a new digital accelerograph network for el salvador

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N e w D i g i t a l c c e l e r o g r a p h N e tw o r k f o r

E l S a l v a d o r

J . J . B o m m e r , 1 A . U d i a s , 2 J . M . C e p e d a , 3 J .C . H a s b u n , 3

W . M . S a l a z a r , 3 A . S u d r e z , 4 N . N . A m b ra s .e y s, 1 E . B u f o r n , 2 J . C o r t in a , 3

R . M a d a r i a g a , ~ P . M 6 n d e z , ~ J . M e z c u a , a n d D . P a p a s t a m a t i o u 6

INTRODU TION

T h e r e a r e fe w c o u n t r ie s w h o s e g e o g r a p h y a n d h i s to r y h a v e

been so a f f ec t ed by ear t hquake and vo l can i c ac t i v i t y as E1

Salvador , the smal lest republ ic in Cent ral America (Figure

1) . The capi tal , San Salvador (Figure 2) , has the unenviable

c l a im o f b e i n g t h e L a t i n A m e r i c a n c i t y m o s t f r e q u e n tl y d a m -

aged by ear thquakes . S i nce 1700 , San Sa l vador has been

severe ly dam aged by ear t hquak es on a t leas t 14 occas ions

( H a r l o w e t a l . , 1993) . The l as t des t ruc t i ve ear t hquake t o

af f ec t San Sa l vador occu r red on 10 Oct o ber 1986 , caus i ng

a b o u t 1 , 5 00 d e a t h s a n d e x t en s iv e d a m a g e o v e r m u c h o f th e

ci ty , as wel l as an ec on om ic loss equiva lent to 31 of E1 Sal -

v a d o r' s G N P ( C o b u r n a n d S p e n c e , 1 9 9 2 ).

E1 Salvador has bee n th e focus o f several s tudies o f seis-

mi c i t y and se i smi c hazard , and t he San Sa l vador ear t hquake

o f O c t o b e r

1 9 8 6

genera t ed r enewed i n t e r es t i n t he a r ea .

Three maj o r hazard s t ud i es have p roduced se i smi c zon i f i ca-

t i ons o f E1 Sa l vador and a fou r t h has been car r i ed ou t a t a

r eg iona l l eve l i n Ce n t r a l A mer i ca . T he s t ud ies fo r E l Sa l vador

have been car r i ed ou t by t he US Geo l og i ca l Su rvey (Al ger -

mi ssen

e t a L ,

1988) , S t an fo rd Un i ver s i t y (Al faro

e t a l . , 1 9 9 0

a n d t h e U n i v e r s id a d N a c i o n a l A u t 6 n o m a d e M ~ x ic o ( S i ng h

e t a l . ,

1 9 9 3 ) . T h e C e n t r a l A m e r i c a n s t u d y h a s b e e n p r o -

duce d as par t o f t he co l l abo ra t i ve research e f fo r t am ong t h e

Cen t ro de Coord i nac i 6n para l a P revenci 6n de Desas t r es en

A m d r i c a C e n t r a l ( C E P R E D E N A C ) , t h e U n i v e r si t y o f

B e rg en , a nd N O R S A R ( R oj a s

e t a l . ,

1 9 9 3; L i n d h o l m

e t a l . ,

1995 ; Gt i ende l and Bun gum , 1995) . F i gu re 3 shows t he

475-year r e t u rn -per i od acce l e ra t i ons p resen t ed i n t hese fou r

s t ud ies , and i t can be seen t ha t t here a r e very cons i derab l e

d i f fe r ences in t he r esu lt s ob t a i ned . T he s t udy by Ro j as

e t a l .

(1993) on l y p resen t s haz ard es t i mat es fo r se l ec ted l oca t ions ,

1. Imperial College of Science, Tec hnolo gy & Medicine, L ondo n, Engla nd.

2 . Universidad Complu tense , M adr id , Spain .

3. Universidad Centro am erica na Jos~ Sim e6n Cafias, San Salvador, El Salvador.

4 . Inst i tu to G eogrf i fico Nacion al , M adr id , Spain.

5 . Inst i tu t du Physique de G lobe, Par is , France .

6. National Technical University, Athens, Greece.

M E X I C

BELIZE

C A R I B B E A N

S A

~ 1 ~ ~.~ H O N D U R A S

-14~ ~ . . - . . . ~ ~ ~ . .

E L S A L V A ~ - i

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C O S T A R I C 4

- 6 0

P A C I F I C O C E A N (

C O L O M B I A ~

9 0 ~ 8 6 ~ 8 2 ~ 7 8

i I I I

9 F i g u r e 1 . C e n t r al A m e r i c a .

and the 475-year accelerat ion for San Salvador i s g iven as

0.76 g . A comparat ive review of these seismic hazard assess-

m e n t s b y B o m m e r

e t a l .

(1996) h i gh l i gh ts t he uncer t a i n t ies

associated wi th the avai lable sei smo logical and geo physical

da t a and t he necess i t y o f r eso l v ing t he m ore i m por t an t d i s -

crepancies be fore at tem pt ing to prod uc e anothe r zoni fica-

t ion of E1 Salvador.

This ar t icle in t roduces an in i t iat ive that i s at tempt ing to

p roduce a m ore r e l iab le da t abase f rom whi c h a se ismi c haz-

ard assessment for Et Salvador wi l l be carr ied out . Through

con t ac t s es t ab li shed du r i ng f i e ld i nves t i ga ti ons o f the 1986

San Salvador ear thquake , a col lab orat ive research pro ject has

been es tab l ished wi t h supp or t f rom t he In t e rna t iona l

Sc ien ti fi c Coo pera t i on p ro g ram o f the Co mm i ss i on o f the

European Communi t i es . The par t ner i n t he r esearch co l l ab -

o ra t i on i n E1 Sa l vador is t he U n i ver s i dad Cen t roam er i cana

426 Seismo logical Research Let ters Volume 68, Num ber 3 May/June 1997



T . . . . . - ,

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-~_I @ ' l ' , , ~ ~ , o @ o . ~ o S t~ . ~ l . V I C E N T E ~ _ , S A N l T . . . . ~ . . . ..

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0 C

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9 Figure 2. El Salvador.

Algermissen et al. (1988)

3

4 ~

_ Alfaro et al. (1990)

5

. ~ . Lindhom e l al. (1995)

9 Figure 3. Hazard maps for El Salvador, showing 475-year return period accelerations (as a proportion of the acceleration due to grav-

ity g) from different studies (Bommer, 1996).

Seismological Research Letters Volume 68, Number 3 May /Jun e 1997 427



Josd Sime 6n Cafias (UCA ), the second-old est university in

the country, w hich is run by the Society of Jesus. Th e project

is coordinated by the Univers idad Complutense de Madrid

(UCM), and the other partners are the Instituto Geogrlifico

Nacional (IG N) in Madrid; Imperial College of Science,

Technology and Medicine (ICSTM) in London; the Inst i tut

du Physiq ue de Globe (IPG) in Paris; and the National Tech-

nical University of Athen s ( NT UA ). The project has a total

duration of three years and has two general objectives: to

compile reliable seismological and strong-motion databases

in order to deter min e a ne w zonation o f El Salvador with

associated seismic design loads, and sim ultaneou sly to estab-

lish in the U CA a teaching and research capacity in seismol-

ogy and earthquake engineering. This second objective is of

particular importance since prior to the initial work through

which this project was established, there was no formal train-

ing in these areas in any of the universities in E1 Salvador.

The project will establish earthquake engineering as a central

discipline within the curricu lum of civil engineering, which

is a vital co mp on en t o f any long-term plan for seismic risk

mit igat ion. A major com pone nt o f the project , which is cen-

tral to bo th of the broad objectives, is the installation o f a

new digital accelerograph network in E1 Salvador, and it is

this aspect of the wo rk th at is described in this report.

TECTONICS AND SEISMICITY

El Salvador is located on the western edge of the Caribbean

plate. The tectonics o f the region are comp lex and in som e

aspects poo rly defined, which is reflected in the differences in

the delimitation of seismic sources in the hazard studies

referred to earlier. Th e dete rminatio n o f the max imu m mag-

nitude for each seismogenic zone is another source of dis-

crepancy among the previous hazard assessments (Bommer

et al.

1996).

The subduct ion of the Cocos plate defines the Middle

American Trench, and this thrust interface is a major source

of earthq uake activity, with steep Benioff-Wadati zones

descending to about 300 km. The largest ins t rumental ly

recorded earthquakes on this interface have had magnitude

of about 8 (D ewey and Suarez, 1991), but the sect ion of the

trench o ff the coast of E1 Salvador has been sh own to have a

relatively low seismic slip rate and the largest events this cen-

tury have had magni tudes between 7.1 and 7.3 (Ambraseys

and Adams, 1996). Nonetheless , of the 14 earthquakes that

have caused destruc tion in San Salvador since 1700, only five

were direct ly associated with the subduct ion of the Cocos

plate. The most damaging events are the moderate magni-

tude, shallow focus earthquakes that coincide with the chain

of Quaternary volcanoes that extends from Guatem ala to

northern Costa Rica. These events reach magnitudes of

about 6 .5 , but their coincidence with major pop ulat ion cen-

ters often results in very severe destruction. Occasionally

these events are a ccom panie d b y volcanic activity, as was the

case in San Salvador in 1917 when the Boquer6n volcano

last erupted, but generally they are tectonic in origin. The

16"

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i ~ i j i i i i i i i i i I i r i r r ~ [ E

-92 -91 --90 -89 -88 -87 --86 -85

17_

15

14-

13

12

o o 9

/o~ ~ oo f~ o

o ~ 9

o

o

o o

o ~ o1 7 6 o ~ 1 7 6 " ~ "

-92 -91 -90 -89 -88 -87 -86 -85

9 Figure 4. Seismici ty of El Salvador and surrounding areas. Cir-

cles show epicenters from 1898 to 1994, small circles are events

with magnitude 5.0 _< M s < 7.0 and large circles M s >_ 7.0. Top

upper-crustal seism icity (focal depth < 25 kin); triangles are vol-

canoes and straight lines show major faults: CP = Chixoy-

Polochic, M = Motagua, JC = Jocot~n-Chamelec6n.

Bot tom

earthquakes with focal depth greater than 25 km.

volcanic arc seismicity has been interprete d as resulting from

a right-lateral shear zone driven by an oblique com po nen t of

convergence between the Caribbean and Cocos plates

(White, 1991).

The principal tectonic structures and the seismicity of

El Salvador and surro undin g areas are shown in F igure 4.

STRONG MOTION DATABASE

A comparative review of hazard studies carried o ut for E1 Sal-

vador (Bommer e t aL 1996) has revealed, rather surpris-

428 Seismolo gical Research Letters Volume 68, Number 3 May/June 1997



l l l l ] l l l l l l l l t l l l l D / l l r W J J l l l l p e l l l l l l l ] l l l t l l l l l l f l l l l l l l ~ l l l l l l l1 ~1 1 [ 1 1 1 1 1 1 1 1 ~ J l l l [ l l t r ] l [ l l l l l l l J l l l l l l J l l l l ]

~ ~ ~

0.5 0.5

I I I I I I I ; { ~ I L ~ I } I ; I I ] I I t l ( I I I I l l l ; ~ I I I I I I I J ~

0 5 10 15 20 2 `5 30 35 40 45 50 55 60 6 `5 70 7 `5 80 8 `5 90 5 10 15 20 25

I ] l l l l l l l l l f r r l l l l ~ l l l l l t J t f f l ] l l J l [ J t l l [ l l l l [ l ~ f l l l r l l l l l J [ ~ l l l l l r r f e l l l l J ] l l f t I f I I ~ l l l J ] l ~ [ l l f [ I r l l l l l l l l l f / l l t i l l (

0 5 10 15 20 25 ;50 3 `5 40 4-`5 50 55 60 6`5 70 7 `5 80 8 `5 90 0 5 10 15 20 25

TI~ sEc)

9 F i g u r e 5 . Strong-motion records obtained in San Salvador from an off-shore earthquake on 19 June 1982 recorded at Observatorio

Sismol6g ico le f t) and the loca l ear thquake of 10 October 1986 recorded at Inst i tu to Geogr~ f ico Nacional r ight ) . L = long i tud ina l,

V = vertical, T = transverse.

ingly , that the at tenuat ion relat ions for peak accelerat ion

employed in the d i f ferent s tudies are qui te s imi lar , wi th the

e x c e p ti o n o f t h o s e e m p l o y e d b y L i n d h o l m

e t a l . 1 9 9 5 ) ,

which predict s igni f icant ly lower values. Nonetheless , al l of

t he a t t enua t i on equat i ons have been based on r a ther l imi t ed

and he t e rogeneous da t ase t s , wh i ch i nc l ude r eco rd i ngs o f

bo t h upper -crus t a l ear t hquakes i n t he vo l can ic cha in r eg i on

and even t s assoc i a t ed d i rec t l y wi t h t he subduct i on zone . On e

recen t s t udy o f s t rong-m ot i on a t t enua t i on charac te ri st ics i n

C e n t ra l A m e r i c a D a h l e

e t a l . , 1 9 9 5 ) ,

us i ng a da t ase t o f 280

t ri ax ia l acce l e rog rams f rom t h rough ou t Cen t r a l Amer i ca and

f rom t he Guer rero a r r ay i n Mex i co , conc l uded t ha t t here i s

no s i gn i f ican t d i f f e r ence be t ween t he a t t enua t i on charac te r -

i st ics of the two types of events . Non etheless , the in te nsi ty

d i s t r ibu t i on o f Cen t r a l A mer i can ear t hquakes sugges ts t ha t

t here a re d i f f e r ences wh i ch i n par t a r e obscu red b y o t her f ac-

tors that af fect the s t ron g-m ot io n recordings, p ar t icular ly the

so il r esponse . O n l y one o f t he hazard s t ud ies fo r H Sa l vador

Alfaro e t a l . , 1 9 9 0 ) has used di f ferent at tenuat ion relat ions

fo r l oca l and sub duc t i on zone ear t hquakes , bu t each r e l a t ion

was der ived f rom regression on only 20 accelerograms. O ne

aspect of the curr ent research p roject is the invest igat ion o f

d i f f e r ences be t ween t he r eco rd i ngs f rom subduct i on ear t h -

quakes and f rom sha l l ow crus ta l ear t hquakes .

Figure 5 sho ws exam ples of accelerograms f rom a

subdu ct i on e ven t and f rom a vo lcan i c cha i n even t . The sub -

d u c t i o n e v e n t o c c u r r e d o n 1 9 J u n e 1 9 8 2 , a b o u t 6 0 k m

sou t hwe s t o f San Sa l vador wi t h a focal dep t h o f 80 km and

su r face-wave ma gn i t ude M, = 7 .3 Ambraseys and Adams ,

1 9 9 6) . T h i s e a r t h q u a k e w a s f e l t t h r o u g h o u t m o s t o f E 1

Sal vador and r esu l t ed i n 8 dea t hs and da mage t o abo u t 1 ,630

dwel l ings Alvarez, 1982 ) . Th e ear thq uak e al so t r iggered a

n u m b e r o f l a n ds l id e s a n d c a u s e d st r u ct u r a l d a m a g e i n s o m e

eng i neered s t ruc t u res Lara , 1983) . Th e l oca l even t occu r re d

on 10 O ct o ber 1986 , j u s t sou t h o f San Sa lvador, w i t h m ag-

n i t ude M~ = 5 .4 and foca l dep t h n o g rea t e r t han 8 km Har -

l ow e t a l . , 1 9 9 3 ) . T h e 1 9 8 6 ear t hquake r esu l t ed i n very

i n t ense dam age over par t o f t he cap it a l c it y, r esu l t ing i n t he

l oss o f 1 ,500 l ives and l eav i ng mo re t han 100 ,00 0 hom el ess

B o m m e r a n d L e d b e t t e r, 1 9 8 7 ; O l s o n , 1 9 8 7 ) .

F i gu re 6 shows t he Ar ias i n t ens it i es o f the c om bi ne d

hor i zon t a l and t he ver t i ca l componen t s f rom t hese t wo

reco rd i ngs. I t i s i n t e r es t ing t o no t e t ha t t hey con t a i n a l mos t

i den ti ca l am oun t s o f energy , bu t i n t he case o f t he vo l can i c

cha i n ear t hquake m os t o f t h is energy is con t a i ned w i t h i n

abou t fi ve seconds o f s t rong shak i ng , wh ereas fo r t he su bduc -

t i on even t t he same energy i s i mpar t ed t o s t ruc t u res over

abou t ha l f a mi nu t e . Th i s par t i a l l y exp l a i ns why t he 1 9 8 6

e a r t h q u a k e w a s s o m u c h m o r e d e s t r u c t i v e t h a n t h e 1 9 8 2

even t and w hy h i s t o r i ca l l y vo l can i c cha i n even t s have cau sed

m u c h g r e a te r d a m a g e t h a n s u b d u c t i o n e v e n ts W h i t e a n d

Har l ow, 1993) . F i gu re 7 shows t he ho r i zon t a l and ver t i ca l

r esponse spec t r a f rom t he t wo r eco rds , wh i ch r evea l i n t e res t -

i ng charac t e r i s t i cs . The r eco rds a r e f rom d i f f e r en t s i t es bu t

bo t h a r e over la i n b y l ayer s o f vo l can i c ash , and t he s i mi la r

shapes in Figure 7 a) may ref lect the inf luence of the soi l .

T h e s p e c tr a l a m p l i t u d e s a g a in i n d i c a te w h y t h e 1 9 8 6 e v e n t

was more des t ruc t i ve . T he ver ti ca l spec t r a i n F i gu re 7 b ) a nd

S e i s m o l o g i c a l R e s e a rc h L e t t e r s V o l u m e 6 8 , N u m b e r 3 M a y / J u n e 1 9 9 7 4 2 9



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0 1 0 2 0 3 0

4

5 0 6 0 7 0 8 0 9 0

T i m e ( s e c )

9 F i g u r e 6 . P l o t s o f A r i a s in t e n s i t y f r o m t h e c o m b i n e d h o r iz o n t a l

c o m p o n e n t s o f m o t i o n a n d t h e v e rt ic a l c o m p o n e n t s ( in d i c a te d b y

V ) o f th e t w o r e c o r d s s h o w n in F i g /J r e 5 f r o m 1 9 8 2 ( s o li d ) a n d

1 9 8 6 ( d a s h e d ) .

t h e r e c o r d s i n F i g u r e 5 s h o w a n o t h e r i n t e r e s t i n g f e a t u r e ,

w h i c h i s t h a t fo r t h e l o c a l v o l c a n i c c h a i n e v e n t s t h e v e r t i ca l

g r o u n d m o t i o n c a n b e v e r y i m p o r t a n t . F o l l o w i n g t h e S a n

S a l v a d or e a r t h q u a k e o f 3 M a y 1 9 6 5 L o m n i t z a n d S c h u l z ,

1966), a l t h o u g h t h e r e w a s n o e v i d e n c e o f s t r o n g v e rt ic a l

m o t i o n s , i t w a s r e c o m m e n d e d t h a t v e r ti c a l s e i s m i c l o a d s b e

c o n s i d e r e d f o r d e s i g n s i n c e i t w a s p o s s i b l e t h a t s u c h l o c a l

e v e n t s c o u l d g e n e r a te s i g n i f i c a n t v e r t i c a l a c c e l e r a t i o n s

R o s e n b l u e t h a n d P r in c e , 1 9 6 6 ) . I n t h e s a m e r e p o r t , R o s e n -

b l u e t h a n d P r i n c e 1966) r e c o m m e n d e d t h e p r o d u c ti o n o f

t w o s e i s m i c z o n i n g m a p s f o r E 1 S a l v a d o r , o n e f o r e v e n t s a s so -

c i a t e d w i t h t h e s u b d u c t i o n z o n e a n d t h e o t h e r f o r e a rt h -

q u a k e s i n t h e v o l c a n i c c h a i n z o n e . T h i s h a s n o t y e t b e e n

c a r r ie d o u t , a n d c le a r l y t h e e l a b o r a t i o n o f s u c h z o n i n g m a p s

a n d t h e s p e c i f i c a t io n o f t h e a s s o c i a t e d d e s i g n s p e c t r a w i l l

r e q u ir e a s i g n i fi c a n t d a t a b a se o f a c c e l e r o g r a m s f r o m b o t h

t y p e s o f e a r t h q u a k e .

A l t h o u g h a n a p p r e c i a b le b o d y o f d a t a h a s n o w b e e n c o l -

l e c t e d a n d s e v e ra l o f th e r e c o r d s h a v e b e e n d i g i t i z e d , th e r e

a r e a n u m b e r o f d e f i c ie n c i e s i n t h e d i s t r i b u t i o n o f t h e r e c o r d -

i n g s t h a t n e e d t o b e a d d r e s s e d . I f t h e s t r o n g - m o t i o n d a ta b a se

i s s e p a r a te d a c c o r d i n g t o t h e s e i s m i c s o u r c e o f t h e g e n e r a t i n g

e a r t h q u a k e s, i t is a p p a r e n t t h a t t h e n u m b e r o f r e c o r d i n g s

f r o m v o l c a n i c c h a i n e v e n t s i s r e l a t iv e l y s m a l l . M o r e o v e r , t h e

d i s t r i b u t io n o f t h e s e d a ta w i t h r e s p e c t t o m a g n i t u d e a n d d i s -

t a n c e i s p o o r, w i t h n e a r l y a l l t h e r e c o r d s o b t a i n e d a t e p i c e n -

t ra l d i s ta n c e s o f n o m o r e t h a n a f e w k i l o m e t e r s f r o m

m o d e r a te e a r th q u a k e s . A s a r e s u l t , i t i s d i f f i c u l t t o o b ta i n a

r e li a b le e s t i m a t e o f t h e a t t e n u a t i o n c h a r a c t e r is t ic s f o r v o l c a -

n i c c h a i n e a r t h q u a k e s f r o m t h e a v a i l a b le d a t a s e t.

( a )

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O . O 0 . 25 O ~ 0 . 75 1 . 0 LZ 5 1 . 5 1 .75 2 . 0

P e r i o d (s e c o n d s )

1.5-

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P e r i o d ( s e c o n d s )

9 F i g u r e 7 . ( a ) H o r iz o n t a l a n d ( b ) v e r t ic a l r e s p o n s e s p e c t ra ( 5 % d a m p i n g ) fr o m t h e r e c o r d s i n F ig u r e 5 f ro m 1 9 8 2 ( s o li d ) a n d 1 9 8 6

( d a s h e d ) . T h e h o r i z o n t a l s p e c t ra a r e t h e e n v e l o p e s o f t h e t w o h o r i z o n t a l c o m p o n e n t s .

4 3 0 S e i s m o l o g i c a l R e s e a r c h L e t te r s V o l u m e 6 8 , N u m b e r 3 M a y / J u n e 1 9 9 7



N E W D I G I T A L N E T W O R K

A net wo rk o f SM A-1 ana l og acce l e rog raphs i s opera t ed i n E1

Sal vador by t he Cen t ro de Inves t i gac i ones Geo t dcn i cas

CIG ) , wh i c h i s par t o f t he M i n i s t ry o f Pub l i c Works. Th i s

netw ork curr ent ly consis t s of 16 acceterographs dis t r ibuted

thro ug ho ut E1 Salvador : four are located wi thin San Salvador

i n t he UC A, t he C IG, t he Obse rva t o r i o S ismol6g ico , and i n

Soyapango , and t he o t her s d i sper sed t h rough ou t t he coun t ry

i n Acaj u t la , Ahuachapf i n , Cha l a t enango , Coma l apa a ir -

por t ) , Cu tuco , Metap~in , San M iguel , Santa Ana, Santa

Tecla, Sant iago de Maria, San Vicente and Sensuntepeque, as

show n i n F i gu re 8 . In ad d i t i on t o t hese in s t rumen t s , ano t her

eight have been instal led as a three-dimensional ar ray wi thin

San Salvador , wi th ins t rum ents at ground level and at the

bo t t om s o f fou r well s.

Three SM A-1 s t ro ng-m ot i on acce le rog raphs a re a l so

opera t ed by t he Comi s i 6n Ej ecu t i va Hi d roe l dc t r i ca de l Rdo

L e m p a C E L ) o n t h e S a n L o r e n z o 1 5 d e s e p ti em b r e ) D a m

on the Lem pa River. Tw o SSA -2 accelerographs have al so

been i n s ta l l ed r ecen t l y as par t o f a mon i t o r i ng ne t wo rk

arou nd the geo therm al p lan t rece nt ly in i t iated in Ber lin , in

t he Dep ar t m en t o f Usu l u t fi n . Th ree SMA-1 i n s t rumen t s a r e

a l so opera t ed by t he Cami no Real Ho t e l i n San Sa l vador ,

appa rent ly the only large bui ld ing to have fulf il led Article 30

of t he 19 66 Regu l a t i ons fo r Se i smic Des i gn t ha t spec i fi ed

t ha t s t rong-mot i on i n s t rumen t s shou l d be i n s t a l l ed i n a l l

bu i l d i ngs wi t h an a rea exceed i ng 1 0 ,000 m 2 o r he i gh t g rea t e r

t h a n 4 5 m . T h e s e i n s t r u m e n t s r e c o r d e d t h e g r o u n d m o t i o n

and s t ruc t u ra l r esponse i n t he Oct ober 1986 ear t hquake

Shakal e t a l . 1987) .

T h e n e w s t r o n g - m o t i o n n e t w o r k e s t a bl i sh e d w i t h i n t h is

p ro j ec t cons i s t s o f t en Ki ne met r i cs SSA-2 d i g i ta l acce l e ro -

g r a ph s , f i t te d w i th T C G - 2 t i m e - c o d e g e n e r a to r s . T h e n e t -

w o r k h a s b e e n g i v e n t h e n a m e T A L U L I N , w h i c h i n t h e

i n d i g e n o u s N a h u a t l l a n g u a g e m e a n s e a r t h q u a k e . T h e

i ns t rumen t s have been dep l oyed t o p rov i de coverage o f se i s -

m i c i t y f r o m b o t h t h e v o l c a n i c c h a i n z o n e a n d t h e s u b d u c -

t i on zone . At t he same t i me, t he ne t work i s des i gned t o

cover d i f f e r en t su r face geo l og i es in ce n t r a l an d so u t hern E1

Salvador, a l t hough t he geograph i ca l d i s t r i bu t i on o f the

i ns t rumen t s was g i ven h i gher p r i o r i t y and t he s i t e geo l ogy

was on l y spec if i ca ll y cons i de red t o exc l ude rock ou t crop

s it es . G i ve n t ha t a s i gn if i can t num be r o f reco rds have a l ready

b e e n o b t a i n e d i n S a n S a l v a d o r a n d t h a t t h e C I G n e t w o r k

g i ves goo d coverage o f t he me t ropo l i t an a rea , i t was dec i de d

t o i n s ta l l on l y one i n s t rum en t w i t h i n t he cap i ta l . Fo r t he

areas outs ide San Salvador , i t was necessary to ident i fy a

l a rge nu m ber o f poss ib l e si tes , p r e ferab l y und er t he d i r ec t i on

a n d m a n a g e m e n t o f a s in g le o r g a n i z a ti o n , w h i c h c o u l d p r o -

v i de secu re l oca t i ons fo r t he s t a t i ons . An i dea l so l u t i on was

f o u n d t h r o u g h t h e e s t a b l i s h m e n t o f a n a g r e e m e n t w i t h t h e

M i n i s t e r i o de Sa l ud P tl b l ica y As i s t enc i a Soc ia l Hea l t h

Mi n i s t ry ) , wh i ch gave access t o hosp i t a l s and hea l t h un i t s

9 0 o 8 E jo 8 8 o

]

G U A T E M A L A . . . . . ,%

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/

/

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] . . . _ . . ; . .

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i

A F igure 8 . S ta t ion locat ions o f new d ig i ta l network ( t r iang les) O pen squ ares ind icate the locations o f SMA-1 inst rum ents operated by

the Cent ro de Invest igac iones Geot~cn icas (C IG) .

S e i s m o l o g ic a l R e s e a r c h L e t t e r s V o l u m e 6 8 , N u m b e r 3 M a y / J u n e 1 9 9 7 4 3 1



1 4 . 0 "

1 3 . 5

1 4 . 0 "

1 3 . 5

8 g . 6 ~ 8 g . O 9

I I

B . . . 9 e ~ '~e 180 0-1 899

,'::-..' . . . . . . .

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1 8 5 7

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. . . . - -

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,",:::- : :'" ,.. . ~_ - ,~ j ,~ ~ , ~,"

9..: .... ' "~ ~ It ~ ~ ,,~ _,-,~

~ ' :~ . ' f ~' : . ' . : : . . . ~ | 1 9 8 5 ~ O , r

, ~ ,oD -+ | ' ' I

~. ~:-:~'~,,, ~. ,* ~+

9,,:~.,..- I, J L%<.- '":.' I~C :g,,X~..~ ' ' &c,~

.... ..-.V X

,,, ~F 1818 ~.....::::... %./

~ , - - ~ \

9 Figur e 9. Maps sh ow ing contours of Modified Mercalli intens ity >VII for upper crustal earthquakes in central El Salvador for the periods

1800-1899 and 1900 -1990 (Harlow

etal.

1993). The dashed line on the lower map shows the MM VII isoseisrnal associated with the

largest subdu ction event this century, the

Ms

= 7.3 earthquake tha t occurred off the coast of El Salvador on 19 June 1982 (AIvarez, 1982).

th roug hout El Sa lvador . The agreement was encouraged by

the Disas ter s Un i t o f the Hea l th Minis t ry , and pa r t o f the

agreement i s tha t the UC A wi l l under take eva lua t ions of the

seismic vulnerabil i ty of some of the health care establish-

ments.

Pr ior i ty was given to hospita ls ra ther th an he alth centers

because of the better infrastructure and greater security that

they can provide. The locations were a lso chosen to enable

reasonably easy access for maintenance of the instruments

and data recovery, since despite its small geographical extent,

travel in E1 Salvador is of ten diff icult and t ime-consuming.

The network was designed according to these cr i ter ia in

order to cover three i mp orta nt sources of se ismic activity in

the volcanic-chain zone: San Salvador and Lake I lopango,

the area southeast o f Lake Coatepeq ue to the west of San Sal-

vador , and San Vicente to the east of San Salvador (Figure 9) .

Stations have a lso been located to the south as far as the

coast, and a single sta tion has been placed to the n orth of San

Salvador. Th e distr i buti on provides coverage of bot h the sub-

duction zone and the volcanic chain, and in par ticular will

a l low recording of volcanic chain ear thquak es c lose to the

source and at distances of the order of 10 to 50 kilometers,

which wi l l he lp to r eso lve the unce r ta in ty a ssoc iated wi th th e

a t tenua t ion o f s t rong mo t ion f rom sha l low c rus tal events .

The d is t r ibu t ion o f the new ne twork i s shown in F igure 8 .

The locations are presented in Table 1 the coordinates have

been measu red using portable GPS, and the e levations corre-

spond to the c losest geodesic benchmarks o f the Salvadorean

Ins t i tu to Geogr~f ico Nac iona l ( IGN) . Al l o f the ins t rumen ts

are insta lled inside structures an d at g roun d level.

In a ll the sta tions, the in strum ents have been set to tr ig-

ger with an acceleration level of 0.010 g horizontally or

0.006 g vertically. Th e UC A manu factu red an d insta lled

shelters for each of the accelerographs.

At this stage, deta iled informa tion ab out si te conditions

is not known, a lthough this information is being recovered

from a numbe r of sources, incl udin g a few borehole logs

available for the actu al sta tion sites as well as the logs from

water wells dr il led by the Ad minis traci6n Nacional de Acue-

ductos y Alcantarrillad os (A ND A), several of wh ich are close

to the accelerograph sta tions. This information is presented

in full in the f irst annu al report on the op eration o f the net-

work (Cepeda

et al .

1997) . I t is important to extrapolate

such informat ion wi th caut ion s ince the topography o f E1

Salvador is such that a great deal of constru ction involves

extensive use of landfill, particu larly to reclaim steep ravines.

The general geological characteristics of the sites presented

here are as interpreted from the 1:100,000 geological maps

of El Salvador prepared by the Geological Mission of the

Bundesanta l t ft ir Bodenforschung, H annover , Germany, and

published in 1978 b y the I GN in E1 Salvador; these maps d o

not provide inform ation abou t local soil cover . Descr iptions

of the different formations are a lso presented by Schmidt-

Thom~ (1975) , Weyl (1980) , and Baxter (1984) .

E1 Salvador consists of four morphological-geological

units , which exist as approximately paralle l str ips running

east-west across the country (Weyl, 1980). Along the border

with Honduras are the northern mountain ranges, consist ing

mainly of plutonic rocks from the Tertiary. The historical

record suggests that seismic hazard is relatively low in this

432 Seism ological Research Letters Volume 68, Num ber 3 May/June 1997



T A B L E 1

S t a t io n s c h a r a c t e r i s t ic s o f n e w d i g i t a l n e t w o r k

Code Locat ion Coordinates Type of structure

S n Elevat ion

Datea

ESJO Externado San Jos6 13.707~ Library of secondary school. One-story annex of two -story re inforced

2396 San Salvador 89.207~ br ick maso nry wal ls.

19/2/96 675 m

HSRF 13.671~

2397 89.279~

23/2/96 912 m

UARM 13.744~

2404 89.501~

26/2/96 570 m

ULLB 13.486~

2398 89.327~

26/2/96 16 m

HSTR 13.517~

2402 88.869~

27/2/96 253 m

USPN 13.602~

2399 88.927~

27/2/96 658 m

HSGT 13.642~

2403 88.784~

28/2/96 373 m

CSBR 13.704~

2395 89.106~

28/2/96 622 m

UPAN 13.614~

2401 89.179~

1/3/96 613 m

UTON 13.778~

2400 89.114~

11/7/96 b 607 m

Hospital San Rafael

Santa Tecla

La Liber tad

Unidad de Salud

Armen ia Sonsonate

Unidad de Salud

La Liber tad

Hospita l San ta Teresa

Zacateco luca La Paz

Unidad de Salud

San Pedro Nonua lco

La Paz

Hospita l S anta Gertrudis

San Vicente

Centro de Salud

San Barto lo

San Salvador

Unidad de Salud

Panchimalco

San Salvador

Unidad de Salud

Tonacatepeque

San Salvador

Large hospi ta l warehouse. Single-story construct ion with steel f rame

and brick walls.

Storage room in heal th center . One-story bui ld ing of re inforced br ick

masonry l ight roof .



Single story ann ex of re inforced br ick maso nry adjacent to 5 -story RC

frame bui ld ing.





Storage room in heal th center . One-story bui ld ing of RC frame hol low

brick walls l ight roof.

Administrat ion of f ice in one-story bui ld ing of re inforced br ick masonry

walls.



a. D ate of installation of the accelerograph.

b. UTONstation wa s n ot installed un til July 1996 due to relocation of Health Center.

part of the country and none of the instruments are located

in this area. The central part of E1 Salvador is the Great Inte-

rior Valley a heterogeneous basin and low mountain topogra-

phy; the southern part of the Valley contains the chain of

Pleistocene volcanoes six of which are active. Nearly all of the

major cities of E1 Salvador and hence most o f the population

are located within the Great Interior Valley. To the south o f

the Valley there are three coastal mountain ranges: the Tacuba

in the west on the border with Guatemala the Balsamo in the

central and western area and the Jucuar~in range on the east-

ern side bordering the Golfo de Fonseca. In between the

coastal ranges are the coastal plains in the western and central

parts of the country which consist of alluvial deposits with

intercalated pyroclastics spits and mangrove swamps.

The UTON station is located within the Great Interior

a l l e y to the north of the axis of the volcanic chain whereas

the ESJO CSBR and HSGT stations are located along the

volcanic chain. The surface geology at these four sites is t ierra

blanca

a young poorly consolidated volcanic tuff which

originated from eruptions within Lake Ilopango during the

Holocene.

The station UARM is located on the boundary between

the Great Interior Valley and the Balsamo range and it is also

overlain by Holocene volcanic tuf f known as tabas color caf~

which is slightly older than the

t ierra blanca.

The HSRF station is also situated on the boundary

between the Great Interior Valley and the Balsamo range

but in an area of effusive rocks from the Late Pleistocene

which together with the

t ierra blanca

and the

tabas color caf~

constitute the San Salvador format ion which is the most

recent in the country.

The stations USPN and UPAN are located within the

Balsamo range the former on the northern side close to the

volcanic center of Chichontepeque and the latter on the

southern side where the mountains descend to the coastal

plain. The geology at these two sites consists of pyroclastics

and volcanic rocks that belong to the CuscatLin formation

and are associated with acidic and intermediate volcanism

during the Pleistocene which produced ignimbri tes on the

southern slopes of the Balsamo range.

The HSTR station is located in the coastal plain on the

older Balsamo format ion Tertiary pyroclastics and volcanic

S e i s m o l o g ic a l R e s ea rc h L e t t e rs V o l u m e 6 8 N u m b e r 3 M a y / J u n e 1 9 9 7 4 3 3



T A B L E 2

S t r o n g - m o t i o n r e c o r d s o b t a i n e d d u r i n g f ir s t s i x m o n t h s o f o p e r a t i o n

a m a x g )

Sta t ion db

N ~ W ~ M a C o d e k m ) N - S V e r t E - W

T i m e

D a t e U T C )

3-3-96 14:55 11.77 86.93 5.4 B USPN 297 0.011 0.006 0.008

3-3-96 16:37 11.12 86.69 6 6 8 HSGT 360 0.013 0.009 0.016

HSTR 356 0 009 0.010 0 010

UARM 421 0.012 0.008 0.011

USPN 367 0.023 0.013 0.021

28-5-96 15:25 13.03 89.11 4.8 B ULLB 56 0.019 0.008 0.011

30-5-96 14:27 12.85 89.14 4.1 B USPN 86 0.014 0.010 0.016

HSRF 92 0.014 0.009 0.021

7-6-96 I4:2 8 12.71 88.11 4.9 B USPN 133 0.011 0 012 0.0 t6

HSGT 127 0.012 0.006 0.010

HSTR 121 0.009 0.008 0.010

7-7-96 23:47 12.8 88.7 4.3 D USPN 92 0.008 0.006 0.006

HSGT 94 0.008 0.008 0.009

HSTR 82 0.016 0 009 0.016

18-7-96 05:22 12.99 88 54 4.5 B USPN 80 0.016 0 010 0.013

HSTR 68 0.016 0.009 0.012

22-7-96 08:30 12.94 88.91 5.5 B UARM 110 0.027 0.023 0.032

CSBR 87 0 020 0.011 0.015

ULLB 75 0.048 0.018 0.027

USPN 73 0.031 0.023 0 028

HSRF 90 0 021 0 013 0.027

UTO ~ 95 ~3.02~ 0 01 4 0.04 8

HSGT 78 0.022 0.015 0.023

ESJO 91 0.015 0.016 0.013

HSTR 64 0.014 0.009 0.013

a. Magnitude:

8 - - M s B - - m o D - - M o .

b. Epicentral distance.

r o c k s, w h i c h i n m a n y l o c a t i o n s a r e o v e r l ai n b y t h i c k c o v e rs

of r ed so i ls .

T h e s t a t i o n U L L B i s l o c a t e d o n t h e c o a s t to t h e s o u t h o f

t h e B ~ l s a m o ra n g e o n Q u a t e r n a r y s e d i m e n t a r y d e p o s i ts

w h i c h a r e p a r t o f t h e S a n S a lv a d o r f o r m a t i o n .

I n t h e l i g h t o f t h e l a c k o f a g r e e m e n t o n t h e c l a s si fi c at i on

o f si t e c o n d i t i o n s i n a t t e n u a t i o n s t u d i e s a n d t h e p r e l i m i n a r y

n a t u r e o f t h e s e d a t a , n o a t t e m p t i s m a d e t o c l a ss if y t h e s i te s

as r ock or so i l un t i l f u r t he r inves t iga t io ns have been ca r -

r i ed ou t .

PRE LIMIN RY RESULTS

T h e n e w a c c e l e r o g r a p h n e t w o r k w a s t e s t e d a l m o s t i m m e d i -

a t e l y a f t e r in s t a l l a ti o n b y a n e a r t h q u a k e t h a t o c c u r r e d o n 3

M a r c h 1 9 9 6 , t w o d a y s a f t e r t h e l a s t i n s t r u m e n t h a d b e e n

p l a c e d i n P a n c h i m a l c o . T h e e a r t h q u a k e o c c u r r e d o f f t h e

c o a st o f N i c a r a g u a w i t h a m a g n i t u d e r e p o r te d a s M , 6 . 6 a n d

was wide ly f e lt in E l S a lvador . O f the n ine s ta t io ns ope r a -

t iona l a t the t im e of the ea r thq uake , f o ur wer e t r igge r ed :

U A R M , H S T R , U S P N a n d H S G T . T h e fiv e s t at io n s t h a t

d i d n o t t r ig g e r w e r e al l c h e c k e d a n d f o u n d t o b e f u n c t i o n i n g

iden t ica l ly to those tha t t r igge r ed , wi th th e excep t ion of

E S J O , w h i c h a t t h e t i m e h a d b e e n i n i t i a ll y s e t w i t h t h e t r ig -

ge r cha r ac te r i st i c s o f an S MA - 1 ( 0 . 0 lg ve r t i ca l acce le ra t ion) .

T h e s t a t i o n U S P N a l s o r e c o r d e d a s m a l l e r e v e n t t h a t

o c c u r r e d a b o u t a n h o u r a n d a h a l f e a rl ie r , l o c a te d i n t h e s a m e

g e n e ra l a r ea a n d f o r w h i c h t h e N E I C r e p o r t s a m a g n i t u d e o f

m b 5 . 4 , w h i c h w a s a l so w i d e l y f e lt t h r o u g h o u t s o u t h a n d

eas tern E1 Salvador .

D u r i n g t h e f i rs t s ix m o n t h s o f o p e r a t i o n o f th e n e t w o r k

( un t i l the end o f Aug us t , 1996) , a to ta l o f 25 r ecor ds wer e

g e n e r a t ed b y e ig h t e a rt h q u a k e s , a n d a l l b u t o n e ( U P A N ) o f

the s ta t ions have been t r igge r ed a t l eas t once . Th e cha r ac te r -

i st ic s o f the r ecords ob ta in ed a r e g iven in Tab le 2 . Al l o f these

r ecor ds have been t r igge r ed by ea r thquakes a s soc ia ted wi th

434 Se ismo log ica l Research Le t te rs Vo lume 68 Numbe r 3 May /June 1997



0 . 9 -

0 . 8

~ , 0 . 7 -

e " 0 . 6

_

121

0 . 5

o

o

~ I ~ 0 . 4

0 .3 -

0 .2 -

0.1-

0 . 0

0

H S T R

U L L B

~. ~,~ ~ : ~ ~ H S G T

. .. _ . ~ C S B R

HSRF

E S J O

~ U T O N

~ U A R M

l l r l l i l l i l l l l J I l l l l l l l I I L I I I I l l i l l l l l l I

5 10 15 35 400 2 5 3 0

Time seconds)

9 F igure 10. Acce lera tion t ime h is tor ies recorded by the new network on 22 Ju ly 1996. The NS comp one nt o f each record i s show n.

t he subduct i on zone , and t he i r hypocen t er s a r e a l l l oca t ed

o f f sho re wi t h r epo r t ed foca l dep t hs be t w een 30 and 80 km.

F i gu re 10 shows t he N-S com pone n t s o f t he accel erat ion

t i me-h i s t o r i es r eco rded on 22 Ju l y

1996.

The t ime scale i s

only relat ive s ince there were problems wi th the absolute

t i mes r eco rded by t he TCG-2 . The r eco rds have been

arranged in orde r of increasing epicent ral d i stance, and the

inf luence of the local s i te con di t ion s i s imm ediately apparent .

In order to ma ke so m e inferences regarding the s ite con -

d i t ions , we have em pl oye d t he spec tr a l r a t io m et hod used by

Theo du l i d i s and Ba rd 1995) , p l o t t i ng t he ra t io o f ho r i zon -

t a l t o ver ti ca l ampl i t udes o f t he Four i e r spec t ra as a func t i on

of f r equency Na kam ura r a t i o ) . The m et hod i s based on t he

assumpt i on t ha t t he ho r i zon t a l mo t i on may be ampl i f i ed by

the presenc e of soi l l ayers bu t that ver tical m ot io n i s not ,

hence t he r a t i o o f t he t wo shou l d r ep resen t the i n f luence o f

the soil l ayers or o th er s i tes ef fects . Figure 11 show s the rat ios

obtained f rom the average of a) seven recordings at San

Pedro No nua l co US PN ) and b ) f ive r eco rd i ngs a t Zaca t e -

co l uca HS TR ) ; i n bo t h cases the r a ti os have been fi r st

smo o t hed by 50 passes o f a 3 -po i n t runn i ng average and

then the averages o f the rat ios f rom each record determined .

Both o f the s tat ions sho w clear ampl if ication in cer tain

frequency ranges w hich m ay ref lect the inf luence of soi l l ayers

at the s i tes . Both records show some peaks at f requencies of

abou t 7 -8 Hz, wh i c h cou l d r e f lec t th i n so il layer s. The excep -

t io n a ll y n a rr o w p e a k s o b se r v e d o n t h e U S P N r e c o rd m a y b e

related to topography; th i s s tat ion i s located atop a narrow

and e l ongat ed r i dge , o r i en t ed app rox i mat e l y E-W, and i t i s

poss i bl e t ha t t he apparen t ampl i f i ca t ion o f mo t i on a t t h is s i te

i s mainly due to topographical ef fects . As fur ther records are

obtained, these character i s t ics wi l l be explored fur ther and

cor re l a ted wi t h t he geo t echn i ca l da t a . The i mpor t ance o f soi l

deposi t s in H Salvador, par t icular ly poo r ly conso l idated volca-

nic tuffs such as the

tierra blanca

in the capital, in ampl i fy in g

ear t hquake mo t i ons has been t he focus o f severa l s tud i es

Rymer, 1987; Faccioli

etal .

1988 ; At akan andT or res , 1994) .

The l ong- t e rm ob j ec t ives o f t h is r esearch i nc l ude t he m i crozo -

nat ion of the main urba n centers in E1 Salvador, par t icular ly

those that l ie wi th in the relat ively smal l zones that are s t rongly

af f ec ted by vo l can ic cha i n ear t hquakes . E l

A C K N O W L E D G M E N T S

W e are part i cu la r l y i ndeb t ed t o t he fo l l owi ng i nd i v idua l s fo r

t he i r generous coopera t i on i n f ac i l i t a t i ng t he es t ab l i shmen t

o f t he new s t ron g-m ot i on ne t work : E An i ba l Meza , S . J. ,

Rec t o r o f the Ex t erna do San Josd ; and i n t he Mi n i s t e r io de

Sal ud P t l b l i ca y As i s t enc i a Soc i a l , Dr . Eduardo In t e r i ano ,

H e a l t h M i n i s t e r ; D r a . A n a M a r f a A l f a r o d e G a m e r o , V i c e -

M i n i s t e r o f H e a l t h ; D r . M a n u e l E n r i q u e A n g u l o , G e n e r a l

Di rec t o r ; Dra . Dy na de Navar ro , S ub-d i r ec t o r o f P reven t a-

S e i s m o l o g ic a l R e s ea rc h L e t te r s V o l u m e 6 8 , N u m b e r 3 M a y / J u n e 1 9 9 7 4 3 5



~0

9

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i

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~

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t l V

0 I ; i ; I I I i i I ; ; ~ i ; f ~ ; I ~ ~ ; I ~ ; ~ I

GO 2.5 5 .0 7 .5 lO.O 12.5 15.0

FR E Q U E N C Y (HZ)

I - -

<

w

>

T

1 0

9

8

7

6

5

4

3

2

1

0

0 . 0

b)

ii

tl

t I x

i r i J I i i = ~ I i i i i I i i i q I i ; i r I J f r r I

2.5 5 .0 7 .5 10.0 12.5 15.0

FR E Q U E N C Y ( H Z)

9 F igure 11 . Rat ios o f hor izonta l to ver tica l Four ier ampl i tude spect ra (Nakam ura ra t ios) for the (a) sev en record ings f rom USPN and

(b) f ive record ings f rom HS TR sta t ions. The so l id l ines represent the NS compo nents and the dashed l ines the EW.

t iv e M e d i c i n e ; D r . O d i l i o A l c i d e s L i na r es , C h i e f o f E n v i r o n -

m e n t a l A t t e n t i o n ; D r a . P a t r i c i a d e Q u i n t e r o s , S u b - d i r e c t o r

o f P r i m a r y H e a l t h C a r e ; D r a . E s t el a P ar a d a, S u b - d i r e c t o r o f

H o s p i t a l s ; a n d n u e s t r o a n g e l L i c . G l o r i a d e C a l l e s , N a t i o n a l

C o o r d i n a t o r o f th e D i s a st e rs U n i t , w h o s e e n t h u s i a s m , e ff i-

c ie n c y , a n d e n e r g y h a v e b e e n c r it ic a l in m a k i n g t h is c o o p e r -

a t i o n p o s s i b l e . W e w o u l d a l s o l i k e t o t h a n k a l l t h o s e

m e m b e r s o f s t a f f i n t h e h o s p i ta l s a n d h e a l t h u n i t s w h o

a l lo w e d u s t o i n t e r r u p t t h e i r b u s y s c h e d u le s a n d g e n e r o u s l y

a s si st e d u s i n m a n y d i f f e r e n t w a y s . W e a r e a ls o g r a t e f u l f o r

t h e c o o p e r a t i o n o f t h e C e n t r o d e I n v e s ti g a c io n e s G e o t d c n i -

c as , e sp e c ia ll y th e C h i e f o f t h e D e p a r t m e n t o f S e i s m o lo g i c al

R e s ea r ch , I n g . D o u g l a s H e r m i n d e z , a n d I n g . R o d o l f o T o rr e s .

W e a re al so h i g h l y i n d e b t e d t o M a u r i c i o C i u d a d R e a l, O g i e

K u r a i c a , R u d y D l a z , E n r i q u e G u e v a r a O r t f z , a n d M a r k

S e re c i f r o m K i n e m e t r i c s I n c . , w h o h a v e p r o v i d e d i n v a l u a b le

s u p p o r t a n d a s s i s t a n c e . T h a n k s a r e a l s o d u e t o K a r l a

R o d r l g u e z f o r h e r a s s i s t a n c e w i t h t h e i l l u s t r a t i o n s . T h e

r e se a rc h p r o j e c t h a s b e e n s u p p o r t e d b y t h e N a t i o n a l C o u n c i l

f o r S c i en c e a n d T e c h n o l o g y ( C O N A C Y T ) i n E l S a lv a d o r.

T h i s p r o je c t is s u p p o r t e d b y C E C c o n t r a c t C I 1 " - C T 9 4 -

0 1 04 ( D G 1 2 H S M U ) .

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I m p e r i a l C o l l e g e

L o n do n S W 7 2 BU , U K

e-m ai l : j .boramer@ic .ac .uk

( J .J . B ., N . N . A . )

U n i v e r s id a d C o m p l u t en s e d e M a d r i d

D e p a r t a m e n t o d e G e o fi si ca

2 8 0 4 0 M a d r i d , S p a i n

( A . U . , E . B . )

U n i v e r s i d a d C e n t r o a m e r i c a n a J o se S i m e d n C a ~ a s

A .P . ( C 1 ) 1 6 8 , S a n S a l v a d o r, E l S a l v a d o r

J.M.C., J.C.H ., W.M .S., J .C., P.M .)

I n s t i t u t o G e o g r A f i c o N a c i o n a l

G e n e r a l I b a n e z d e I b er o 3 , 2 8 0 0 3

M a d r i d , S p a i n

A.S., J .M .)

I n s t i t u t d u P h y s i q u e d e G l o b e

D e p a r t m e n t d e S i sm o l o gi e

R u e P l a c e J u s s ie u 4

7 5 2 5 2 P a r i s C X 0 5 , F ra n c e

( R . M . )

N a t i o n a l T e c h n i c a l U n i v e r s i t y

L a b o r a t o ry f o r E a r t h q u a k e E n g i n e e r in g

Z e g r a f o s , 1 5 7 0 0 A t h e n s

Greece

( D . R )

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