftersheks f the San Fernand earthquake f magnitude 3. and greater, thrugh February 23, 1971 (reprduced by permissin frm the Divisin f Gelgical and Planetary Sciences, Califrnia Institute f Technlgy, 1971). a small permanent tilt f the instrument during the earthquake, which culd be estimated t be f the rder f?5 in the NW directin. The amunt f this permanent tilt culd be estimated with fair accuracy frm the adjustments required after the earthquake t re-level the aceelergraph (Dielman, persnal cmmunicatin). A view f the aeeelergraph munted n its cncrete pedestal within the circular huse after the earthquake is shwn in Figure 8. A plan view f the dam, and the abutment area, with the instrument lcatins indicated, is shwn in Figure 9. Als shwn is the lcatin f the Wilmt Seismsepe n the crest f the dam. During the first few secnds f earthquake mtin, the mtin f

4 1396 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA the crest f the dam was s severe that the seismscpe glass recrd plate was disldged frm its retaining ring, s t h a t n useable seismscpe recrd was btained. As a further indicatin f the setting f the site within the aftershek regin, Figure 2 may be referred t. The site is apprximately 8 k m suth f the instrumentallydetermined epicenter snd is nearly in the center and abve the tentative fault dislc~- FIG. 3. Oblique aerial view lking suthwest ver the Paeima Dam site. Lin surface striking N72 W and drpping abut 45 tward the nrth (Kamb, et al., 1971). ACCELEIaOGI~AP~ PERFORMANCE The AR-24 strng-mtin accelergraph at the Pacima D a m site recrds tw hrizntal and ne vertical cmpnents f acceleratin n 12-in-wide phtgraphic

5 ANALYSIS OF THE PACOIMR DRM ACCELEROGRAM 1397 paper. The aceelergraph transducers have natural frequencies f abut 19 eps and damping f apprximately 6 per cent critical (Hudsn, 197). The instrument is ne f several wned by the Ls Angeles Cunty Fld Cntrl District and is a part f the Suthern Califrnia strng-mtin aeeelergraph netwrk maintained by the Seismlgical Field Survey f the NOAA Natinal Ocean Survey. :FIG. 4. Clse v i e w f t h e P a c i m a D a m. As mentined abve, after the earthquake, the aecelergraph fundatin remained tilted thrugh a small angle which was estimated t be abut?5. This small angle was sufficient t actuate the starting pendulum and the instrument recrded cntinuusly fr sme 6 rain until it ran ut f paper. During this interval, at least 3 aftershcks were recrded. In ne sense, the small permanent tilt f the fundatin can be cnsidered t be a frtunate ccurrence, since it permitted this recrding f the beginning f the aftershek sequence, and indicated the exact sequence f aftershek

6 1398 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA events in the epicentral regin. These details f the aftershck sequence will be f imprtance in investigating the mechanism f energy release. In rder t check the instrument perfrmance, tilt, free vibratin, and damping tests were perfrmed after the earthquake. The tilt test shwed that the sensitivity f FIG. 5. Strng-mtin AR-24 accelergraph site and the smm1rck slide.

7 FIG. 6. View f the dam and the i n s t r u m e n t site. FIG. 7. Cracks in the f u n d a t i n f the i n s t r u m e n t huse. 1399

8 14 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA the acce]ergraph had nt changed significantly. The alignment f the transducer axes relative t the instrument base was als checked during the tilt test (Trifunac and Hudsn, 197). It was fund that the tw hrizntal transducers were well within a FIG. 8. AR-24 accelergraph after the earthquake. 1 alignment. The vertical transducer sensitivity vectr was abut 5 frm the vertical in the lngitudinal directin. Judging frm the pint f view f the accuracy f the typical strng-mtin accelergraph (Trifunac and Hudsn, 197), it may be cncluded that the AR-24 accelergraph at the Pacima Dam site perfrmed essentially t specificatins and that the

9 _&N/kLYSIS OF THE PACOI~IA DAlai ACCELEROGRAM 141 recrded acceleratin traces may be adpted as representative f the actual mtin f the instrument fundatin. The peak acceleratin values remained n scale n the phtgraphic paper, and there is n evidence f appreciable nnlinear respnse at the maximum amplitudes invlved. After the earthquake, the instrument base was tilted in apprximately the NW directin, relative t its psitin prir t the earthquake. Since the preliminary calculatins FIG. 9. Pacima Dam site with AR-24 strng-mtin accelergraph and Sg 1 Wilmt Seismscpe. TABLE 1 ~:~ESULTS OF TESTS TO ASCERTAIN DISPLACEMENT LIMITS Directin f Baseline Shift fr Tilt in NW Directin Baseline Shift (cm/sec 2) N 74 E 13.3 Dwn 7.7 N 16W 2.6 f the grund displacement indicated a significant shift in the accelergraph base line, clearly a cnsequence f such tilt, tests were cnducted t ascertain likely limits fr such displacements. The accelcrgraph was tilted in the NW directin thrugh an angle which just clsed the starter pendulum gap, and in this way a lwer bund estimate f the acceleratin base-line shift culd be determined. The results f this test are given in Table 1. The integratin f the digitized accelergrams (Figure 1) including the first aftershck (abut 42 sec) indicates that the tilt must have ccurred within the first 1 t 15 scc f the strng mtin. This can be cncluded frm the behavir f the integrated

10 142 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA t l GROUND ACCELERATION 2/9/71 6 PST PACOIMA DAM, CALIFORNIA S 74 W COMPONENT E z F- li w w f.d Q) I 1 I I I SECONDS I GROUND ACCELERATION 2/9/71 6 PST PACOIMA DAM, CALIFORNIA DOWN COMPONENT Z [IZ w w <~ I I I I I I SECONDS I GROUND ACCELERATION 2/9/71 6 PST PACOIMA DAM, CALIFORNIA S 16 E COMPONENT % _ n- Ld W (D (D <[ IO 1 I I I 2 3 SECONDS FiG. 1. Plt f digitized accelergrams recrded at the Pacima Dam. I 4

11 ANALYSIS OF THE PACOIMA DAM ACCELEROGRAM 143 velcity curves. If a straight line fitted t the velcity curves (Trifunac, 197), ver the interval between 12 and 42 sec, is extraplated back t the zer time, the resulting displacement curves indicate the permanent displacements after the earthquake as shwn in Table 2. It might be tempting t interpret these results in terms f the bserved surface fault displacements (Figure 1, Kamb et al., 1971). Hwever, if it is assumed that the tilting indeed tk place during the first 1 sec, the lwer bunds fr the "permanent displacements" btained nly frm the tilt wuld be thse shwn in Table 3 (based n data f Table 1). Fr this calculatin it was assumed that the acceleratin zer base line is determined by its fixed psitin after the tilt is cmpleted, 1 sec after the instrument has triggered, and that the tilt ccurred unifrmly ver the 1 sec interval. Cmparing the amplitudes TABLE 2 INDICATED PERMANENT DISPLACEMENTS AFTER THE EARTHQUAKE Permanent Displacement in Directin Permanent Displacement Amplitude (m) N 74 E 1. Up 1.3 S 16E 1.7 TABLE 3 VALUES BASED ON ASSUMPTION THAT TILTING OCCURRED DURING }2IRST 1 SECONDS Permanent Displacement in Directin Lwer Bunds n Permanent Displacement Amplitudes Caused by Tilt (m) S 74W 4 Up 3 $16E 1 given in Tables 2 and 3, it may be cncluded that the tilt was large enugh t prevent any estimatin f the permanent displacements assciated with the earthquake. DATA PROCESSINa AND G~OUND MOTION CALCULATIONS Figure 1 is a plt f the first 42 sec f the digitized accelergrams including the first aftershck. The strng mtin representing the main energy release lasted fr abut 7 sec and the first aftershck was recrded abut 29 see after the instrument ~vas triggered. The first 15 sec f the acceleratin were chsen fr the analysis f grund mtin. The AR-24 accelergram was digitized at Caltech n a Bensn Lehner 99D data reducer and prcessed by the standard methds develped in recent years fr strngmtin accelergram analysis (Hudsn et al., 1969). The quality f the riginal recrd was excellent. The trace was clear and cntinuus with the exceptin f ne 1.25 g peak n the $16 E cmpnent at 7.6 sec. At this pint, the trace was lst abve the 1 g level and had t be extraplated. Because f the excellent phtgraphic quality f the trace, this extraplatin culd be carried ut with cnfidence. The errr in the peak is

12 144 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA believed t be less than.1 g, which wuld nt appreciably influence any calculatins based n the accelergram. The digitizatin f the Pacima Dam accelergram is GROUND ACCELERATION z d,,~ I, I, I, l 5 I 15 I GROUND VELOCITY 5 E >- I.- (D, I.iJ > 5 I t I I I ~ I 5 I 15 5 e i---- z hi ILl --I {2. "-I GROUND DISPLACEMENT 5, I, I 5 I SECONDS FIG. 11. $74 W mtin, Pacima Dam--San Fernand, Califrnia, Earthquake February 9, 1971, 6: P.S.T. believed t be as accurate as may be achieved by presently available techniques and equipment. The base-line crrectin was perfrmed by high-pass filtering the uncrrected data abve the frequency.7 cps. This means that all perids lnger than apprximately

13 ~knalysis OF THE PACOIM)~ DAM ACCELIgROGRA_M see have been remved frm the recrd, and, hence, that n infrmatin n permanent displacements is t be expected frm the analysis. This filtering prcedure, and the least-square fitting f a straight line t the grund velcity, which gives an esti- % I GROUND ACCELERATION z rr" L~ L) IOO g I, I 5 I I g 5 GROUND VELOCITY E I--,_J i,i 5 I I i I ' I 5 I 15 5 Z DA UA (D <~.J CL U) g GROUND DISPLACEMENT SECONDS FIG. 12. Dwn mt, in, Pacima Dam February 9, 1971, 6: P.S.T. San Fernand, Califrnia, Earthquake, I 15 mate f the initial velcity, cnstitute a new methd recently prpsed fr standard base-line crrectin f the strng-mtin aceelergrams (Trifunae, 197). The resulting acceleratin, velcity and displacement curves are shtvn fr the three recrded cmpnents in Figures 11, 12 and 13. As may be seen in these figures, the maximum ac-

14 146 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA. celeratin is 1.25 g fr bth hrizntal cmpnents and.7 g fr the vertical cmpnent, and the peak velcity is 115 cm/sec. GROUND ACCELERATION z (2_ n- w J w.) < I, I, t 5 I 15 g I 5 GROUND VELOCITY F- J w > 5O IO O 5 I i [, I 5 I 15 E i--- z w 2~ w (_3 n 3 GROUND DISPLACEMENT 5O I I, I, 5 I SECONDS FIG. 13. $16 E mtin, Pacima Dam--San Fernand, Califrnia, Earthquake February 9, 1971, 6: P.S.T. As already mentined, the tilting f the instrument, base must have taken place during the first 15 see f the strng mtin. Thus, the displacements in Figures 11, 12, and 13 may cntain an unknwn cntributin frm the tilting f the instrument in additin t the actual grund mtin. Nevertheless, the cmputed grund mtin

15 ANALYSIS OF THE PACOIMA DAM ACCELEROGRAM 147 L ~ I I 1 _ I I _ [ I _ I cj RELRTJVE VELOCITY RESPONSE SPECTRUM /9/71 6PST PACOIMA DRM, CAL S7~W z z > I 1 I I _ P I I I I I 2 3 ¼ PERIOD IN SECONDS I I ] I_ I I I I J t z RELRTIVE VELOCITY RESPONSE SPECTRUM /9171 6OOPST PQCOIMR DAM, CAL,DOWN Z >- cj > L I I I I I I J I I 2 3 '~ I PERIOD IN SECONDS 1 i I i I I I I I RELATIVE VELOCITY RESPONSE SPECTRUM /9/72 OGOOPST PACOIMA ORM, CRL 516E : ~ i ~ 1 I I I I i 2 3 ~ PERIOD IN 9 IO FIG. 14. Relative velcity respnse spectra, Pacima Dam. The curves are fr, 2, 5, 1, and 2 per cent damping.

16 148 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA J L L 1 J J L c_ >- I-- f,d LU Sd ~L RESPONSE SPECTRUM /9/71 6 PST PACOIMA DAM, CALIFORNIA $74 W COMPONENT Ld CO Z EL O9 LIJ ry 2 3 II I PERIOD IN SECONDS 9 _~ 1 I I I I I I I,g RESPONSE SPECTRUM l / Sd ~ FI /9/71 6 PST PACOIMA DAM, CALIFORNIA /I f 2 > g % / I I I I I I I I 2 3 ~ PERIOD IN SECONDS I ~ I I I I I I I.E >_" Sd RESPONSE SPECTRUM /9/71 6 PST PACOIMA DAM, CALIFORNIA Sl6 E COMPONENT (D hi Z O3 hi ry f I I I I I I I I q PERIOD IN SECONDS FX(~. 15. Sd2~r/T respnse spectra, Pacima Dam. The curves are fr, 2, 5, 1, and 2 per cent damping.

17 ANALYSIS OF THE PACOIMA DAM ACCELEROGRAM 149 indicates that the biggest displacements were vertical and in the nrth-suth directin, in general agreement with bserved surface faulting (Figure 1). In terms f acceleratin amplitude, the grund mtins recrded at the Pacima Dam site are the largest s far measured during any earthquake. The relatively shrt duratin f the severe shaking is a cnsequence f the shrt fault rupture. RESPONSE SPECTRA The cmputed relative velcity and Sd2rr/T respnse spectra (Sa = displacement spectrum; T = perid) are shwn in Figures 14 and 15. Fr each acceleratin cmpnent, the respnse spectrum curves were calculated fr, 2, 5, 1 and 2 per cent f critical damping. As expected, the relative velcity and Sd2~r/T spectra are very similar fr shrt perids while the Sd2rr/T spectrum falls ff mre rapidly fr lnger perids. It may be recalled that the zer damped relative velcity respnse spectrum is an apprximate representatin f the Furier amplitude spectrum f the accelergram. TABLE 4 COMPARISON OF EARTHQUAKE DATA Distance* Peak Acceleratin Peak Velcity Apprx. SV fr Earthquake Magnitude (kra) (g) (in/sec) T (in/sec) ~ 3 see San Fernand, ~ E1 Centr, :~ Parkfield, Kyna, India, ** 9 15 * Estimated distance frm accelergraph t prtin f fault surface assciated with maximum energy release. t Divisin f Gelgical and Planetary Sciences, Califrnia Institute f Technlgy (1971). :~ M. D. Trifunac and J. N. Brune (197). G. W. Husner and M. D. Trifunac (1967). II K. Aki (1968). H. G. Gupta, B. K. Rastgi, and H. Narain (1971). ** J. Krishna, A. R. Chandrasekaran, and S. S. Saini (1969). The spectrum curves fr the hrizntal S16 E and $74 W cmpnents shw peaks at abut.4- and 1.4-see perids, while the spectra fr the vertical cmpnent indicates predminant perids near.3 and 2 sec. The shrt duratin f the strng mtin is reflected in the nature f the respnse spectra f Figure 14, which shws a relatively flat character fr perids lnger than 5 r 6 see. In the perid range.5 t abut 3 sec, the spectral amplitudes are similar t thse calculated fr the E1 Centr 194 accelergram (Alfrd et al., 1951). The high-frequency spectral amplitudes in the Pacima Dam recrd are nt incmpatible with past experience. Similar high-frequency characteristics can be nted n recrds frm the Parkfield, Califrnia, earthquake f June 27, 1966, (Husner and Trifunac, 1967) and fr the Kyna, India, earthquake f December 1, 1967 (Gupta, el al., 1971). Direct cmparisns with these ther earthquakes are difficult because f significant differences in the lcatin f the accelergraphs, ~4th respect t the pattern f faulting, and in the sizes f the events. Table 4 gives an apprximate idea f such cmparisns. The strng earthquake grund mtin recrded during the Parkfield, Califrnia, 1966 earthquake (Husner and Trifunac, 1967) may be cnsidered as a typical example f a shrt, impulsive-type grund mtin. On the ther hand, the mtin re-

18 141 BULLETIN OF THE S~ISMOLOGICAL SOCIETY OF AMERICA crded at E1 Centr during the Imperial Valley, Califrnia, 194 earthquake (Trifunae and Brune, 197) is an example f the relatively lng shaking prduced by multiple events successively ccurring alng a fault abut 4 miles lng. The grund acceleratin, velcity, and displacement curves pltted in Figures 11, 12 and 13 shw that frm the engineering pint f view, the duratin f the energy release during the San Fernand earthquake is smewhere between that f Parkfield and E1 Centr. TI~E ENGINEERING SIGNIFICANCE OF TI-IE PACOIMA RESULTS One f the imprtant facts abut, strng earthquake grund mtin is that large grund acceleratin amplitudes in themselves d nt necessarily indicate severe damage t structures. It is als clear that high spectral acceleratins d nt always tell the whle stry. The respnse spectrum curves alne cannt give a cmplete picture f the effects f the time duratin f the acceleratin histry. These facts have been clearly demnstrated by the spectra calculated fr the Parkfield earthquake (Ilusner and Trifunac, 1967) and the El Centr earthquake (Affrd et al., 1951). Thus the high spectral amplitudes in Figures 14 and 15 d nt necessarily mean that this mtin was very destructive fr structures f all types. Pacima Dam, fr example, apparently suffered n significant damage. The San Fernand earthquake with strng mtin lasting abut 7 see nw becmes an excellent example f a strng grund acceleratin f shrt t mderately lng duratin. If the shaking had cntinued fr anther few secnds, much greater damage wuld have resulted, and many buildings and bridges s far nly partially damaged wuld have cllapsed. It is mainly this effect f the duratin f shaking n structural damage that calls fr detailed investigatins f the pattern f earthquake energy release in time. ACKNOWLEDGMENTS The existence f the Pacima accelergram is a tribute t the success f the lng-range prgram establishing the Suthern Califrnia strng-mtin accelergraph netwrk under the supervisin f W. K. Clud, Chief f the Seismlgical Field Survey, and R. P. Maley, in charge f the Ls Angeles ffice, f the NOAA Natinal Ocean Survey. We much appreciate the numerus cntributins f Richard J. Dielman f the Earthquake Engineering Research Labratry f the Califrnia Institute f Technlgy t all aspects f the prgram f instrument installatin, servicing, recrd cllectin and develpment, and labratry instrument evaluatin. V. Perez f the NOAA Seismlgical Field Survey assisted with the digitizatin f the Pacima accelergram; and Dr. A. G. Brady f the Caltech grup carried ut details f the data prcessing. We are indebted t the Ls Angeles Cunty Fld Cntrl District fr their frward-lking prgram f instrumentatin and in particular t Mr. E. J. Zielbauer f that rganizatin fr cperatin with instrument siting, site visits after the earthquake, and fr prviding maps and infrmatin n the site and the dam. We wish t thank Prfessrs C. Allen and B. Kamb f the Califrnia Institute f Technlgy fr permissin t reprduce their data in Figures I and 2. Special thanks are due t Engineering Mechanics ]Divisin f the Natinal Science Fundatin fr their very prmpt actin after the earthquake in making supprt available t carry ut data prcessing f the unprecedented bdy f strng-mtin accelergrams. This research was als supprted in part by the Natinal Science Fundatin Grant GA 2279 t the Lamnt-I)herty Gelgical Observatry f Clumbia University. REFERENCES Aki, If. (1968). Seismic displacements near a fault, r. Gephys. Res. 73, Alfrd, J. L., G. W. Yiusner, and R. R. Martel (1951). Spectrum Analysis f Strng-Mtin Earthquakes, Califrnia Institute f Technlgy, Pasadena.

19 ANALYSIS OF THE PACOIMA DAM ACCELEROGRAM 1411 Divisin f Gelgical and Planetary Sciences, Califrnia Institute f Technlgy (1971). Preliminary seismlgical and gelgical studies f the San Fernand, Califrnia, Earthquake f February 9, 1971, Bull. Seism. Sc. Am. 61, Gupta, H. K., B. K. Rastgi, and H. Narain (1971). The Kyna Earthquake f December 1, 1967: A multiple seismic event, Bull. Seism. Sc. Am. 61, Hanks, T. C., T. H. Jrdan, and J. B. Minster (1971). Precise lcatins f aftershcks f the San Fernand Earthquake 23 (GMT) February 1 t 17 February 11, 1971, U. S. Gelgical Survey, Prfessinal Paper 738, pp Husner, G. W. and M. D. Trifunac (1967). Analysis f accelergrams--parkfield Earthquake, Bull. Seism. Se. Amer. 57, Husner, G. W. (197). Strng grund mtin, Chapter 4 in Earthquake Engineering, R. L. Wiegel, Editr, Prentice-Hall, 197. Hudsn, D. E., A. G. Brady, and M. D. Trifunac (1969). Strng Mtin Earthquake Accelergrams, Digitized and Pltted Data, Vl. 1, Earthquake Engineering Research Labratry, EERL 69-2, Califrnia Institute f Technlgy, Pasadena. Hudsn, D. E. (197). Grund mtin measurements, Chapter 6 in Earthqua}ce Engineering, R. L. Wiegel, Editr, Prentice-Hall, 197. Kamb, B., L. T. Silver, M. J. Abrams, B. A. Carter, T. H. Jrdan, and J. B. Minster (1971). Pattern f Faulting and Nature f Fault Mvement in the San Fernand Earthquake. U. S. Gelgical Survey, Prfessinal Paper 788, pp Krishna, J., A. R. Chandrasekaran, and S. S. Saini (1969). Analysis f Kyna Accelergram f December 11, Bull. Seism. Sc. Am., 59, Trifunac, M. D. and J. N. Brune (197). Cmplexity f energy release during the Imperial Valley, Califrnia, Earthquake f 194, Bull. Seism. Se. Am. 6, Trifunac, M. D. and D. E. Hudsn (197). Labratry evaluatins and instrument crrectins f strng-mtin aceelergraphs, Earthquake Engineering Research Labratry, EERL 7-4, Califrnia Institute f Technlgy, Pasadena. Trifunac, M. D. (197). Lw frequency digitizatin errrs and a new methd fr zer baseline crrectin f strng-mtin accelergrams, Earthquake Engineering Research Labratry, EERL 7-7, Califrnia Institute f Technlgy, Pasadena. LAMONT-DOHERTY GEOLOGICAL OBSERVATORY EARTHQUAKE ENGINEERING I:~ESEARCH COLUMBIA UNIVERSITY LABORATORY PALISADES, NEW "YORK AND CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA (D.E.H.) EARTHQUAKE ENGINEERING I:~ESEARCI-I LABORATORY CALIFORNIA INSTITUTE OF TECHNOLOGY PASADENA, CALIFORNIA (M.D.T.) LAMONT-DOHERTY GEOLOGICAL OBSERVATORY CONTRIBUTION 7~N~O. 171 Manuscript received May 3, 1971.