Basin structure estimation by waveform modeling: Forward and inverse methods

We introduce a technique for using broadband seismograms recorded from earthquakes at local and regional distances to refine basin structure, For the region outside the basin, we assume a one-dimensional (ID) crustal model and analytical techniques (GRT) to propagate the energy from sources to the basin edge where the motions are then interfaced with a (2D) finite-difference algorithm (Wen and Helmberger, 1996). We parameterize the basin section by isovelocity layers with linear dipping segments between control points. The control point depths are allowed to vary to improve the modeling of waveform data of stations inside the basin. The comparison between data and synthetics is qualified by a fitness function defined by two factors: the timing shift required for best alignment and the correlation coefficient. The procedure was applied to a strong-motion waveform profile across the extended Los Angeles Basin produced by the 1992 Landers, California earthquake to refine the velocity structure using sensitivity testing and forward modeling. Only the correlation coefficient and amplitude were used because absolute timing was unknown. The procedure was extended to a direct waveform inversion by employing a conjugate gradient approach, which uses numerical derivatives. Numerical tests using the new inversion process with synthetic data demonstrate that it is possible to recover a detailed basin structure, if a sufficient amount of high-quality data exists.