Wave propagation from a line source embedded in a fault zone containing densely distributed parallel cracks

We compute the synthetic seismograms of the displacement field radiated from a seismic source embedded in a fault zone. It is revealed from analyses of shear-wave splitting and P-wave polarization anomalies that parallel cracks are densely distributed in a fault zone. Moreover observations of fault zone trapped waves revealed low-velocity and low-Q fault zone structure (e.g. Li et al., 1994, JGR 99). A fault zone is modeled as a low-velocity zone with densely distributed parallel cracks in it. We investigate SH wave propagation in a 2-D elastic medium. A seismic line source is located at the center of the fault zone and its radiation is assumed to be isotropic. Observation stations are located near the center of the fault zone. All the cracks are assumed to have the same length 2a and to be periodically distributed in a zone. Synthetic seismograms are computed by the method introduced by Murai & Yamashita (1998, GJI 134). We use the Ricker wavelet as the source time function. The seismograms show headwave refracted along the cross-fault material contrast, fault zone trapped waves and the waves scattered by cracks. Next, we investigate the amplitude spectra. We calculate the amplitude spectrum for each Ricker wavelet source time function in a time window including the direct wave and the trapped and scattered wave trains. The amplitude spectrum for each Ricker wavelet is normalized by that of each source time function to eliminate the contribution of source spectra. The amplitude spectra show the prominent peaks in relatively low and high wavenumber ranges. The peak in the low-wavenumber range is formed by the waves trapped in the low-velocity zone. The peak wavenumber becomes lower for a low-velocity zone with the larger width. The peak in the high-wavenumber range is formed at ka∼1 by the scattered waves, where k is the wavenumber. If the spectral peak is observable, we can estimate the crack length in a fault zone from the peak frequency in the high frequency range. The amplitude spectra also depend on the crack density. We investigate the amplitudes of the spectral peaks for various crack densities. The spectral peak amplitudes in the low-wavenumber range become larger for the higher crack density and its dependency is heavier for a fault zone with the smaller width. On the other hand, the spectral peak amplitudes at ka∼1 show considerable variation among the spatial distributions of cracks and the observation stations even if the same crack density is assumed and do not obviously depend on the crack density. This means that the spectral peak amplitude in the low-wavenumber range becomes larger relative to that in the high-wavenumber range for the higher crack density. Therefore it will be possible to estimate the crack density by modeling a fault zone to satisfy the observed spectral peak amplitudes in both the low and high wavenumber ranges.