Rw. Graves, PRELIMINARY-ANALYSIS OF LONG-PERIOD BASIN RESPONSE IN THE LOS-ANGELESREGION FROM THE 1994 NORTHRIDGE EARTHQUAKE, Geophysical research letters, 22(2), 1995, pp. 101-104
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.