PRELIMINARY-ANALYSIS OF LONG-PERIOD BASIN RESPONSE IN THE LOS-ANGELESREGION FROM THE 1994 NORTHRIDGE EARTHQUAKE

Long-period (1-10 sec) ground motions recorded in the Los Angeles basi n region during the Northridge earthquake show complex waveforms, exte nded durations and multiple sets of arrivals which cannot be attribute d solely to source processes or wave propagation within a plane-layere d medium. These features suggest a strong interaction between the prop agating seismic wave field and the laterally varying subsurface geolog ic structure of this region. Recorded motions at a hard rock site in t he Santa Monica Mountains (scrs) are smaller by nearly a factor of thr ee in peak velocity compared to recordings at more distant sites in th e adjacent, northwest portion of the Los Angeles basin. In addition, a lthough the rock site recording has a relatively simple wave shape, th e basin site recordings are dominated by late arriving, large amplitud e pulses of energy. We interpret these arrivals to be surface waves, w hich are generated by body waves interacting with the thickening margi n of the basin. A preliminary modeling analysis of these data indicate s that a combination of both large-scale (deep basin) and small-scale (shallow micro-basin) structures are needed to explain the observed re sponses.