GROUND MOTIONS DURING THE 1994 NORTHRIDGE EARTHQUAKE

The seismological background of the Northridge earthquake on a blind t hrust fault is presented. The extensive data base of recorded ground m otion provided the opportunity to review the effectiveness of some wid ely used attenuation relations for peak horizontal acceleration and ps eudo-spectral velocity. Large pulses of velocity and displacement were noted in some records and the potential implications of these for tal l buildings are noted. Some major damage centres coincided with alluvi al basins. Generation of surface waves within the basins during the ma in shock contributed to increased amplification and duration. Surface waves were detected at the site of the collapse of the I-10 interchang e and in the damaged area of Sherman Oaks using aftershock data. Cedar Hill seismographic station in Tarzana recorded the highest accelerati ons of the earthquake, much higher than motions at nearby stations. Th ese are currently attributed to topographic effects of the hill on whi ch the station is located. Aftershock data from two major arrays found that the low amplitude motions were magnified significantly at the cr est of the hill compared to motions at the base.