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