Relation Between Site Amplifcation Factor and Surficial Geology

7/30/2019 Relation Between Site Amplifcation Factor and Surficial Geology

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Bul letin of the Seismological Society of America, Vol. 82, No. 2, pp. 580-602, Apr il 1992

T H E R E L A T I O N B E T W E E N S I T E A M P L I F I C A T I O N F A C T O RA N D S U R F I C I A L G E O L O G Y I N C E N T R A L C A L I F O R N I ABY F. Su ., K . AKI, T. TENG, Y. ZENG, S. KOYANAGI, AND K . MAYEDA

ABSTRACTSite ampli f ication factors at frequencies of 1.5, 3, 6, and 12 Hz were deter-

mined for 132 stations of th e USGS seismic network in central Cali fornia fromcoda waves of 185 local earthquakes in this area using a recursive stochasticinversion method. We found that the site ampli f ication at a station is systemati-cal ly related to the geology u nderlying that station. The site ampli f ication is highfor young, Quaternary sedim ents and decrease s with increasing geolog ic age atal l frequencies between 1.5 and 12 Hz. The rate of decrease varies with fre-quency where site ampli f ication at low frequencies shows a faster rate ofdecrease with age than at higher frequencies. To quanti fy the relation betweensite ampli f ication factor and site geology, the surface geology of station siteswas classif ied into f ive groups, namely, (1) Quaternary sediments, (2) Pl iocenesediments, (3) Miocene through Cretaceous sediments, (4) Franciscan formationand Mesozoic granit ic rocks, and (5) Pre-Cretaceous metamorphic rocks. Thestation site ampli f ication factors for each group were logarithmical ly averagedand the mean value was assigned to the median geologic age of that group. Asmooth power law relation was found between the mean site ampli f ication andthe median geologic age. This relation provides a simple way of estimating siteeffect at a specif ic site with known surficial geology. Our site ampli f icationfactors were compared with magnitude station residuals determined by Eaton(1990) for the same network. A remarkable l inear correlation was found betweenthe logarithmic coda amplif ication factor and the magnitude station residualvalue, confirming that the coda method provides an effective means of estimat-ing site ampli f ication factor. Comparison of our site ampli f ication factor withstrong-motion results obtained from the Loma Prieta earthquake (Boore et al.,1989; Maley et al. , 1989; Shakal et al. , 1989; Chin and Aki, 1991) suggests thatweak and strong-motion site ampli f ication factors correlate well in the regionoutside the epicentral source region beyond epicentral distances of about 50km. Within the epicentral source region, however, the weak motion ampli f icationfactors estimated from coda waves do not agree with the observed for strongground motion, suggesting a nonlinear behavior at sediment sites. These resultstogether with studies on the correlation between weak-motion site ampli f icationand earthquake intensity (Borcherdt, 1970; Borcherdt and Gibbs, 1976;Chavez-Garcia et ah, 1990; King et ah, 1990) demonstrate the importance of insitu determination of weak-motion site ampli f ication for seismic zonation.

INTRODUCTIONT h e i m p o r t an ce o f r eco rd i n g s i t e e f fec t o n s e i s m i c g ro u n d m o t i o n h as b een

w e l l r eco g n i zed b y s e i s m o l o g i s ts an d ea r t h q u ak e en g i n ee r s . T rad i t i o n a l l y , d i -r ec t w av es w ere u s ed fo r t h e s i t e e f f ec t s t u d y (G u t en b e rg , 1 9 5 7 ; Bo rch e rd t ,1970; Rogers et al . , 1 9 7 9 , 1 9 8 4 ) . By co m p ar i n g t h e r e s u l t s f ro m d i r ec t S w av esan d co d a w av es , T s u j i u ra (1 97 8 ) an d T u ck e r an d K i n g (1 98 4 ) co n c l u d ed t h a tu s i n g co d a w av es p ro v i d es m o re s t ab l e e s t i m a t e s o f s i te am p l i f i ca t io n . S i n ceco d a w av es a r e co n s i d e red t o b e co m p o s ed of b ack - s ca t t e r ed w av es co m i n g f ro mm an y h e t e ro g en e i t i e s s u r ro u n d i n g t h e s o u rce an d r ece i v e r , t h e r e s u l t i s an

580


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S I TE A M P L I F I C A T I O N F A C T O R A N D S U R F I C I A L G E O L O G Y 5 8 1

av e rag e o v e r a ll d i r ec t i o n s . Fu r t h e rm o re , t h e s ep a ra t i o n o f s o u rce, p a t h , an ds i t e e f fec t s i s d r a s t i ca l l y s i m p l e r fo r co d a w av es t h an fo r S w av es .

A n o t h e r i m p o r t an t i s s u e r eg a rd i n g t h e s t u d y o f s i t e e ff ec t i s t h e l o ca l geo l o gy .I t h a s l o n g b een k n o w n t h a t s u r f ace g eo l o g y ex e r t s s i g n i f i can t i n f l u en ce o n th es i t e ampl i f i ca t ion (Borcherd t , 1970; Borcherd t and Gibbs , 1976; Rogers et al . ,1 9 79 ) . H o w ev er , f o r a q u an t i t a t i v e ev a l u a t i o n o f t h i s i n f l u en ce , w e n eed t oaccu m u l a t e m o re o b s e rv a t i o n s u s i n g d a t a r eco rd ed a t s i t e s w i t h v a r i o u s t y p es o fg eo l o g y . In t h i s p ap e r , w e u s ed a r ecu r s i v e s t o ch as t i c i n v e r s e m e t h o d t o d e t e r -m i n e t h e s i t e am p l i f i ca t i o n f ac t o r s f ro m co d a w av es u s i n g s t a t i o n s i n cen t r a lCa l i fo rn i a l o ca t ed a t a v a r i e t y o f s u r f i c ia l g eo l o gy . T h e U SG S cen t r a l Ca l i fo rn i as e i s m i c n e t w o rk o p e ra t e s h u n d red s o f s t a t i o n s i n t h i s a r ea , p ro v i d i n g an ex ce l -l en t d a t a s e t fo r t h i s s t u d y . We fo u n d t h a t t h e s i t e am p l i f i ca t i o n o f a s t a t i o n i ss y s t em a t i ca l l y r e l a t ed t o t h e g eo l o g y u n d e r l y i n g t h a t s t a t i o n . T h e s i t e am p li f i -ca t i o n is h i g h fo r y o u n g , Q u a t e rn a ry s ed i m e n t s an d d ec reas es w i t h i n c reas i n gg eo lo g i c ag e . T h e r a t e o f d ec reas e v a r i e s w i t h f r eq u en cy . S i t e am p l i f i ca t i o n a tl o w f r eq u en c i e s s h o w a f a s t e r r a t e o f d ec reas e w i t h ag e t h an a t h i g h e r f r eq u en -c i e s . O u r s i t e am p l i f i ca t i o n f ac t o r s w ere co m p ared w i t h E a t o n ' s (1 9 9 0 ) m ag n i -t u d e s t a t i o n r e s i d u a l s . A r em ar k ab l e l i n ea r co r r e l a t i o n w as fo u n d b e t w een t h el o g a r i t h m i c am p l i f i ca t i o n f ac t o r s f ro m co d a w av es an d t h e m ag n i t u d e r e s i d u a lv a l u es , co n f i rm i n g t h a t o u r m e t h o d p ro v i d es an e f f ec t i v e m ea n s o f s i t e am p l i f i-ca t i o n e s t i m a t i o n . T h e co m p ar i s o n o f o u r w eak -m o t i o n s i t e am p l i f ica t i o n r e s u l t sw i t h t h a t o f s t ro n g -m o t i o n r e s u l t s i s d i s cu s s ed i n a l a t e r s ec t i o n .

D A T A A N D A N A L Y S I S

D a t aReco rd s of 1 8 5 ea r t h q u ak e s o b t a i n ed b y t h e U SG S cen t r a l Ca l i fo rn i a s e i s m i c

n e t w o rk d u r i n g t h e p e r i o d f ro m 1 9 8 4 t o 1 9 9 0 w ere co l l ec t ed fo r t h i s s t u d y . T h em ag n i t u d e o f t h es e e a r t h q u a k es r an g ed f ro m 1 . 8 t o 3 . 5 an d t h e i r f o ca l d ep t h sa re l e s s t h an 2 0 k m . F i g u re l a s h o w s t h e d i s t r i b u t i o n o f t h e ea r t h q u ak e s (o p enc i rc l es ) . A l l w av e fo rm s o f t h es e ev en t s w ere d i g i ti zed a t 1 0 0 s am p l es p e r s econ d .T h e s o l i d s t a r s i n F i g u re l a a r e t h e s t a t i o n s u s ed t o ca l cu l a t e s i t e am p l i f i ca t i o nfac t o rs . T h es e s t a t i o n s a r e l o ca t ed o n a w i d e v a r i e t y o f g eo l og i c s e t t i n g s r an g i n gfrom al lu v ium to Mesozoic rocks . A s impl i f i ed geo logy ma p for cen t ra l C al i fo r-n i a i s s h o w n i n F i g u re l b . A l l t h e s t a t i o n s u s ed w ere s t an d a rd U SG S v e r t i ca li n s t ru m en t s w i t h a n a t u ra l f r eq u en cy o f 1 H z . U s i n g ca l i b r a t i o n i n fo rm a t i o np ro v i d ed b y E a t o n (1 9 8 0 ) , a l l r eco rd ed s e i s m o g ram s w ere co r r ec t ed fo r i n s t ru -m en t r e s p o n se . S i n ce i n s t ru m en t g a i n s e t t i n g s ch an g ed f r eq u en t l y o v e r t h et i m e p e r i o d s p an n i n g o u r co l l ec t ed ev en t s , s p ec i a l ca r e w as t ak en t o en s u re t h a tp ro p e r g a i n co r r ec t i o n s w ere t r an s c r i b ed f ro m s t a t i o n h i s t o ry f i l e s ( J . E a t o n ,p e r s o n a l co m m . , 1 9 9 0 ). T h e s h ap e o f t h e i n s t ru m en t r e s p o n s e cu rv e b e t w een 1an d 2 0 H z i s t h e s am e fo r d i f f e r en t s t a t i o n s , w h i l e i t b eco m es co m p l i ca t edb ey o n d 2 0 H z , d ep en d i n g o n t h e t y p e o f d i s c r i m i n a t o r u s ed a t t h e s t a t io n . T oav o i d an y p o s s i b l e m i s t ak e i n th e i n s t ru m en t co r r ec ti o n , o u r o q tav e b an d s w erel i m i t ed t o cen t e r f r eq u en c i e s b e t w een 1 . 5 t o 1 2 H z .M e t h o d

Si n ce t h e p i o n ee r i n g w o rk o f A k i (1 9 6 9) , t h e fu n d am en t a l s ep a rab i l i t y o fs o u rce , s i te , an d p a t h e f f ec t s i n t h e co d a w av e p o w e r s p ec t ru m h as b ee nco n f i rm ed b y m a n y r e s ea rch e r s (A k i an d Ch o u e t , 1 9 75 ; Ph i l l i p s an d A k i , 1 9 8 6;


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5 8 2 F . S U E T A L .

S u et al., 1 9 91 ) . T h e p o w e r s p e c t ru m P(co I t ) o f co d a w av es can b e ex p res s ed a sP(co[ t ) = source(co) -s i t e( co ) . pa th(co l t ) , (1)

w h e re co i s t h e c i r cu l a r f r e q u en c y an d t i s t h e l ap s e t i m e m eas u red f ro m t h eev en t o r i g i n t i m e . T h e ab o v e fo rm u l a i s v a li d , a s a ru l e o f t h u m b , fo r t g r ea t e rt h a n t w i c e th e t r a v e l t i m e o f s h e a r w a v e s f r o m t h e s o u rc e t o t h e s t a t i o n(Rau t i an an d K h a l t u r i n , 1 9 78 ) . T ak i n g n a t u ra l l o g o f b o t h s i d es o f (1) , w e h av e

l l n Pij(~z, tk) = dijkl = ri(wl) + sj(~l) + c(~l, tk), (2)w h e r e ri(cot) i s t h e s i t e t e rm , sj(col) i s the source t e rm, and c(coz , tk) is the codap ro p ag a t i o n t e rm . In d i ces i , j , k , an d l r ep re s en t t h e s t a t i o n , s o u rce, lap s e t im e ,an d f r eq u en cy , r e s p ec t i v e l y . A f ac t o r o f o n e h a l f i s i n t ro d u ced s o t h a t l a t e rr e s u l t s can b e ex p res s ed i n t e rm s o f r e l a t i v e am p l i t u d e .S i n ce t h e co d a en e rg y i s a s s u m ed t o b e t h e s u m o f b ack - s ca t t e r e d w av e en e rg yf ro m h e t e ro g en e i t i e s d i s t r i b u t ed m o re o r l e s s u n i fo rm l y t h ro u g h o u t t h e l i t h o -s p h e re , t h e p a t h t e rm c (coz, th ) d ep en d s o n l y o n t h e r eg i o n a l av e rag e p ro p e r t i e so f t h e m ed i u m an d i s i n d ep en d e n t o f an y p a r t i cu l a r s o u rce - r ece i v e r l o ca ti o n .T a k i n g a n a v e r a g e o f dijkl i n eq u a t i o n (2 ) o v e r a l l av a i l ab l e s t a t i o n i i n t h es am e r eg i o n w i t h f i x ed in d i ces j , k , an d l an d t h en s u b t r ac t i n g i t f ro m t h eor ig inal d~jkl v a l u es , w e a r r i v e a t t h e fo l l o w i n g eq u a t i o n

d, jkt - dJ~l : r i - r~, (3)w h ere Fi i s t h e av e rag e o f r i o v e r t h e av a i l ab l e s t a t i o n s fo r f i x ed i n d i ces j , k ,an d l. We can fu r t h e r w r i t e r i = ~ mSi mrm a n d ri = (1/Njkl)Z~Imrm, w h e r e5 ira = 1 i f i = m an d 0 o the rwise , Njk z i s the to ta l nu mb er of usa b le s t a t ion s forf ixed ind ices j , k , and l and Im = 1 i f s t a t io n m i s used , o r I m = 0 o therw ise .Su b s t i t u t i n g t h es e ex p res s i o n s i n t o (3 ) , w e o b t a i n

dijk - = E ( im -- mlgjk )r . (4)m

Wri t i n g (4 ) i n t o m a t r i x fo rm , w e g e tG r = z~d, (5)

w h e r e h d i s t h e d a t a v e c t o r t h a t c a n b e o b ta i n e d f r o m t h e c o d a p o w e r s p e c t r u man d r i s t h e m o d e l v ec t o r , w h i ch co n t a i n s t h e r e l a t i v e s i t e am p l i f i ca t i o n f ac t o r st o b e d e t e rm i n ed . G i s a m a t r i x o f r ea l e l em en t s . I n p rac ti ce , t h i s m a t r i x co u l db e h u g e an d v e ry s p a r s e . A s i n g u l a r v a l u e d eco m p o s i t i o n an d g en e ra l izedi n v e r s e t ech n i q u e u s ed b y Ph i l l i p s an d A k i (1 9 8 6 ) i s t o o ex p en s i v e an d i n e f f i -c i e n t w h e n a l a r ge n u m b e r o f m o d e l p a r a m e t e r s a n d o b s e r v a t i o n d a t a p o i n ts a r ei n v o l v ed . In t h i s w o rk , w e ap p l y a f a s t r ecu r s i v e s t o ch as t i c i n v e r s i o n m e t h o d(Z en g , 1 9 9 0 ) t o o b t a i n s o l u t i o n s o f o u r i n v e r s e p ro b l em .By t ak i n g t h e n o i s e i n t o co n s i d e ra t i o n , w e can w r i t e e q u a t i o n (5 ) a s

Gr + n = (6)


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S I T E A M P L I F I C A T I O N F A C T O R A N D S U R F I C I A L G E O L O G Y 583

38

37

0

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~ o ~.

\ *

* Station

o E a r t h q u a k e

20 KM

122 121 o 120 la )FIG. 1. (a) M a p o f t h e c e n t r a l C a l i f o r n i a s h o w i n g t h e d i s t r i b u t i o n o f t h e e a r t h q u a k e s ( o p enc i rc les ) and se i smic s ta t ion s ( so l id s ta r s ) u sed in th i s s tud y a s w e l l a s f au l t t r ace s ( so l id l ines ) . (b ) As i m p l i f ie d g e ol o gy m a p f o r c e n t r a l C a l i f o r n i a . T h e n a m e s o f t h e f a u l t s a n d p l a c e s a r e r e f e r r e d i n t h etex t .


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5 8

3 7

5 6

584 F. SU ET AL .

3 5

[ ~ Q u o t e rSed im ,~ Plioce,

S e d i m'~ Miocew

Sedim'~ Fronci~Gr a n i t~ P r e - CMeton"

i122 121 (b}

FIG. 1. (Continued),1 2 0


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SITE AMPLIFI CATION FACTOR AND SURFICIAL GEOLOGY 585

where n is a noise vector. The I th component of this eq uatio n can be writte n asGIr + n I = Adi, I = 1, 2, .. . , N. (7)

where G1 is the Ith row of matrix G, and n I and Adl are the Ith element ofvectors n and Ad, respectively.

By applyin g the recursive stochastic inverse fo rmula (Zeng, 1990) to our case,we get

[2i + 1 = 21 + K I+ I( Ad I+ I - GI+12t)T T 02~K1+1 : RIGx +I/ (Gz+ IRIG x+I + 1+1)

| Rz + 1 = R z + Kz+ tGI +I RI(s )

where 21 is the stochastic inverse solution of r given A dl ,. . . , Adl, RI is itscorresponding error covariance matrix and z2 is the noise var ian ce for n I.Superscript "T " means ma trix transpose. We define the initial estimate 2 o = 0,and give R o a priori . Then we compute the updated r according to the recursiveprocedure given by (8).

From (8), we can see tha t t his rec ursive inverse process is a sequence of scalarinverses that require only matrix and vector additions and multiplications. Forthe i nversion of a large sparse ma trix, we just compute the term RIGT+I bytracing only the nonzero elements in G1+1, which can avoid a large number ofunnecessar y calculations and grea tly reduce the computation time. It is particu-larly effective for inverse problems with a large numb er of model para mete rsand observations, such as our site amplif ication inversion. In addition, thisrecursive scheme provides some insight to the inversion process. From therecursive process given in (8), we see th at our process can stop at an y step I andthe result is stored. These intermediate results give us a good perspective ofhow each observation improves our inv erse solution. Because resul ts of ourinversion can be inc rementall y stored, we can al ways update our solution withnew data points witho ut computation over the whole data set.

Another a dvantag e of the inversion method, as compared to the spectral ratiomethod (Mayeda et al., 1991), is that it can be applied to a seismic network overa large region, which enable s us to compare the site amplificat ion factors overthe large regi on with a common reference. The details of coda power spectrumcalculation, noise eliminatio n and data processing technique can be found in Suand Aki (1990).

RESULTSCoda S i te Ampl i f i ca t ion in Centra l Cal i forn ia

The final inve rsion result s of site amplif ication factors are listed in Table 1.The standard errors, in natural log, are about 0.065 for frequency 1.5 Hz, 0.056for 3 Hz, 0.051 for 6 Hz, and 0.050 for 12 Hz. To show the spatial distribution ofsite amplification in central California we plot these site amplification factorson maps (Fig. 2a to d) for the freq uency bands centered at 1.5, 3.0, 6.0, and 12.0Hz, respectively. In these figures, we used six symbols to indicate six differentsite amplificatio n ranges. For 1.5 Hz, we can see th at t he hig hest site amplifica-tions are in the vicinity of Watsonville and fault zone near Hollister. Thesurface geology of these areas consists mainl y of Quaternar y sediments. The


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586 F . S U ET AL .T A B L E 1

STATION LOCATION, SITE SURFICIAL GEOLOGY, AND SITE AMPLIFICATION FACTORDETERMINED IN THIS STUDY

Sta~onSite AmplificationLn ( A / A)

Location 1.5 Hz 3 Hz 6 Hz 12 Hz Geological SymbolJ P R VP P T VB H R VC D U VP S A VH C O VJ P L VH P H VJ L T VM Y L VB E H VH F H VJ S G VJ S J VN H M VH K R VB S L VH O R VN O L VN C F VC M C VC D O VC R A VB B N VJ T G VJ B Z VJ P S VJ R G VH P R VP B W VP L O VJ B G VC B R VC B W VC M J VC A C VH Q R VH S F VJ S M VB P I VB R V VH C B VC S P VP A N VB V L VP J L VC M H VJ E C VJ S F VJ S C V

3 7 4 7 .7 0 1 2 2 2 8 .4 3 -0 .0 5 0 .1 8 0 .4 4 0 .5 036 6 .50 120 43.27 0 .52 0 .98 1 .01 0 .5536 43.67 121 15.83 1 .18 0 .57 0 .46 0 .6538 1 .78 122 0 .05 1 .95 2 .04 1 .77 1 .5136 1 .52 120 53.30 1 .75 1 .36 0 .97 1 .3136 53.31 121 42.34 1 .32 1 .07 0 .76 0 .7036 58.62 121 49.93 1 .59 1 .35 1 .04 0 .8336 51.38 121 24.37 1 .98 1 .78 1 .56 1 .3737 21.22 122 12.25 0 .45 0 .91 1 .15 0 .8037 23.02 120 25.16 0 .17 0 .31 0 .62 0 .5036 39.88 121 10.45 0 .82 0 .55 0 .47 0 .3436 53.29 121 28.13 1 .70 0 .95 0 .76 0 .7137 16.96 122 3 .00 0 .45 0 .24 0 .28 0 .1837 20.03 122 5 .48 0 .65 0 .60 0 .57 0 .7338 9 .28 121 48.02 1 .01 0 .68 0 .11 0 .3236 54.10 121 25.56 1 .77 1 .02 1 .11 0 .9436 46.53 121 20.96 1 .69 1 .24 0 .99 0 .8736 55.03 121 30.46 1 .64 0 .89 0 .53 0 .6638 2 .50 122 47.64 0 .65 0 .79 0 .80 0 .7138 19.28 122 47.73 0 .05 0 .26 0 .69 0 .963 7 4 6 .8 8 1 2 2 1 0 .5 5 0 .1 0 -0 .3 7 0 .0 0 0 .5 137 43.80 121 50.12 0 .77 0 .76 0 .44 0 .383 7 4 6 .0 3 1 2 1 5 6 .2 5 0 .6 4 0 .3 5 -0 .0 3 -0 .3 33 6 3 0 .6 0 1 2 1 4 .5 3 0 .3 4 0 .2 1 0 .1 4 -0 .0 837 1 .71 121 52.58 0 .91 0 .75 0 .58 0 .9537 1 .07 121 49.15 0 .99 0 .98 1 .45 1 .253 7 1 1 .9 4 1 2 2 2 0 .9 0 0 .4 5 0 .4 0 0 .1 9 -0 .1 337 2 .22 121 57.87 0 .57 1 .03 0 .94 1 .0436 57.19 121 41.70 1 .00 0 .53 0 .49 0 .4136 18.90 120 55.75 0 .41 0 .25 0 .38 0 .8036 14.79 121 2 .55 0 .56 1 .22 1 .01 0 .8537 20.52 122 20.34 0 .94 0 .63 0 .53 0 .413 7 4 8 .9 7 1 22 3 .7 2 - 0 . 0 9 - 0 . 6 6 - 0 . 7 8 - 0 . 8 03 7 5 5 . 4 5 1 2 2 6 . 4 0 0 . 3 5 - 0 . 1 5 - 0 . 3 6 - 0 . 1 33 7 3 1 . 25 1 2 1 5 2 . 23 - 0 . 5 4 - 0 . 5 6 - 0 . 2 8 - 0 . 7 537 58.57 121 45.62 0 .94 0 .43 0 .10 0 .903 6 5 0 .0 2 1 21 1 2 .7 6 - 0 . 4 5 - 0 . 3 8 - 0 . 5 2 - 0 . 9 036 48.72 121 29.97 0 .78 0 .70 0 .32 0 .033 7 1 2 .7 4 1 2 2 1 0 .0 6 0 .4 7 0 .2 6 0 .4 2 -0 . 0 23 6 2 9 .4 0 1 21 1 0. 11 - 0 . 7 9 - 0 . 6 0 - 0 . 4 4 - 0 . 4 736 25.49 121 1 .10 0 .32 0 .23 0 .03 0 .423 6 5 5 .8 8 1 2 1 3 9 .6 3 0 .4 2 0 .4 2 0 .3 5 -0 .1 83 7 5 7 . 45 1 2 2 1 8 .6 5 0 . 0 9 - 0 . 0 7 - 0 . 0 4 - 0 . 4 43 5 4 6 .7 8 1 2 0 5 4 .4 4 0 .9 9 0 .6 5 0 .4 0 0 ,6 93 6 3 4 .5 1 1 2 1 1 1 .3 4 0 .2 3 0 .1 5 -0 .2 0 -0 .3 536 5 .39 121 9 .33 0 .10 0 .17 0 .09 0 .073 7 2 1 .5 7 1 21 4 5 .3 8 - 0 . 2 8 - 0 . 2 6 - 0 . 3 8 - 0 . 3 53 7 3 . 0 4 1 2 1 4 8 . 5 6 - 0 . 1 1 0 . 1 1 - 0 . 0 4 - 0 . 4 637 24.31 122 10.55 0 .65 0 .37 0 .02 0 .113 7 1 7 .0 7 1 2 2 7 , 4 2 - 0 . 5 3 - 0 . 2 1 - 0 . 0 6 - 0 . 2 1

QsQ a l , P m lQ fQQ t , P m lQcQcQ pQpQp , PmlcQ pQ pQ pQpQpcQ pP cP cPPP v rP m l cP m l cP m l cP m lP m lP m lP m lP m lP m lP m lP m lM uM uM uM uM vM vM v bM v rM m cM m , P m lM m , P m l , Q a lM mM mM mM m , M uM m , P m lM mM1, Kjf


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S ITE AMP LIF ICATION F ACTOR AND S URF ICIAL GEOLOGYTABLE 1 - -C o n t i n u e d

587

StationSite Amplification Ln ( A / A)

Locati on 1.5 Hz 3 Hz 6 Hz 12 Hz Geological SymbolJP PV 37 15.81 122 12,78 0.29 0.34 -0 .0 1 0.15HBTV 36 51.01 121 33.04 -0 .05 -0. 22 -0 .58 -0. 32JBCV 37 9.62 122 1.57 0.40 0.44 0.38 0.07JH PV 37 26.65 122 18.09 0.25 0.43 0.12 0.08NGVV 38 16.84 122 12.89 -0 .09 -0 .14 -0. 15 -0. 26NTYV 38 23.37 122 39.70 0.86 0.15 -0 .0 9 -0 .3 5NSPV 38 10.96 122 27.20 -0. 02 -0 .10 -0. 05 -0 .26CPLV 37 38.25 121 57.64 -0 .44 -0. 63 -0 .55 -0. 40HCAV 37 1.52 121 29.02 -0. 07 -0 .34 -0. 48 -0. 42HSPV 37 6.91 121 30.94 -0 .1 0 0.08 - 0. 20 -0 .46CMOV 37 48.68 121 48.15 0.33 0.15 0.08 -0 .2 2BMSV 36 39.78 120 47.51 -0 .2 6 0.29 0.15 -0 .3 6NAPV 38 26.34 122 14.99 0.75 -0 .1 6 -0 .5 2 -0 .5 0BBGV 36 34.70 121 2.31 0.88 0.82 0.59 0.19CMPV 37 21.45 121 18.51 -0 .69 -0.6 1 -0 .74 -0. 75HCPV 37 11.67 121 11.08 -0 .4 7 0.27 0.50 -0 .0 9NLHV 38 7.19 122 8.87 0.21 0.46 0.39 -0 .1 3HSLV 37 1.16 121 5.13 -0. 08 0.49 0.39 0.09HFEV 36 59.00 121 24.09 -0. 02 -0 .25 -0. 33 -0. 42CLCV 37 44.28 122 3.83 0.25 0.47 0.36 0.30HJSV 36 48,99 121 17.92 0.07 -0 .3 2 -0 .5 0 -0 .45CBSV 37 49.06 121 38.43 0.44 0.87 0.77 0.59HLTV 36 53.07 121 18.49 -0 .34 -0. 62 -0 .71 -1. 02JHLV 37 6.54 121 49.99 -0. 87 -0. 82 -0 .40 0.05BRMV 36 50.70 120 49.40 0.01 0.56 0.72 0.25JALV 37 9.50 121 50.82 -0 .83 -0. 53 -0. 05 -0. 05CMRV 37 35.68 121 38.22 -0 .60 -0. 73 -0. 79 -0. 81HGSV 37 5.75 121 26.83 -0. 78 -0. 78 -0. 79 -0. 85JSSV 37 10.17 121 55.84 -0 .64 -0. 41 -0. 61 -0. 47JBMV 37 19.09 122 9.16 -0. 52 -0. 52 -0. 51 -0. 47HGWV 37 1.02 121 39.20 -0. 76 -0. 78 -0. 82 -0. 91COSV 37 30.51 121 22.44 -0. 87 -0 .69 -0. 66 -0. 55CAOV 37 20.96 121 31.96 -0. 74 -0 .63 -0. 76 -0. 96ADWV 38 26.35 120 50.89 -0. 69 -0 .38 -0. 14 -0 .69CSCV 37 17.11 121 46.35 0.34 0.18 -0 .0 2 -0 .1 4JLXV 37 12.11 121 59.17 -0 .63 -0 .39 -0. 15 -0. 02NLNV 38 9.15 122 42.75 - 0. 48 -0 .2 0 0.05 0.39BPFV 36 13.82 121 46.32 -0. 51 -0. 20 -0 .12 0.04NTAV 37 55.43 122 35.70 -0 .63 -0. 70 -0. 61 0.10HPLV 37 3.13 121 17.40 -0 .60 -0. 46 -0 .32 -0. 59BEMV 36 39.68 121 5.76 -0 .5 6 0.31 -0 .1 3 -0~27PAPV 35 54.77 121 21.70 -1 .05 -0. 55 -0. 41 -0. 53CMLV 37 28.64 121 39.09 -0 .90 -0. 90 -0. 81 -0 .26PHRV 36 22.38 120 49.10 -0. 05 -0. 29 -0. 51 -0, 54CMMV 37 27.34 121 29.62 -0. 75 -0. 87 -1. 07 -0. 92JRRV 37 3.27 121 43.61 -0. 97 -0. 86 -0. 17 0.03BAVV 36 38.75 121 1.79 -0. 67 -0 .86 -0. 88 -1 .19CALV 37 27.07 121 47.95 -0. 71 -0. 65 -0. 61 -0 .77CAIV 37 51.68 122 25.77 -0 .6 7 -0 .5 2 0.17 0.12JMG V 37 38.22 122 28.43 - 0.46 - 0.42 - 0.28 0.44CCYV 37 33.10 122 5.45 -0. 98 -1 .04 -0. 76 -0. 27HCRV 36 57.46 121 35.01 -0. 65 -0 .92 -0. 70 -0. 22JSAV 37 34.95 122 25.03 -0 .30 -0 .23 -0. 06 0.26JEGV 37 30.84 122 27.74 -0 .5 2 -0 .2 1 0.10 0.37MHDV 37 7.36 119 53.60 -0 .2 0 0.06 0.42 0.29

M1, E&, gr, QtEETvTvTvKKKKKuKu, Tv, UmKuKuKuKuKuKuKuKuKuKuKuKuKjfgjfKjfgjfKjfKjfKjfKjfKjfKjfKjfKjfKjfKjfKjfKjfKjfKj fKjfKjfKj fKjfKjfKjfvKjfvKjfvKjfvKjfvgrgr


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588 F. SU ET AL.TABLE 1--Continued

Site AmplificationLn (A/A)Station Location 1.5 Hz 3 Hz 6 Hz 12 Hz Geological ymbolBPCV 36 34.32 121 37.56 0.30 0.16 0.16 0.19 grBVYV 36 44.96 121 24.80 - 0.6 6 -0 .80 -0. 73 -0 .44 grHMOV 36 36.03 121 55.06 - 0.8 6 -0 .89 -0. 62 -0. 07 grHAZV 36 53.08 121 35.45 -0 .26 -0. 71 -1. 05 -0 .87 grBAPV 36 10.55 121 38.56 - 0. 79 -0 .0 8 0.36 0.12 gr, mBJCV 36 32.82 121 23.53 -1 .56 -1 .4 3 -1. 15 -0 .69 grBPPV 36 10.12 121 22.68 -1 .22 -0. 61 -0. 49 -0 .76 grBPRV 36 24.42 121 43.77 -0. 72 -0 .38 -0 .44 -0 .25 gr, mBJOV 36 36.65 121 18.81 -0 .9 6 -0 .7 3 0.44 0.62 grHDLV 36 50.12 121 38.64 - 0.6 8 -0 .55 -0. 24 -0 .20 grBSCV 36 38.50 121 15.59 -0 .11 -0 .35 -0. 63 -0 .60 grBSGV 36 24.83 121 15.22 -1. 05 -0 .8 0 - 0. 18 0.63 grJBLV 37 7.69 122 10.08 -0.8 8 -0 .89 -0. 72 -0 .36 grBCGV 36 42.55 121 20.60 0.07 -0 .0 6 0.13 0.07 grHJGV 36 47.88 121 34.43 -0 .52 -0 .7 4 -0. 66 -0 .42 grBSRV 36 39.99 121 31.12 -0.9 2 -0 .5 8 0.20 0.63 grCRP V 37 54.75 121 54.33 0.20 0.79 0.41 0.07 ub, ku, grCADV 37 9.83 121 37.55 -0.2 1 -0 .7 9 -0 .3 5 0.03 ubJCBV 37 6.71 121 41.33 -0 .4 9 0.17 0.05 -0 .2 8 ubJSTV 37 12.41 121 47.84 -0 .26 -0. 40 -0 .38 -0. 13 ub, KjfCSHV 37 38.88 122 2.57 -0 .5 4 -0 .5 4 -0 .4 4 0.30 ub, JkCCOV 37 15.46 121 40.35 - 0.0 8 -0. 25 -0 .58 -0. 72 JkBHSV 36 21.35 121 32.41 -0 .6 8 0.28 0.66 0.58 m, Mm, grBCWV 36 18.40 121 33.96 - 1.0 1 -0. 66 -0. 11 -0. 31 mBSMV 36 23.03 121 25.63 - 0.6 8 -0 .24 -0 .06 -0. 31 mJUCV 37 0.07 122 2.91 -0 .43 -0. 53 - 0.3 2 -0 .05 msHFPV 36 45.22 121 29.43 -1 .14 -1. 22 -1. 24 -1. 14 1S

Diablo range of the Fran ciscan form ation has low amplification at all frequencybands studied (Fig. 2a to d). The Gabilan range and the southwest segments ofthe Naci miento faults have the lowest site amplification at 1.5 Hz, but t heir siteamplification gradually increases with increasing frequency. These areas aremostly composed of competent Mesozoic granitic rocks and Pre-Cretaceousmetamorphic rocks. The site amplification along the San Andreas fault is lesssystematic since the geologic conditions along the fault zone changes dramati-cally. In general, we found that the site amplification of a station is controlledby its underlying surface geology.The Relation of Coda Site Amplification Factors with Surficial Geology

To investigate how the site geology affects the station site amplification, wehave obtained the surface geology underli ng each station from CDMG 1:250,000scale geologic maps. The surficial geology at each station is listed in Table 1.From Table 1, we can see that most stations fall in six categories of surficialgeology: Quate rna ry sediments, Pliocene sediments, Miocene sediments, Creta-ceous Marine sediments, Franciscan formation, and Mesozoic granitic rocks.Figur e 3 shows a plot of site amplif ication as a function of frequency for each ofthe six geologic categories. Each line in the figure repres ents site amplificationat one station and the average is shown by the dotted line. From Figure 3, wecan see that the site amplification decreases with increasing geologic age anddifferent geologic settings show different frequ ency dependencies. The Q uater-


10/23


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11/23

590 F . SUE T AL.

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12/23


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13/23

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14/23


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Documents

Relation Between Site Amplifcation Factor and Surficial Geology