1980bssa pga postscript

8/12/2019 1980bssa Pga Postscript

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B u l l e t i n o f t h e S e i s m o l o g i c a l S o c i e t y o f A m e r i c a , V o l . 70 , N o . 6 , p p . 2 2 9 5 - 22 9 7 , D e c e m b e r 1 98 0

L ET T E RS T O T HE EDI T ORPEAK ACCELERATION FROM STRONG-MOTION RECORDS:A POSTSCRIPT

BY DAVID M. BOORE AND RONALD L. PORCELLAIn a recent paper, Boore et al. 1980) used a regression analysis of strong-motiondata to derive relations giving peak amplitudes as a function of distance to the faultsurface for earthquakes in the three magnitude ranges 5.0 to 5.7, 6.0 to 6.4, and 7.1

to 7.6 the peculiar limits of the ranges were dictated by the magn itudes of theavailable events). The data came from earthquakes before 1976. Additions to t hestrong-motion data set from several recent earthquakes Table 1) have prompted usto test the regression curves of Boore et al. 1980) for peak horizontal accelerationrecorded at the base of small structures {less than 3 stories high) agains t the newdata. The comparison, presented in Figure 1, shows generally good agreementbetween the new data add the predictions of the mean line from the regression

TABLEEARTHQUAKES PROVIDING DATA USED IN THIS PAPER

Date Origin Thne Location Magnitudey /m/d) h :min UTC)79/10/15 23:19 Imperial Valley, California 5.0 ML,PAS)78/08/13 22:54 Santa Barbara, California 5.1 ML, PAS)80/01/24 19:00 Livermorealley, California 5.5 ML,BRK)80/01/27 02:33 Livermorealley, California 5.8 ML,BRK)79/08/06 17:05 Coyote Lake, California 5.9 ML,BRK)79/10/15 23:17 Imperial Valley, California 6.6 ML,PAS)79/02/28 21:27 St. Elias, Alaska 7.1 Ms, NEIS)

analysis. The magnitude 6 data all of which are from the Imperial Valley earth-quake) are in almost perfect agreement with the prediction intervals. The magnitude5 data Figure la) indicate that t he prediction intervals based on the prior data aretoo broad. Dat a from the Horse Canyon, California, ear thquake of 25 Februa ry 1980,not shown for the sake of clarity, substantiate this.

Even with smaller prediction intervals, however, a large unce rta inty still exists inthe prediction of peak acceleration. One factor contributing to the scatter is sug-gested by the data from the Livermore Valley earthquakes. We plotted th e ratio ofpeak accelerations from both events, recorded at the same site without regard tostruc ture size) and corrected for distance, against the mean az imuth from the closestpoints on the rupture surfaces to the recording site Figure 2). Using the same sitesshould eliminate variations due to site effects. The results show a strong dependenceon azimuth and are most easily interpret ed as the result of directivity Boore andJoyner, 1978). A similar dependence is suggested by plots of the residuals about theregression line in Figure 1. The results in Figure 2 are consistent with a simple modelof rupture in which the 1/24 event ruptured to the southeast and the 1/27 eventruptured to the northwest. If the directivity persists at lower frequencies as mightbe seen, e.g., in peak velocities), the ML determined from instruments at Berkeleyazimuth =290 ) might be biased to low and h igh values for the 1/24 and 1/27events, respectively.

2295


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96 LETTERS TO THE EDITOR

M A G N I T U D E 5 . 0 - 5 . 7SM LL STRUCTURES

M A G N IT U D E 6 . 0 - 6 . 4T . . . . . I . . . . . . . . I ' ' ' ' ' q

o 1

. o , : O ' ' P E 'A L V A L L E Y L6O O l ' ' . . . . ' . . . . . . . . . . ' I , . . . . I , , r , r , , , l , , , , , T , , II 0 I 0 0 I I 0 I O O@ D I S T A N C E k m ) ~ D I S T A N C E k m )

M A G N I T U D E Z I - 7.6

Q C ) \ \

I 0 I 0 0~ ) D I S T A N C E k r n )

FIG. 1. Peak horizontal acceleration against closest distance to fault, compared with the meanregression line and 70 and 95 per cent p rediction intervals inner and outer pair, respectively) given byBoore e t a l 1980). Distance range used in regression analysis was determined by the requirement ofsufficient data an d the absence of operational stations tha t did not trigger in the distance range indicated.The range of magnitudes shown at the t o p of each figure are the ranges used in the regression analysis.The magnit ude range of the new data can be obtained from Table 1. The new data are the largest peakacceleration of the two horizontal components at a site, in most cases scaled by one of the authorsR.L.P.) fr om the original records. Locations of rupture surface used in distance calculations were takenfrom ongoing, unpublis hed studies of the source paramet ers by several different workers.

o o

S O M =6 . 0

,4.0 0d

2 .0

O S

o.60 4

0.2 I I I I I I I I I ~ I I I )~ I I 240 80 120 160 200 240 280 :520 360A V E R A G E A Z I M U T H D E G R E E S )

FIG. 2. Ratios of peak acceleration from the 1/27/80 and 1/24/80 Livermore Valley earthquakesrecord ed at the same sites, plott ed against mean azim uth clockwise from north) from source to station.The data were correc ted for distance using a power law with exponent of -0.93 the value of the slope ofthe regression line in Figure la). T heoreti cal curves are based on a simple directivity formula [e.g., Booreand Jo yner , 1978, equat ions 2) and 3)] and assum e unilateral ru ptu re at azimuths of 170 and 314 forthe 1/24 and 1 / 2 7 events, respectively. The azimuths are taken from the fault plane solutions ofCockerham e t a l 1980). Horizontal takeoff of the rays is assumed and the radiation pa tter n is neglected.M is the ratio of rupture velocity to shear-wave velocity.


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L E T T E R S T O T H E E D I T O R 2297Although in general the new data give support to the prediction equations of

Boore et al 1980), their real value lies in providing da ta at close distances; inparticular, for the magnitude 6.0 to 6.4 class, the new data confirm our expectationthat an extrapolation of the mean regression line to shorter distances is inappro-priate. We are currently working on a revision of the regression equations which willinclude these new data. Th e report of this research will include tabulations of thedata used in this note.

A C K N O W L E D G M E N T SW e t h a n k R . A r c h u l e t a , J . B o a t w r i g h t , a n d C . S t e p h e n s f o r i n f o r m a t i o n c o n c e rn i n g t h e l o c a t i o n o f t h eea r t h q u ak e ru p t u re s u r f ace s, an d W . B . J o y n e r an d R . A . P ag e fo r c r i t i c a l r ev i ew s .

R E F E R E N C E SB o o re , D . M . an d W . B . J o y n e r 1 9 78 ). T h e i n f l u en ce o f ru p t u re i n co h e ren ce o n s e i s m i c d i r ec t i v i t y , B u l l .

S e i s m . S o c . A m . 68, 283-300.B o o re , D . M . , W . B . J o y n e r , A . A . O l i v e r , I I I , an d R . A . P ag e 1 9 8 0 ). P ea k a cce l e r a t i o n , v e l o c i t y , an d

d i s p l a c e m e n t f r o m s t r o n g - m o t i o n r e c o rd s , B u l l . S e i s m . S o c . A m . 70, 305-321.C o c k e rh am , R . S ., F . W . L es t e r , an d W . L . E l l s w o r t h 1 9 8 0 ). A p re l i m i n a ry r ep o r t o n t h e L i v e rm o reV a l l e y e a r t h q u a k e s e q u e n c e J a n u a r y 2 4 - F e b r u a r y 2 6, 1 98 0 , U . S . G e o l . S u r v . O p e n - F i l e R e p t . 80 -714.

U.S . GEOLOGICALSURVEY345 M IDDLEFIELD ROADMENLO PAR K, CALIFORNIA94025

M a n u s c r i p t r e c e i v e d J u n e 1 7, 1 98 0

1980bssa pga postscript

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