A model for the shape of the Fourier amplitude spectrum of acceleration at high frequencies

At high frequencies f the spectrum of S-wave accelerations is characterized by a trend of exponential decay, e (super -pi kappa f) . In our study, the spectral decay parameter kappa increases gradually as the epicentral distance increases. For multiple recordings of the San Fernando earthquake, kappa increases slowly with distance, and kappa is smaller for sites on rock than for sites on alluvium. Under the assumption that the Fourier spectrum of acceleration at the source is constant above the corner frequency (an omega (super -2) source model), the exponential decay is consistent with an attenuation model in which Q increases rapidly with depth in the shallow crustal layers.--Modified journal abstract.