High-frequency decay of Fourier spectra of strong motion acceleration and nonlinear site response

We examine how the decay of high-frequency amplitudes of Fourier spectra of strong earthquake ground acceleration (described in seismological literature by the slope parameter k) depends on the soil and geological site conditions at the recording stations in the Los Angeles metropolitan area. Then, we introduce a Gedanken numerical experiment, in which we analyze nonlinear wave motion in shallow soil and show that it also leads to similar decay of the high-frequency spectral amplitudes. The results of both of our studies are consistent with the interpretation that the decay of high-frequency Fourier spectra of strong motion acceleration, described by k, occurs at the recording site near the ground surface