Ground motions during the 1994 Northridge earthquake

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 I-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.