Long-period (1--10 sec) ground motions recorded in the Los Angeles basin region during the Northridge earthquake show complex waveforms, extended durations and multiple sets of arrivals which cannot be attributed solely to source processes or wave propagation within a plane-layered medium. These features suggest a strong interaction between the propagating seismic wave field and the laterally varying subsurface geologic structure of this region. Recorded motions at a hard rock site in the Santa Monica Mountains (scrs) are smaller by nearly a factor of three in peak velocity compared to recordings at more distant sites in the adjacent, northwest portion of the Los Angeles basin. In addition, although the rock site recording has a relatively simple wave shape, the basin site recordings are dominated by late arriving, large amplitude pulses of energy. We interpret these arrivals to be surface waves, which are generated by body waves interacting with the thickening margin of the basin. A preliminary modeling analysis of these data indicates that a combination of both large-scale (deep basin) and small-scale (shallow micro-basin) structures are needed to explain the observed responses. ¿ American Geophysical Union 1995