Ground motions during the 1994 Northridge earthquake W.D. Liam Finn Carlos E. Ventura and Norman D. Schuster Abstract: The seismological background of the Northridge earthquake on a blind thrust fault is presented. The extensive data base of recorded ground motion provided the opportunity to review the effectiveness of some widely used attenuation relations for peak horizontal acceleration and pseudo-spectral velocity. Large pulses of velocity and displacement were noted in some records and the potential implications of these for tall buildings are noted. Some major damage centres coincided with alluvial basins. Generation of surface waves within the basins during the main shock contributed to increased amplification and duration. Surface waves were detected at the site of the collapse of the 1-10 interchange and in the damaged area of Sherman Oaks using aftershock data. Cedar Hill seismographic station in Tarzana recorded the highest accelerations of the earthquake, much higher than motions at nearby stations. These are currently attributed to topographic effects of the hill on which the station is located. Aftershock data from two major arrays found that the low amplitude motions were magnified significantly at the crest of the hill compared to motions at the base. Key words arthquake, thrust fault, ground motion attenuation, fault fling, surface waves, basin effects, topographic effects. RCsurnC Le cadre sismologique du tremblement de terre de Northridge, qui s est produit sur une faille chevauchante, est prCsentC. La base de donnees exhaustives relatives aux mouvements du sol enregistrks durant le sCisme, a permis d exam iner l efficacitk de certaines Cquations d amortissement utilisCes grande Cchelle dans le calcul de l accC1Cr at ion horizontale maximale et de la vitesse pseudospectrale. D importantes impulsions de vitesse et de dkplacement ont CtC observCes dans certains rapports d e sCisme et leurs consCquences possibles pour les bdtiments en hauteur sont discutCes. Certains principaux centres de dommages correspondent des bassins d alluvions. La production d ondes superficielles I interieur d es bassins durant le choc principal a contribuC accroitre l amplification et la duree. Des ondes superficielles ont CtC dCtect Ces au lieu d effondrem ent de 1 Cchangeur 1- 10 ainsi que dans la zone sinistree de Sherman Oaks l aide des donnCes de rCplique sismique. La station sismographique de Cedar Hill, Tarzan a, a enregistre les plus fortes accClCrat ions du tremblement de terre, beaucoup plus ClevCes que les mouvements enregistrCs aux stations plus rapprochCes. Cette situation est attribuable aux effets topographiques de la colline sur laquelle la station est situCe. Les donnees de replique sismique provenant de deux importantes stations complexes indiquent que les mouvements de faible amplitude ont CtC caractCrisCs par u ne plus grand e amplification au som met d e la colline qu au pied de celle-ci. Mots cl is tremblement de terre, faille chevauchante, amortissement des mouvements du sol, ondes superficielles, effets de bassin, effets topographiques. [Traduit par la rCdaction] Introduction The California Northridge earthquake, w = 6.7, occurred on January 17, 1994, at 4:30 a.m. Pacific Standard Time. The epicentre was located about 30 m west-northwest of downtown Los Angeles. The earthquake is the largest of the Received June 21, 1994. Revised manuscript accepted January 5, 1995. W.D.L. Finn C.E. Ventura and N.D. Schuster. Department of Civil Engineering, The University of British Columbia, 2324 Main Mall, Vancouver, BC V6T 124, Canada. Written discussion of this paper is welcomed and will be received by the Editor until August 3 1, 1995 (address inside front cover). significant earthquakes that have occurred in the area since the 197 1 San Fernan do earthq uake, w hich was also of m agni- tude w = 6.7. However, the Northridge earthquake was much more damaging because its epicentre was located in a densely populated area and very strong groun d motions were generated. A powerful impression of the overall seismic threat to Los Angeles is given by Fig. 1 EERI 1994), which shows that potential source zones of major earthquakes blanket metropolitan Los Angeles and the surro unding cities. Signifi- cant historical earthquakes that have occurred at these sources and Northridge 1994. The Northridge earthquake triggered a very large number of strong motion instruments throughout southern California, providing the most extensive strong motion data for any earthquake to date. These data will be reviewed here with Can. J. Civ. Eng. : 300-315 (1995 ). Printe d in Canada Imprim t au Canada C a n . J . C i v . E n g . D o w n l o a d e d f r o m w w w . n r c r e s e a r c h p r e s s . c o m b y C h i n a U n i v e r s i t y o f S c i e n c e a n d T e c h n o l o g y o n 0 6 / 0 6 / 1 3 F o r p e r s o n a l u s e o n l y .
