EFFECTS OF THE HANGING WALL AND FOOTWALL ON GROUND MOTIONS RECORDED DURING THE NORTHRIDGE EARTHQUAKE

Systematic differences in ground motion on the hanging wall and footwa ll during the Northridge earthquake are evaluated using empirical data . An empirical model for the hanging-wall effect is developed for the Northridge earthquake. This empirical model results in up to a 50% inc rease in peak horizontal accelerations on the hanging wall over the di stance range of 10 to 20 km relative to the median attenuation for the Northridge earthquake. In contrast, the peak accelerations on the foo twall are not significantly different from the median attenuation over this distance range. Recordings from other reverse events show a simi lar trend of an increase in the peak accelerations on the hanging wall , indicating that this systematic difference in hanging-wall peak acce lerations is likely to be observed in future reverse events.