Frequency-dependent source processes for the 1989 Loma Prieta earthquake by complex spectral inversion

Low- and high-frequency wave radiations contain significant information for interpretation of earthquake source physics and it is important to try to clarify the frequency-dependence of the generation of seismic waves over as wide a frequency range as possible. Based on complex spectral inversions proposed by Olson and Anderson (1993) and Cotton and Campillo (1995), we have developed an approach of fitting the complex source spectra with frequency-dependent phase weighting that models both the coherent and stochastic summation of waveforms using empirical Green's functions. We apply this technique to estimate the distribution of slip-velocity intensity for the 1989 Loma Prieta earthquake in the frequency range of 0.4-1Hz (λ =1.0), 1-2Hz (λ =0.5), and 2-4Hz (λ =0.0), where λ is the relative weight of phase and amplitude source spectra in the matrix for the complex spectral inversion. The lower-limit of the available frequency range depends on the noise level of the aftershock records. Ground velocity records for the mainshock and aftershocks as empirical Green's functions were selected at 6 stations. We assumed 36km length by 16km width fault plane and a single time window for rupture, propagating 2.5km/s from the hypocenter. The subfault size was calibrated by strong ground motion simulation using the empirical Green's function method formulated by Irikura (1986). We obtained low-frequency (0.4-1Hz) wave radiations corresponding to two asperities seen in previous time-domain waveform inversions (e.g., Beroza, 1991; Wald et al., 1991), and high-frequency (2-4Hz) wave radiations mainly generated at the breaking points of the asperities. The distributions of slip-velocity intensity indicated a progression of wave generation from the center to edge of each asperity with increasing frequency.