SITE RESPONSE MODELING BY NONLINEAR WAVE-FORM INVERSION

Microearthquake waveforms and spectra are well known to be strongly in fluenced by the shallow subsurface conditions at the recording site. W e make use of this fact to determine optimum site models from SH-wave microearthquake seismograms using a non-linear waveform inversion tech nique based on simulated annealing. Site models are optimized in terms of thicknesses, S-wave velocities, and densities of the uppermost lay ers (< 100 m) for 1D plane layer models. In addition, the source param eters in terms of Brune model source time functions are optimized simu ltaneously The synthetic seismograms are calculated using a modificati on of Haskell's plane SH-wave algorithm. Using aftershock recordings o f the Loma Prieta earthquake, a 1D plane layer model has been obtained for a selected site in Central California which has been occupied dur ing the aftershock experiment. For this dataset, cross-correlation coe fficients between optimized synthetics and observed records reach valu es above 0.8. The site model at the selected station for varying sourc es is fairly stable, especially for the uppermost layers. We observe s ystematic variations of layer parameters with back-azimuth which would be in agreement with slightly dipping layer interfaces.