HR: 0830h
AN: S51E-0086    [PDF]
TI: Seismic Amplitude Variations due to Site and Basin Edge Effects in the Los Angeles Basin
AU: * Husker, A L
EM: uskerhay@moho.ess.ucla.edu
AF: UCLA, Dept of Earth and Space Science Geology Building, Los Angeles, CA 90095 United States
AU: Kohler, M
EM: kohler@ess.ucla.edu
AF: UCLA, Dept of Earth and Space Science Geology Building, Los Angeles, CA 90095 United States
AU: Davis, P M
EM: pdavis@ess.ucla.edu
AF: UCLA, Dept of Earth and Space Science Geology Building, Los Angeles, CA 90095 United States
AB: An empirical model of S-wave amplitude variations throughout the Los Angeles and San Gabriel basins is developed using simple attenuation and scattering theory. The model incorporates geometrical spreading, magnitude, and soil effects at the surface and takes into account basin edge effects. The model is developed using data from the 1997 Los Angeles Basin Passive Seismic Experiment (LABPSE). LABPSE was a passive seismic experiment consisting of 18 seismic stations along a line crossing the Los Angeles and San Gabriel basins recording local, regional and teleseismic earthquakes for nine months. The data consist of spectral amplitudes for S waves from 39 local events. Events are chosen for which Sg is the first S wave arrival and at least half the stations had a good signal-to-noise ratio. The spectral amplitude is the mean of the FFT of a +/- 2 sec window around the beginning of the S arrival. The spectral amplitudes are broken into frequency bands (1-5 Hz, 5-10 Hz) to determine the frequency dependence of the model parameters, however little variation is seen across the bands. Raypaths are determined by ray tracing through the SCEC 3D velocity model. The variable parameters of the model include event amplitudes and Q. Station site terms are determined from the S wave coda; 1/r geometric spreading is assumed. The model was fit to the data using a non-linear least-squares inversion. The fit was not improved by inclusion of double couple fault plane radiation patterns. This may be due to the use of spectral amplitudes above 1 Hz and the more difficult detection of radiation patterns in S waves. Fault plane radiation patterns are not included in the final model. The results indicate that for most stations the model is a good first order approximation of the amplitude variation. However arrivals from a restricted range of azimuths for stations near the basin edge have amplitudes that are several times larger than the model predicts. One interpretation of these results is that the Whittier Fault at the edge of the Los Angeles basin creates a zone of increased amplification for earthquakes along the path of the fault. This effect has subsequently been included in the model.
DE: 7205 Continental crust (1242)
DE: 7212 Earthquake ground motions and engineering
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2003 Fall Meeting