Nonlinear Soil Response— 1994 Northridge, California, Earthquake

Peak ground accelerations recorded during the Northridge, California, earthquake of January 17, 1994 are described in relation to the observed, nonlinear soil response. Four attenuation relationships were developed specifically for this earthquake, for horizontal and vertical motions, and for “soft” and “hard” soil conditions. The results show that, within 20 km from the epicenter, seven (possibly 10) stations with “soft” soil conditions recorded smaller horizontal peak accelerations than expected. No such trend could be detected for the vertical motions. At the “soft” sites, where a reduction of peak horizontal acceleration was observed, the peak strains were larger than 10 −3 . The widespread nonlinear response of soil in the same general area (sliding, liquefaction, cracked pipes, and roads) suggests that the observed reduction of peak accelerations is most probably due to nonlinear soil response.