Raghu Kantu and Iyenga 2004

8/4/2019 Raghu Kantu and Iyenga 2004

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A S T IE H N I I ~ I D E M I I i I [ I I I l lSEISMOLOGICALI R E S E A R C H ~

A ttenuation of Strong Ground M o tio n inP eninsu lar Ind iaR. N . lyengar and S. T . G. R ag hu KanthD e p t . o f C i v i l E n g i n e e r i n g , I n d i a n I n s t it u t e o f S c i e n c e , B a n g a l o r e

ABSTRACTE a r t h q u a k e s i n I n d i a o c c u r i n t h e p l a t e - b o u n d a r y r e g i o n o ft h e H i mal ay a s a s we l l as in t h e i n t r ap l a t e r eg i o n o f p en i n su l a rIn d i a (P I ) . Dev as t a t i n g ev en t s h av e o ccu r r ed i n P I i n t h erecen t p as t, wh i c h i s a wa rn i n g a b o u t t h e p o ss i b i l i t y o f su chear t h q u ak es i n t h e fu t u re . B u t v e ry l i mi t ed r ec o rd ed d a t a a r eav a i l ab l e ab o u t g ro u n d mo t i o n i n P I fo r en g i n ee r s t o r e l yu p o n . Th e p resen t p ap er , a f t e r a r ev i ew o f av a i l ab l e d a t a ,d ev e l o p s an a t t en u a t i o n r e l a t i o n sh i p b ased o n a s t a t i s t i ca l l ys i m u l a t e d s e i s m o l o g i c a l m o d e l . T h e p r o p o s e d e q u a t i o n f o rp e a k g r o u n d a c c e l e r a t i o n ( P G A ) , u n d e r b e d r o c k c o n d i t i o n s ,i s o f t h e fo rml n ( P G A / g ) = c 1 + c 2 ( M - 6 ) + s 6) 2 - In(R) - c4R + lne, (1)w i t h c1 - - 1 . 6 8 5 8 , c2 = 0 . 9 2 4 1 , c3 = - 0 . 0 7 6 0 , c 4 = 0 . 0 0 5 7 ,an d o ' (l n t3 ) - 0 .4 6 4 8 . Co r r ec t i o n f ac t o rs fo r o t h e r s i t e co n d i -t i o n s a re al so co m p u t e d . I n t h e ab sen ce o f a ro b u s t d a t ab aseo f s t ro n g -m o t i o n r eco rd s , s e ismo l o g i ca l m o d e l i n g i s a r a t io -n a l a l t e rn a t i v e u n t i l su f f i c i en t i n s t ru men t a l r eco rd s b eco meav a i lab l e i n P I . I t i s o b se rv ed t h a t a t t e n u a t i o n o f s t ro n gmo t i o n i n P I i s s i mi l a r t o t h a t i n o t h e r i n t r ap l a t e r eg i o n s o ft h e wo r l d .INTRODUCTIONIn d i a h as f aced sev era l d ev as t a t i n g ea r t h q u ak es i n t h e p as t .T h e l a rg e s t o f t he s e h a v e o r i g i n a te d i n t h e H i m a l a y a n p l a t e -b o u n d a ry r eg i o n , wh i ch r em ai n s a reg i o n o f g r ea t s c i en t if i can d en g i n ee r i n g i n t e r es t . No t su rp r i s i n g l y , co n s i d e rab l e d a t aan d ea r t h q u ak e r e l a t ed l i t e r a t u r e a r e av a i l ab l e ab o u t t h en o r t h e r n p a r t o f I n d ia . O n t h e o t h e r h a n d , v e r y l it tl e s e is m o -l o g i ca l i n fo rmat i o n i s av a i l ab l e ab o u t p en i n su l a r In d i a (P I ) ,wh i ch i s t ak en h e re as so u t h o f 2 4 ~ l a t i t u d e . Th i s s i t u a t i o ni s ch an g i n g , i n r e sp o n se t o t h r ee r ecen t d ev as t a t i n g ev en t s :

t h e Kh i l l a r i ( 2 9 S ep t emb er 1 9 9 3 ) , J ab a l p u r (2 2 M ay 1 9 9 7 ) ,an d K u t ch (2 6 Jan u ary 2 0 0 1 ) sh o ck s . Bu t t h e av a i lab l e q u an -t i f ied in format ion i s so sparse, eng ineers p resen t ly face ad a u n t i n g p r o b l e m i n e s t i m a t i n g g r o u n d - m o t i o n l e v e l s f o rfu t u re ev en t s i n P I . Th e p resen t p ap er i s mo t i v a t ed b y t h en e e d t o h a v e a s i m p l e a p p r o a c h t o u n d e r s t a n d a t t e n u a t i o n i nP I f ro m t h e en g i n ee r i n g p o i n t o f v iew.

Fi rs t , the database avai lab le fo r PI i s b r ief ly rev iewed . PIi s n o t h o mo g en o u s wi t h r e sp ec t t o i t s s e i smo g en i c ch a rac t e r .T h e a v a i l a b l e s t r o n g - m o t i o n a c c e l e r o g r a p h ( S M A ) d a t a d on o t co v er t h e en t i r e r eg i o n an d l ack mu l t i p l e S M A reco rd i n g so f i n d i v i d u a l ev en t s . Hen c e an emp i r i ca l eq u a t i o n o b t a i n edso l el y wi t h t h e h e l p o f t h e av a i lab l e i n co mp l e t e S M A d a t a wi l lb e u n re li ab l e . Here , t h i s d i f f icu l t y is c i r cu mv en t ed b y ad o p t -i n g a s e i smo l o g i ca l mo d e l fo r sy n t h e t i c g en era t i o n o f P GAv al u es fo l l o wi n g Bo o re (1 9 8 3 ) . Th e p resen t s t u d y i n c l u d esreg i o n a l d i f f e r en ces o f q u a l i t y f ac t o r wi t h i n P I an d e f f ec t o funce r tain t ies in st ress d rop , rad ia t ion coeff ic ien t , cu t -o ff f ie-q u en cy , an d fo ca l d ep t h . W i t h t h e h e l p o f a l a rg e sy n t h e t i cd a t ab ase , an a t t en u a t i o n r e l a t i o n i s o b t a i n ed b y a t wo -ways t r a t i f i ca t i o n ap p ro ach ( Jo y n er an d Bo o re , 1 9 8 1 ) . Th e p ro -p o sed a t t en u a t i o n r e l a t i o n i s f o u n d t o ma t ch f av o rab l y wi t ht h e e s t im a t e d P G A v a l u e s o f t h e K u t c h e a r t h q u a k e o f 2 0 0 1 .PENINSULAR IND IAT h e r e g i on o f t h e I n d i a n s u b c o n t i n e n t s o u t h o f 2 4 ~ l a t i tu d ei s t ak en h e re a s p en i n su l a r In d i a (P I ) . Th i s l an d mass i s f a ra w a y fr o m t h e H i m a l a y a n c o l li s io n z o n e , t h e w e l l k n o w nb o u n d a r y b e t w e e n t h e I n d i a n a n d A s i a n p l at e s. N o n e t h e l e s s ,i t is r e c o gn i z e d t h a t C a m b a y a n d R a n n o f K u t c h i n G u j a r a ta r e amo n g t h e ac t i v e r eg i o n s o f In d i a . Ap ar t f ro m t h i s r eg i o na n d t h e A n d a m a n - N i c o b a r I s l a nd s , th e r e m a i n i n g p a r t o fco n t i n en t a l P I h as r e l i ab l y ex p er i en ced so me 4 0 0 ea r t h q u ak esi n a p e r i o d o f 6 0 0 y e a r s. T h i s n u m b e r w o u l d b e m u c h l a rg e ri f a ll i n s t ru m en t a l l y r eco rd ed sh o ck s o f smal l m ag n i t u d es

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TABLE 1Ea r thqua k e s o f Pe n ins u la r Ind ia

Locat ion Date F o c a l M a g .E p ic e n te r ( N / E ) D e p t h k m ) ( M w )Epicentra lIntens ity( I o ) Re fe re nc e

C0imbat0re 7 February 1900Vi jayanagaram 17 Apr i l 1917Anjar 21 July 1956Ong01e 13 Oc tober 195 9Ong01e 27 Ma rch 1967K0yna 13 September 1967K0yna 11 Decem ber 1967Bhadrachalam 13 April 196 9Broach 23 Ma rch 1970C0imbat0re 29 July 1972Shim0ga 12 May 1975Kr ishnagiri 20 Ma rch 1984Idukki 7 June 1988Khi l lar i 29 September 1993B0naigarh 29 Ma rch 1995Jabalpur 21 May 1997Bhuj, Kutch 26 January 2001Bhuj, Kutch 28 January 2001

10.8~ ~ - - 6 .118 .0~ ~ - - 5 .523 .0~ ~ - - 6 .115.6~ ~ ~ 5.015.6~ ~ ~ 5.817 .4~ ~ - - 5 .817.5~ ~ 10 6.517.9~ ~ 25 5.721.70/72.9~ 8 5.81 1 .0 ~ ~ - - 5 . 013.8~ ~ 10 5.012.58~ ~ 6 4.59.81 ~ o 5 4.518 .06~ ~ 7 6 .221.660/84.59~ 21 4.723.0o/80.0~ 35 5.823.40/70.28~ 24 7.723.61~ ~ 15 5.7

VIII Kaila et aL (1978)VII IMDIX Kaila et a l . (1978)VII Chandra (1977)VII IMDVIII Guha e t a L (1970)

VII I - IX Guha e t a L (1970)VII Kaila et a l . (1978)VII Kaila et a l . (1978)VI IMDV Kai la et aL (1978)VI Guha e t a L (1993)V Rast0gi e t aL(1995)VIII Narula e t a L (2000)V Narula e t a L (2000)

VIII Narula et a l . (2000)Xl IMDVII IMD

IMD: India Meteorological Department, New De lhi , India.w er e a lso inc luded . A l i s t o f damaging ea r thquakes of engi -neer ing impor tance tha t have occur r ed in P I in the la s t 100years is presented in Table 1.

The seismici ty of PI f rom a seismological perspective hasbeen d i scussed in the pas t no tab ly by Chandr a ( 1977) andRao and R ao ( 1984) . A ca ta log of P I ea r thquakes o f magn i -tude gr ea te r than 3 w as compi led by G uha and Basu ( 1993) .I t is general ly held tha t seismic act ivi ty is greater a t the inte r -sec tions of the D h ar w ad , A r ava l i, and S inghb hum pr o tocon -t inen ts , w h ich tog e ther con s t i tu te P I ( F igur e 1A ) . W i th da taavailable up to 1984, Rao and Rao (1984) f i t ted the f re-q u e n c y - m a g n i t u d e r e la t io n s h i pLog10 N = a - b M (2 )to ge t a = 4 .4 and b = 0 .85 f or P I . The y a l so dem ons t r a tedtha t the in te r va l 1 870 - 19 20 w as a pe r iod of qu iescence ,w her eas pr ior to a nd a f ter th i s t ime w ind ow PI show ed h igherleve ls o f se i smic ac tiv ity . I t may be no ted tha t the I nd ian codeon ea r thquake- r es i s tan t des ign o f s t r uc tur es , I S - 1893, p r e -sen ts a zona t ion map of I nd ia w hich w as r ev ised a f te r theK oyna and K hi l la r i ea r thquakes . The r ev is ion has been toupgr ade par t s o f P I f r om zero and low leve ls o f s e i smic hazar dto higher levels . Thus , there is a consensus among scientis tsand engineers that PI is pass ing through a peak in i ts act ivi ty.

Seeber e t a l . ( 1999) s tud ied the se i smic i ty of P I w i th pa r t i cu-la r r e f e r ence to Mahar ash t r a . They conc luded tha t be tw een1960 an d 1990 the se i smic i ty of P I show ed a th r ee f o ldincr ease . This w as the pe r iod dur ing w hich indus t r ia l deve l -op me nt a l so inc r eased severa l fo ld in P I. Thus , engineer s m us tr ecognize tha t the looming se i smic r i sk to man- made s t r uc -tur es in P I i s mo r e th an w ha t w as pr ev ious ly be lieved .GROUND-MO TION DATABASEThis i s a b r ie f rev iew of s t r ong- m ot ion da ta ava i lab le f or P I.T h e o n l y r e g i o n w i t h S M A d a t a i s t h e K o y n a - W a r n a r e g i o nof w es te r n I nd ia ( F igure 1B) . The ea r li e st ava i lab le PG A va luef or P I i s f r om the K oyna ea r thquake r ecor d of 11 D ecember1967. A f te r th i s, a l a rge nu mb er o f recor ds of smal le r magni -tudes w er e ob ta ined in the K oyn a r eg ion . This s e t o f da ta ,t a k e n f r o m t h e r e p o r t s o f G u p t a e t a l . ( 1992) , i s p r esen ted inTable 2 . For the main shoc k of the K hi l la r i ea r thqu ake o f1993, no near - sour ce gr ound mot ion r ecor ds a r e ava i lab le .S M A r e co r d s h av e b e e n o b t a i n e d b y B a u m b a c h e t a l . ( 1994)f or a f ew a f te r shocks of th i s event , how ever . Thr ee such va luesa r e g iven in Table 3 . A f ew ins t r umenta l ve loc i ty r ecor dsw i th in ep icen t r a l d i s tances of 300 km ar e avai lable f or theJ a b a l p u r e a r t h q u a k e o f 1 9 9 7 ( S in g h e t a l . , 1999) . S imilar ly, af ew da ta a r e ava i lab le f or the main event and a f te r shocks of

Seismo log ica l Research Le t te rs Ju ly /Augus t2004 Vo lum e 75 , Num ber4 531


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TABLE 2Instr ument a l PGA f or Ko yna- War na RegionEpicenter (N/E) Date E p ic e n tr al F o c a l D e p t h R e c o r d e dP G A E s t i m a t e dP G A ( g )M w D i s t a n c e k m ) ( k m ) ( g ) ( E q . 1 1 )17.50~ ~17.48~ ~17.50~ ~17.28~ ~17.29~ ~17.35~ ~17.35~ ~17.33~ ~17.36~ ~17.36~ ~17.32~ ~17.48~ ~17.36~ ~17.49~ ~17.27~ ~17.23~ ~17.23~ ~17.20~ ~17.25~ ~17.24~ ~17.24~ ~17.35~ ~17.32~ ~

13 September 196713 September 196710 December 196712 December 196713 December 196724 December 196724 December 19674 M arch 19684 M arch 19681 Janu ary 197027 May 197026 September 197017 February 197429 July 19742 September 19802 September 198020 September 198020 September 198020 September 198025 Apri l 198225 Apri l 198212 M arch 199513 M arch 1995

5.6 13 3 0.1640 0.26514.3 11 5 0.020 0 0.07266.5 13 10 0.4860 0.48274.5 14 13 0.0360 0.05464.4 12 15 0.0410 0.04834.8 7 20 0.0500 0.06764.8 7 20 0 .0350 0.06764.0 4 10 0.0190 0.05764.0 9 10 0.0090 0.04504.1 9 11 0.0130 0.04794.2 11 3 0.0698 0.06884.2 11 13 0.0420 0.04394.5 17 19 0.0214 0.03874.1 8 24 0.03 60 0.02444.1 18 13 0.0293 0.02864.1 18 13 0.0110 0.02864.5 21 8 0.0310 0.04504.7 17 8 0.0220 0.06974.7 17 8 0.0219 0.06974.1 18 13 0.0293 0.02864.1 18 13 0.011 0 0.02864.5 20 10 0.0130 0.04524.2 25 10 0.0055 0.0255

TABLE 3Instrum ental PGA for Wes tern-Cen tral RegionEpicenter (N/E) Date E p ic e n tr a l F o c a l D e p t h R e c o r d e dP G A E s t i m a t e dP G A ( g )M w D i s t a n c e k m ) ( k m ) ( g ) ( E q . 1 2 )17.93~ ~ 8 October 199317.93~ ~ 8 October 199317.93~ ~ 8 October 199323.8o/80.06~ 21 May 199723.8o/80.06~ 21 May 199723.61 ~17 6 28 January 200123.61 ~17 6 28 January 2001

4.3 21 5 0.0640 0.03 734.3 11 5 0.0103 0.07094.3 13 5 0.0655 0.06085.8 237 36 0.0125 0.00435.8 271 36 0.0086 0.00305.7 101 15 0.0079 0.02185.7 249 15 0.0017 0.0035

t he K utch ea r thquake o f 2001. PGA va lues obta ined by d if -ferent iat ing the ins t rumental veloci ty records of the Jabalpurear thquake and Kutch a f t e rshock (S ingh et al., 2003) are alsoshown in Table 3 . Th e m om ent magni tudes ( M w) of eventsfor which ins t rum ental data are avai lable in PI varies from 4to 7.7. The database does not have ins t rumental PGA valuesin al l dis tance ranges of engineering importance, however,and hence can not be used for empirical a t tenuat ion s tudies .SEISMOLOGICAL MODELIn regions lacking s t rong-motion data , seismological models(Boore, 1983) are viable alternat ives an d are used worldwidefor deriving at tenuat ion relat ionships (Atkinson and Boore,

1 9 9 5 ; H w a n g a n d H u o , 1 9 9 7 ; T o r o et aL, 1997). Singh et aL(1999) used a seismological model for es t imat ing groundmo t ion in pa r ts of P I , bu t n o spec if ic a t t enua t ion equa t ionhas been proposed by them. The theory and appl i ca t ion ofse ismologica l mode ls for es t imat ing gro und mo t ion has beendiscussed in detai l by Boore (1 983, 200 3). Briefly, the Fo urieram pli tude spectrum of accelerat ion at bed rock is expressed asA( ) = C [ S( f ) ] D ( f ) P ( f ) (3 )where S ( f ) i s the source spectral funct ion, D ( f ) is thedim inut ion func t ion charac te r iz ing the a tt enua t ion , P ( f ) isa fi l ter to shape accelerat ion am pli tude s bey ond a high cut-o ff

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f r equency fro, and C is a scal ing factor . In the present s tudy,the s ing le cor ner - f r equency mode l

S ( f ) - ( 2 a f ) 2 M ~l + ( f ) 2

(4 )

of Br une ( 1970) i s used , w her e the cor ner f r equency f ~ , these ismic mo m en t M 0 , and the s t res s dr op A a a r e r e la tedt h r o u g h

4 9 x 1 0 , / 0 / 1 ' . ( 5 )Here the sh ear -wave velo ci ty V~ in the source reg ion is takena s 3 . 6 k m / s. T h e d i m i n u t i o n f u n c t i o n D ( f ) is d e f in e d a s

D ( f ) - G exp V s Q ( f ) , (6 )in w hich G r e f e r s to the geomet r ic a t t enua t ion and the o the rte r m to ane las tic a t t enua t ion . I n th i s equa t ion Q i s the qua l i tyf ac tor o f the r eg ion . The h igh- c u t f i l te r in the se i smologicalmod e l i s g iven by[ 1 ' 2P ( f ' f m ) - 1 + --~m (7 )

w her e f m cont r o l s th e h ig h- f r equen cy f a ll o f the spec t r u m.The scal ing factor C is

- ( 8 )4 rcp Vs 3whe re (R0~) is the radiat ion coe ff icient averaged over anappr o pna te r ange of az imuths an d take- of f angles . Th e coef -f ic ien t a /2 in the above equa t ion a ri ses as the pr od uc t o f thef lee- surf ace ampl i f ica t ion an d par t i t ion ing of ener gy inor thogona l d i r ec t ions .

F o l lo w i n g t h e w o r k o f S i n g h e t a l . ( 1999) , the geomet r i -ca l a t t enua t ion te r m G f or the I nd ian sh ie ld r eg ion i s t aken tobe equal to I / R f or R < 100 km and equa l to 1 / ( 10 f iR ) f orR > 100 km. For P I , w e have th r ee s tud ies r epor ted in the l i t -e r a tur e f or f ind ing the Q va lues . Manda l and Ras togi ( 1998)have s tud ied the K oyna- W ar na r eg ion , w hich has a la r ge nu m-ber of ins t r um enta l r ecor d ings , to a r rive a t Q = 1 69 f . 77. Ra oe t a l . ( 1998) used s t r ong- m ot ion r ecor ds of smal l - m agni tu dee a r th q u a k e s a n d e s ti m a t e d Q v a lu e t o b e 4 6 0 f ~ f o r t h esouther n pa r t o f P I, exc luding the K oyna- W ar na r eg ion .

S ingh e t aL ( 1999) de rived Q as 5 0 8 f ~ f r om the br oad-band r ecor ds of the Jaba lpur ea r thquake , f or the w es te r n- cen-tral par ts of PI . The se three regions which m ake u p PI areshow n in F igure 1B. Th us to deve lop an a t tenua t ion r e la tionapplicable for the wh ole of PI , the above quali ty, factors wh ichbroadly represent three dif ferent regions within PI have to beconsidered.

The se i smologica l mode l i s implemented in the t imedom ain in each reg ion by us ing the meth od o f Boor e ( 1983 ,2003) . Th e s imula t ion pr ocedur e es sen t ial ly cons is t s o f th rees teps . F irs t , a Gaussian s tat ionary random process sample ofs t ro n g g r o u n d - m o t i o n d u r a t i o n ( B o or e a n d A t k i n s o n 1 9 8 7 ),

1T = ~ + 0 . 0 5 R , (9 )i s s imula ted . Th e sample i s w in dow ed b y mul t ip ly ing i t w i tht h e m o d u l a t i n g f u n c t i o n o f Sa r ag o n i a n d H a r t ( 1 9 7 4) a n d i sFour ie r - t r ans f or med in to the f r equency domain . The Four ie ramp l i tude spec t r um i s nor mal ized by the squar e roo t o f them e a n s q u a re a m p l i t u d e s p e c t r u m a n d m u l t i p li e d b y t h e t a r -ge t spec t r um of Equa t io n 3 , de r ived f rom the se i smologicalmode l . This i s t r ans f or med back in to the t ime domain togener a te a s ample of acce le r a t ion t ime h i s tor y . This w ay anensem ble of accelerat ion t im e his tor ies is s imulated .

The PG A samples ob ta ined a r e based on a g iven se t o fmode l pa r amete r s , w hich a r e themse lves uncer ta in . Thus thesample PG A va lues f r om a su i te o f such s imula t ions ma y s t il lno t r e flect a ll the va r iab i l i ty obse r ved in r ea l g r ou nd mot io n .To accoun t f or th i s , the f ou r mo de l pa r amete r s , nam ely s tr es sdrop, focal depth, fm, and the rad iat ion coeff icient , are t reatedas r andom var iab les , d i s t r ibu ted un i f or mly about a meanva lue . The s tr es s dr op i s t aken to va r y be tw een 100 - 300 bar s(Singh e t a l . , 1999) . From Table 1, i t is seen that the focaldepth in P I has va r ied in the pas t a r ound 10 km. W i th th i s inmind , the f oca l dep th i s t aken as a un i f or m r andom var iab lein the r ange 5- 1 5 km. T he cu t - of f f r equency , based on pas tSMA da ta , i s t aken in the in te r va l 20- 50 H z . The r ange ofthe S - w ave r ad ia t ion coef f ic ien t is t aken as 0 .48- 0 .64 ( Boor ea n d B o a t w r ig h t , 1 9 8 4 ) . I t m a y b e n o t e d h e r e t h a t H w a n g a n dH uo ( 1997) a l so cons ider ed uncer ta in t ies in mode l pa r ame-te r s f or de r iv ing synthe t ic a t t enua t ion r e la t ionsh ips f or thecent r a l and eas te r n U ni ted S ta tes ( CEU S) . PG A va lues a r es i m u l at e d fo r m o m e n t m a g n i t u d e s r a n g i n g f r o m 4 t o 8 w i t han inc r em ent o f 0 .5 un i t s . Epicent r a l d i s tance is va ried inintervals of In( r ) = 0.13, w here r s tands for the epice ntral dis-tance. The r and om v ibr a t ion code w r i t t en by Boor e ( 1996) i sused f or gener a t ing the synthe t ic da ta . The magni tude andr ange of ep icen t r a l d i s tance chosen in each case a r e show n inTable 4 . A low er l imi t on the ep icen t r a l d i s tance i s imposeds ince the se i smologica l mode l uses a po in t - sour ce as su mp t ion(Kriniztsky e t a l . , 1 9 9 3 ) . T h e n u m b e r o f d i s ta n c e s a m p le scons ider ed f or each ma gni tude i s a lso show n in Table 4 . I n a llthere are 101 pairs of magn itud es an d dis tances . For eachmagn i tude , 100 se t s o f se i smic pa r amete r s a r e gener a ted ,

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TABLE 4Ran ges of Epicen tral Distance

M o m e n t M a g n i tu d e Epicentral Distance( k m )N u m b e r o fDistanceSamples

4 1-300 204.5 1-300 205.0 5-300 145.5 15-300 106.0 25-300 96.5 35-300 87.0 40-300 77.5 45-300 78.0 60-300 6

wh ere each se t o f se ismic p a ram ete r s d e f in es a sy n th e t i c ea r t h -q u ak e . Th u s , t h e d a t ab ase co n s i s t s o f 1 0 ,1 0 0 P GA samp lesf ro m 9 0 0 ea r t h q u ak es . Th i s sy n th e t i c d a t ab ase is g en era t edsep ara t e ly fo r t h e Ko y n a-Warn a , wes t e rn -cen t r a l , an d so u th -ern reg ions o f PI using th ei r respect ive qual i ty factors .ATTENUATIONA rev i ew o f so me o f th e g en era l a t t en u a t i o n r e l a ti o n s an d t h eme th o d s ap p ro p r i a t e fo r r eg res s io n an a ly s i s h av e b eenrecen t l y p resen t ed b y Camp b e l l ( 2 0 0 3 ) . F o r P I , a mo d e l i sp ro p o sed t h a t acco u n t s fo r g eo met r i ca l sp read in g an d an e l as -t i c a t t en u a t i o n s imi l a r t o t h e eq u a t i o n s d e r i v ed fo r eas t e rnN o r t h A m e r i c a ( E N A ) a n d o t h e r i n t r a p la t e r e g io n s. T h e c h o -sen a t t en u a t i o n r e l a t i o n h as t h e fo rmln y = 6 " 1 + 6 " 2 ( M - 6) + c 3(M- 6 ) 2 - l n R - c 4R + l n e (10)w h e r e y , M , a n d R r e f er to P G A ( g ) , m o m e n t m a g n i t u d e , a n dh y p o ce n t r a l d i s t an ce , r e spec t iv e ly. S in ce P GA i s k n o w n to b ed i s t r i b u t ed n ear ly as a l o g n o rmal r an d o m v ar i abl e , I n y wo u ldb e n o rm al ly d i s t r i b u t ed wi th t h e av erag e o f l n/3 b e in g a lmo s tzero . Hen ce w i th ~3 = 1 , Eq uat ion 10 represen ts a 50 percen-t i l e o r med ian - l ev e l h azard es t imat i o n fo rmu la fo r P GA. Atwo-step s t rat i f ied regress ion analysis o r ig inal ly p roposed byJo y n er an d Bo o re (1 9 8 1 ) i s ca r r i ed o u t o n t h e g en era t ed sy n -th e t i c d a t a to o b t a in t h e p a ramete r s o f t h e a t t en u a t i o n eq u a-t i o n fo r b ed ro ck co n d i t i o n (V s = 3 .6 km /s) asK o y n a - W a r n a R e g i o n :

c = 1 .7615; c2 = 0 .93 25; c3 = - 0 . 0 7 0 6 ; c4 = 0 .0 0 8 6 ;o '0 n e) = 0 .3 2 9 2 ; (11 )

Weste rn -cen t r a l Reg io n :6"1 = 1 .7 2 3 6 ; c2 = 0 .9 4 5 3 ; c3 = - 0 . 0 7 4 0 ; c4 = 0 .0 0 6 4 ;o ' ( lnE) = 0 .3439; and (12)

S o u t h e r n R e g i o n :c I = 1 .7 8 1 6 ; c2 = 0 .9 2 0 5 ; c3 = - 0 . 0 6 7 3 ; c4 = 0 .0 0 3 5 ;o ( l ne ) = 0 . 3 1 3 6 . (13)

Ty p ica l a t t en u a t i o n cu rv es fo r M w 6 a re p resen t ed i n F ig u re 2fo r a l l t h r ee su b reg io n s o f P I . M an d a l a n d R as to g i (1 9 9 8 )h a v e p r e v i o u s l y o b s e r v e d t h a t a t t e n u a t i o n i n K o y n a - W a r n areg ion i s fas ter , s imi lar to what i s expected in a tecton ical lyac t iv e r eg io n . Th e so u th e rn r eg io n sh o ws sl o w a t t en u a t i o n , a si n a s t ab l e co n t i n en t a l r eg io n . Th e wes t e rn -cen t r a l r eg io nap p ear s t o l i e b e tween t h ese two p a t t e rn s . I n t h e p resen ti n v es t i g a t i o n P I h as b een r ep resen t ed b y t h r ee r eg io n s wi thd i f feren t Q factors . Equ at ion s 11 , 12 , and 13 , fo r al l p ract icalp u rp o ses , a r e v e ry s imil a r , ho wev er . Hen c e , i t wo u ld b e co n -v en i en t a n d a l so f a i r ly accu ra te t o h av e a s i n g le co m p o s i t e fo r -m u l a f o r P I . T h e t h r e e r e g i o n s ~ K o y n a - W a r n a ( 8 4 , 9 4 8 k m 2 ) ,so u th ern r eg io n (3 ,3 9 ,7 9 3 k m2 ) , an d wes t e rn -cen t r a l r eg io n( 4 , 2 4 , 7 4 2 k m 2 ) ~ c o v e r P I n ea r l y i n th e r a ti o 1 : 4 :5 . W i t h t h isi n min d , f l e sh P GA samp les h av e b een se l ec t ed f ro m th e t h r eereg io n a l p o p u l a t i o n s i n t h e ab o v e r a t i o t o c r ea t e a n ew sy n -th e t i c d a t ab ase fo r P I . Th i s co n t a in s 1 0 ,1 0 0 samp les as b e fo re ,co v er in g t h e same mag n i tu d e an d d i s t an ce r an g es . F o r t h ep ro p o sed a t t e n u a t i o n r e l a t i o n o f Eq u a t i o n 1 0 , t h e fo ll o win gp aram ete r s a r e o b t a in ed b y s tr a t if i ed r eg res s io n fo r p en in su l a rInd ia:6" = 1 .6 8 5 8 ; c2 = 0 .9 2 4 1 ; c3 = - 0 . 0 7 6 0 ; c4 = 0 .0 0 5 7 ;

a n d o ' 0 n e ) = 0 . 4 6 4 8 . (14)Th e ab o v e a t t en u a t i o n r e l a t io n i s v a l i d fo r h a rd ro ck ex p o sedo n t h e su r f ace , w i th V n ear ly eq u a l t o 3 .6 k m / s . F o r o th e r s i teco n d i t i o n s t h e r esu l t s o f t h e ab o v e eq u a t i o n wi l l h av e to b eco r r ec t ed as d iscu ssed b e lo w.

VALIDATIONI t h as a l r ead y b een p o in t ed o u t t h a t t h e av a i l ab l e S M A d a t a i nP I d o n o t co v er a l l r e l ev an t mag n i tu d e an d d i s t an ce r an g es .Hen ce , i t wo u ld b e r e l ev an t t o see h o w th e ab o v e sy n th e t i ca t t en u a t i o n r e l a t i o n match es wi th av a i l ab l e o b se rv a t i o n s . I nTab l e 2 , t h e p red i c t i o n s o f Eq u a t i o n s 1 0 an d 1 1 a r e co m p are dw i t h o b s e r v e d P G A d a t a o n h a r d r o c k i n t h e K o y n a - W a r n areg io n . S imi l a r l y i n Tab l e 3 , so me co mp a r i so n s a r e p resen t edfo r t h e wes t e rn -cen t r a l r eg io n. In b o th t h ese cases t h e s i te s a r eh ard ro c k wi th h ig h V v a lues an d h en ce n o c o r r ec t i o n s a r eap p l i ed .

A n o t h e r c o m p a r i s o n o f t h e n e w a t t e n u a t i o n r e l a ti o n isp r e s e n t e d i n F i g ur e 3 f o r t h e K u t c h e a r t h q u a k e o f 2 6 J a n u a r y2 0 0 1 . No i n s t ru men ta l o b se rv a t i o n s a r e av a i l ab l e fo r t h i sev en t i n t h e n e ar - so u rce r eg io n ex cep t fo r a f ew r ead in g s f ro msp ec t r a l r e sp o n se r eco rd er s (S RR) . Th ese r eco rd er s g iv e t h ep eak r esp o n se o f a s imp le h a rm o n ic o sc i ll a to r , t u n e d t o a p a r -t i c u la r f r e q u e n c y a n d d a m p i n g v a l u e . I n t h e a b s e n c e o f d ir e c tS M A reco rd in g s , t h e ab o v e d a t a can b e u sed fo r e s t imat i n gP G A . T h i s p r o v i d e s a n o p p o r t u n i t y t o v e r if y t h e n e w a t t e n u -

Seismological Research Letters July/Au gust2 004 Volum e75,Number4 535


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1 0 - ~

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, . : : : : : : ~oyn a- wa r nai i ! ~ s ~1 76

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i i ! i i i " i \i i i i i ! \

i i i ! : : : : \~ ~ ~ ~ ~ : ~ ~ ~ ~ ~ ~ ! ~ : ~ ~. . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . .. . i . i . . i . ; . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . .. . i . . ! . . i . i . . . . . . . . . - ~.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ; . . i . . : . ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . i . . ; . ; . . . . . . . . . \ . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . i i i i . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i i ! i . . . . . . . . . . . . . \ ~ ~ ~ ~ ~ ~ ~

. . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . , . , . . . . : . : . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . , . ~ . . . . . . . . . . . . . .

9 i i i ! . . . . . \i i i i i i ! i i !i i \

. . . . . . . . . . . . . . . . . . . .

10 ~ 10 ~ 10 2E p icent r a l D ist ance ( km)

A Figu re 2. Attenuation in peninsular India: M,, 6. ED = 10 km.- 1 - 1 [ 1 I ,1 0 0 , " 1 ; . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . [ ] P G A ( S R R ) ~ -

" ~ , ) . :- ~- . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . . . ! . . . . . . . . . . . . . . . - ~ P G A ( C r a m e r & K u m a r ( 2 0 0 3 ) ) I -" t J L i l . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . . . . - W e s t e r n - C e n t r a l R e g i o n I" : . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . ! . . . . . . . . . . . . . . . . P I I -

. . . . . . . . i ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ! . . . . . . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . ! . . . . . . . . . . . . . . . . . .

. _ . _ . . . . . . . . . . . . ; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

\ , ,

.( .9 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . : . . . . . . . . . . . . ~ - - - " . . . . . . . . . . . . . . . . .

[]

. . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ~ . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . .

1 0 - 2 ~ so lOO 1so 200 250 300E picent r a l D ist ance ( km)

,A, Fig ur e 3. Analytical attenuation relation and PGA values of Kutch earthquake (B-C site condition).

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T ABL E 5PGA Es t im a te s f ro m SR R Da ta (Ku tc h Re g io n )

StationEpicentra lDis tance( k m )

77 = 0.0 5T = 0 . 4 s T = 0 . 7 5 sS R R S R RS,(g) S, (O)

7 7 = 0 . 1T = 0 . 4 sS R Rs, (g)

T = 0 . 7 5 sS R RSa(g)PGA ( g PGA (g ) B -C(Eq s . 1 5 , S i te (Cra m e r1 6 ) a n d Ku m a r ) PGA (g) B-Csite (authors)

An ar 44 1.62 0.70 0.91Kandla 53 0.86 0.57 0.65Niruna 97 0.81 0.65 0.76Naliya 147 0.72 0.22 0.69Khambal ya 150 O.18 0.07 0.09Jamj odhpu 166 0.22 O.15 O.14Dwaraka 188 0.21 ~ 0.18Po bander 206 O.19 0.25 O.15Junagarh 216 O.19 0.06 O.10Amrel i 225 0.09 0.07 0.07Ahmedabad 238 0.29 0.23 0.24Cambay 266 0.49 0.04 0.29Anand 288 0.14 0.06 0.12

0.69 0.62 0 .58 0.550.53 0.41 0.33 0.380.55 0.44 0 .24 0.330.21 0.40 0.22 0.290.04 0.06 0 .07 0.060.05 0.07 0 .09 0.070.11 0.11 0 .06 0.080.21 0.10 0 .05 0.070.05 0.07 0 .08 0.070.05 0.05 0 .04 0.050.19 0.15 0 .08 0.100.04 0.19 0 .14 0.140.05 0.08 0 .04 0.06

a t ion re l a t ion fo r a s t rong ea r thq uake in P I . In T ab le 5 theSRR va lues a t t h i r t e en s t a t ions a s repo r t ed by the Un ive rs i tyo f R oorkee (2001) a re s hown . T o conv e r t t he SRR va lues in toPGA values , f i rs t a mult ivaria te regress ion between recordedPG A va lues o f 240 s ample s t ro ng-m ot ion acce le rog rams o ft h e P E E R ( h t t p : / /www.peer .be rk e tey .edu) global da tabase andthe i r co r re s pond ing t rue S a ( r , T n ) is per fo rm ed . H er e ,.,ca isthe spectra l accelera t ion , 77 is the viscous dam pin g coeff ic ient ,a n d T , i s the na tura l period of the osc i l la tor . The regress ionequa t ion i s o f the fo rm

In(PGA) = a I + a 2 I n [ S , ( 0 . 0 5 , 0 . 4 ) ] + a , 1 n [ S , ( 0 . 0 5 , 0 . 7 5 ) ] (15 )+a4 In [ Sa (0.1, 0 .4)1 + a4 In [ Sa (0.1, 0 .75)1 + I n &

For the g loba l da ta one f indsa 1 = - 0 . 5 1 5 8 ; a 2 = 0 . 2 5 ; a 3 = - 0 . 2 4 8 8 ; a 4 = 0 .8 5 8 6 ;a 5 = 0 . 2922 ; a nd o ( lnb ') = 0 . 3 42 9 . (16 )T he e s t ima ted PGA va lues ob ta ined fo r the Ku tch ea r th -quake by the above mul t iva r i a t e reg re s s ion a re repor t ed inT ab le 5 . P rev ious ly Kumar e t a l l . (2001) e s t ima ted the PGAva lues us ing o n ly S a (0 . 05 , 0 . 4 ) da ta . T he e s t ima ted PG A va l -ue s re fe r to reco rde rs on d i f fe ren t s o i l cond i t ions and hencecanno t ye t be compared d i rec t ly w i th the de r ived a t t enua t ionre la tion . T o c i rcum ven t th i s d i f fi cu lty , fo llowing C ram er a ndKumar (2003) the s o i l cond i t ions a t t he th i r t e en s t a t ions a rec lass if ied as B-C , C, a nd D s i tes (BSSC, 200 1), an d PG A val-ue s a re e s t ima ted fo r f i rm rock (B-C) cond i t ions us ing the

coe f fi c ien t s g iven by J oyne r and Boore (2000) . T h i s s e t o fa d j u s te d P G A v a lu e s o f th e K u t c h e v e n t w i th M w 7.7 is alsos h o w n i n T a b le 5 . I n F i g u r e 3, a t t e n u a t i o n o f P G A d u r i n g t h ee v e n t, a s e s t im a t e d b y C r a m e r a n d K u m a r ( 2 0 0 3 ) a n d E q u a -t ion 15 , i s compared w i th the ana ly t i c a l r e s u l t s o f E qua t ions10 and 12 , a s co r rec ted fo r B -C type s o i l cond i t ion . I t i sobs e rved tha t t he empi r i ca l equa t ion i s ab le to e s t ima te thePG A va lues rea s onab ly we l l.D I SCUSS I ONIn the abs ence o f s u f f i c ien t pa s t r e co rded da ta , t he us e o f ana -ly t i c a l mode l s i s t he nex t be s t app roach to e s t ima te s t rongground mot ion . Bes ide s deve lop ing a mode l a t t enua t ion re l a -t ion fo r P I , s ome l imi t ed va l ida t ion o f the mo de l ha s a l sobeen p re s en ted he re . I t i s s een tha t fo r the K oyna ea r thq uakeo f 1 1 D e c e m b e r 1 9 6 7 , t h e p r e s e n t e s t i m a t e m a t c h e s w el l w i t hthe reco rded va lue . In T ab le 2 , t he mean and s t anda rd dev ia -t ions o f the d if fe rences be tween the e s t ima ted an d obs e rve dva lues a r e - 0 . 02 3 and 0 . 025 , r e spec tively . In T ab le 3 , it isfound tha t t he mean e r ro r i s nea r ly ze ro , w i th the s t anda rdd e v i a ti o n b e i n g 0 . 0 2 7 . F o r t h e 5 . 8 m a g n i t u d e J a b a l p u r e a r t h -quake , howeve r , t he p red ic t ed va lue s a t l ong d i s t ances a relower than the reco rded va lue s . T h i s pe rhaps can be a t t r ib -u ted to the h igh s t re ss d rop o f 420 ba rs a s soc ia ted w i th th i seven t (S ingh e t a l . , 1 9 9 9 ) . T h e c o m p a r i s o n o f t h e K u t c he a r t h q u a k e P G A d a t a w i t h t h e m o d e l a t t e n u a t i o n i s a l s ofavorab le , w i th s even obs e rva t ions ly ing in the mean p lus /minus s t anda rd dev ia t ion in t e rva l .

I t i s qu i t e we l l know n tha t the re can be cons ide rab le va r i -a t ion in local s it e cond i t ions , and hence the s u r face - leve l PG A

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va lue can be d i f fe ren t f rom the expos ed bed rock va lue . T oaccount for th is i t i s expedient to adopt a c lass i f ica t ion schemeas pe r NE HRP (B SSC, 2001) , wh ich g roups s i t e s ba s ed onthe average shear-w ave veloci ty (1/30) in the top 30 meters .Fo l lowing th i s app roach , co r rec t ion fac to r s to E qua t ions10- 14 have been fo und fo r A , B , and B -C type s it es . T odescribe typica l s i te ve loci ty profi les , the examples given byH w a n g e t a l . (199 7) for A (1 .5 < V30 < 3 .6 km /s) and B(0.76 < 1/30 < 1 .5 km /s) s i tes have bee n used. F or f i rm -rocks i tes , which are be tween s i tes C and B, denoted as B-C types ,the s ample p ro f i l e o f F ranke l e t a l. ( 1 9 9 6 ) w i t hV30 = 0 . 76 k m /s ha s be en us ed . S i t e -depen den t PG A va lues Yshave been again s im ula te d for these soil profi les , us ing the s to-chas t i c s e i s molog ica l mode l o f Boore ( 1 9 9 6 , 2 0 0 3 ) . F r o mthese s imula ted resul ts , a re la t ion between Ys a n d y o f E q u a -t ion 10 a t t he med ian l eve l c an be expre s s ed in the fo rmYs = yFs . T he co r rec t ion coe f f i c i en t F s i s found to be 1 . 429 fo rA sites, 1 . 6 4 3 for B s i tes , and 2 .297 for B-C s i tes in PI . Thes tanda rd e r ro r in the a t t en ua t ion inco rpo ra t ing the s i te e ffec tsw o u l d b e o ' ( l n eA) = 0 .465, o ' ( lneB) = 0 . 467 , andcr(lneBc) = 0 .4 69 .For o the r types o f s i te cond i t ions , s uch a s C , D , an d E ,further s i te -specif ic soi l amplif ica t ion s tudies are needed toa r r ive a t s u r face - l eve l PGA va lues f rom the a t t enua t ion fo r -mu la de r ived he re. In F igu re 3 , r e su l ts o f the com pos i t e a t t en -ua t ion re l a t ion o f P I co r rec ted fo r f irm-roc k (B-C s it e s)

cond i t ions a re s hown fo r an even t w i th M w 7 . 7 . I t i s s een tha tthe ana ly ti c a l e st ima te s com pare favorab ly w i th the ob s e rvedPGA va lues o f the Ku tch ea r thquak e , a s exp la ined p rev ious ly .

T h e e a st e rn p a r t o f N o r t h A m e r i c a ( E N A ) a n d P I s h a r es imilar i t ies in tha t both are in t rapla te regions . With th is inm i n d , a c o m p a r i s o n o f t h e p r e s e n t f o r m u l a w i t h a f ew o t h e rs uch re l a t ions fo r E NA (Atk ins on and Boore , 1995 ; Hwangand Huo , 1997 ; T oro e t a l . , 1997) i s s hown in F igu re 4 . In thes ame figu re , the a t t enu a t ion re l a t ions h ip de r ived f rom w or ld -wide in t rap la t e ea r thquake da ta (Dah le e t a l . , 1 9 9 0 ) is alsoshown. I t i s seen tha t a l l the resul ts a re qual i ta t ive ly s imilarbut show some sca t ter . I t i s in teres t ing to observe tha t thede r ived a t t enua t ion re l a t ions h ip fo r P I ma tches we l l w i thE N A g r o u n d - m o t i o n r e l a t i o n s . S u c h a c o m p a r i s o n b e t w e e nK u t c h P G A d a t a a n d E N A a t t e n u a t i o n w a s al so o b s e r v ed b yC r a m e r a n d K u m a r ( 2 0 0 3 ) . I n v i e w o f th e a b o v e fa v o ra b l epo in t s , it is p ropos e d tha t E qu a t ions 10 and 14 , s u i t ab ly co r -rec ted fo r A , B , and B-C type s i te s , may be us ed fo r e s t ima t -ing PGA in PI un t i l more in s t rumen ta l da ta a re reco rded .SUMMARYOn e o f the key pa ram e te r s eng inee rs look fo r i s t he PG A a t achosen s i te for the des ign-bas is ear thquake. This is region-s peci fi c and hen ce eve ry e f fo r t shou ld be made to und e rs t an d

. . .. . .. . .. . . : . . .. . . : . . .. : : : i i i i . . .. . .. . .. . : . . .. . . : . . .. : + H w a n g & H u o ( 1 9 9 7 )" ~ " ! ! " i ! ! ! . . . . . . . . . . . ! . . . . . . ! . . . . ! - - - - x -- A tk in s o n & B o o r e ( 1 9 9 5 )

- ' ~ - . ~ ~ . ! . ! . .. . . .. .. . . ! . .. .. . ! . .. . ! - . - - - - D a h l e ( 1 9 9 0 )i . . . . . . . . . . : _ _ _ ~ ~ ~ . . . . - - 0 - T o r o e t a l ( 1 99 7 )~ ~ ; . . .. . ; . . . ! : . : i ~ ' x , , x , . . ." : . . . . . . . . . . ! . . . .. ! . . . ! - e - P I

. . . . . . i . . . . . . . . . . . . . .

. . . . .

1 0 - 1

I1.

1 0 - 2

1 0 ~ 1 0 1 1 0 2E p i c e n t r a l D i s t a n c e ( k m )

A Figu re 4. Com parison of SCR attenuation curves: Mw 6, F .D = 10 k in.

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h o w p e a k g r o u n d p a r a m e t e r s a t t e n u a t e w i t h d i s t a n c e f o rIndian earthquakes . At present no specif ic re la t ion is avai lablefo r eng inee rs to u s e in P I , wh ich i s t he in t rap la t e reg ion s ou tho f 24~ l a t i tude . T he s t rong -mo t ion da ta ava i lab le fo r P I a reve ry s pars e and a re conf ined to a l imi t ed reg ion . Fo r the deva -s t a ti n g K u t c h e a r t h q u a k e o f 2 6 J a n u a r y 2 0 0 1 o n l y S R R d a t aare avai lable , which a t bes t can help in s ta t is tica l ly es t imat in gPG A va lues. Add i t iona lly , t he re have been ma jo r ea r thquakesin other regions of PI for which no records are avai lable .Un de r s uch c i rcum s tances , t he s tochas t i c mod e l o f Boore(1983) to gene ra te PGA va lues s yn the t i ca l ly , i nco rpora t ingunce r t a in t i e s in the s e i s molog ical pa rame te r s o f the reg ion , i sa v i ab le a l te rna t ive . T he mo de l c an inco rpo ra te inhom ogene -i ty in the qua l i ty fac to r s and unce r t a in ty in o the r mode lpa rame te r s .

He re PI i s d iv ided in to th ree pa r t s , name ly the Koyna -Warna , wes te rn -cen t ra l , and s ou the rn reg ions , fo r wh ichqual i ty fac tors are available in the l i te ra ture . Syn thet ic gro undm ot ion i s gene ra ted s epa ra tely fo r the th ree reg ions , consi s t-ing o f 10 , 100 s ample s cove r ing the m agn i tu de range o f 4 to8 , fo r bed ro ck (V = 3 . 6 km/s ) s it e cond i t ions . Regre ss ionanalys is is carr ied out on the synthet ic da ta to derive newreg iona l a t t enua t ion re l a t ions h ips . An a t t enua t ion fo rmulaval id for PI in genera l has been proposed. Si te-correc t ion fac-tors for h ard -ro ck (1 .5 < 1/30 < 3 .6 km /s) , rock (0 .76 < 1/30 3.0)in Peninsular India, 1993 , Atomic E nergy Regulatory Board, Tech-nical Document No. TD/CSE-1, 70 pp.Guha , S. K., P. D. Gosari, M. M . Varm a, S. D. Agarw al, J. G. Padale,and S. C. Marwadi (1970). Recent Seismic Disturbances in theShivajisagar Lake Area o f he Koyna HydroelectricProject, Ma harash -tra, India, Report, Central Water and Pow er Research Stat ion,Poona-24, India.Gup ta, I. D., R . G. Joshi, and V. Ram babu (1992). Analysis o f Some Sig-nificant Accelerograms of Koyna Dam Earthquakes Using ImprovedDa ta Processing Techniques,Technical Memorandu m, Ministry ofWa ter Resources, Central Water and Powe r Research Station.Hwang, H. and J.-R. Huo (1997). Attenuation relat ions of groundmo tion for rock and soil sites in eastern Un ited S tates, So i l Dynam-ics an d Earthq uake Engineering16, 363-372.Hwa ng, H ., H . L in, and J.-R. H uo (1997). Site coefficients for designof buildings in eastern United States, So i l Dynamics and Earth-quake Engineering 16, 29-40.Joyner, W. B. and D. M. Boore (1981). Peak horizontal acceleration andvelocity from strong motion records including records from the1979, I mp erial valley, California earthqu ake, Bulletin ofthe Seismo-logical Society o fAmerica 71, 2,011-2,038.Joyner, W. B. and D. M . Bo ore (2000). R ecent developments in earth-quake ground motion estimation, Proceedings of the Sixt h Interna-

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tional Conference on Se ismic Zona tion, N o v e m b e r 1 2 - 1 5 , 2 0 0 0 ,Pa lm Spr ings , Oak land , CA: EERI .Kaila, K. L. and D. Sarkar (1978). Atlas of isoseismal maps o f majorear thquakes in Ind ia , GeophysicalResearch Bulle tin 16 , 233-267 .Kriniztsky, E. L. , J . P. Gould, and E H. Edinger (1993). Fundamentalsof Earthquake-resistant Construction, New York : Wiley .Kum ar, A. , S. Basu, S. K. Thakk ar, M . S hrikhan de, P. Agarwal, J . Das,and D . K . Pau l (2001). S t rong m ot ion records o f Bhuj ea r thquake ,In te rna t iona l Conference on Se ism ic Hazard w i th Par t icu la r Ref-e rence to Bhuj Ear thquake , New Delh i , Oc tober 3 -5 , 2001 ,1 0 p p .Mand a l , E an d B . K . Rastog i (1998). A f requency-dep enden t re la t ion o fcoda Q , fo r Koyna-Warna reg ion , Ind ia , Pure andApp lied Geophys-ics 1 5 3 , 1 6 3 - 1 7 7 .Narula, P. L. , S. K. Acharyya, and J. Banerjee (2000). SeismotectonicAtlas o f India a nd Its Environs, Geolog ica l Survey o f Ind ia .Rao, B. R. and E S. Rao (1984). H istorical seismicity of peninsularInd ia , Bulle tin of the Seismological Society of Americ a 74 ,2 , 5 1 9 - 2 , 5 3 3 .Rao , R . , C . V . Sesham m a, and E Mand a l (1998) . Estimation o f Coda 0rand Spectral Characteristics of Some Modera te Earthquakes o f South-ern Indian P eninsula, unpubl ished repor t .Rastogi, B. K. , R. K. Chadka , and C. S. P. Sarm a (1995 ). Investigationsof June 7 , 1988 ear thquake o f m ag ni tude 4 .5 near Idukk i Da m insou thern Ind ia , Pure andAp plied Geophysics145 , 109-119 .

Saragoni, G. R. and G . C. Ha rt (19 74). Sim ulation o f artif icial earth-quakes, Earthquak e Engineering and Structural Dynam ics 2,2 4 9 - 2 6 7 .Seeber, L. , J . G. Armbruster, and K. H. Jacob (1999). ProbabilisticAssessment of Seismic Hazard for Maharashtra, G o v e r n m e n t o fMaharash t ra , unpubl ished repor t .Singh, S. K. , B. K. Bansal, S. N. Bhattacha rya, J . F. Pacheco, R. S. Dat-tatrayam, M. Ordaz, G. Suresh, Kamal, and S. E. Hough (2003).Es t im at ion o f g round m o t ion fo r Bhuj (26 th January 2001 ,Mw =7.6) and fo r fu tu re ea r thquakes in Ind ia , Bulletin o f the Seis-mological Society ofAmerica 9 3 , 3 5 3 - 3 7 0 .Singh, S. K. , M. Ordaz, R. S. Dattatrayam, and H. K. Gupta (1999). Aspectral analysis of the 21 M ay 1 997, Jabalpur, India, earthqu ake(M w = 5 .8 ) and es t im at ion o f g round m ot io n f rom fu tu re ea r th -quakes in the Indian shield region, Bulle tin o f the SeismologicalSociety ofAmerica 89 , 1 ,620-1 ,630 .Toro , G . , N . Abraham son , and J . Schne ider (1997) . M odel o f s trongground m ot ion in eas te rn and cen t ra l Nor th Am er ica : Bes t es ti -mates and uncertainties, Seismological Research Le tters 6 8 , 4 1 - 5 7 .D e p a r t m e n t o f C i v i l E n g i n ee r i n g

In d ia n In s t i t u t e o f S c i en ceB a n g a lo re 5 6 0 0 1 2

I n d i arn i @ dviL i isc.ernet, in

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