Isotropic high-frequency radiation in near-fault seismic data

We systematically compare Fourier Amplitude Spectra of Fault Normal (FN) and Fault Parallel (FP) seismograms at near-fault sites for 11 strike-slip earthquakes with magnitudes M≥6. For all events we find large FN/FP ratios at low frequencies consistent with the S-wave radiation patterns for strike-slip earthquakes near the faults. However, the difference diminishes with increasing frequency and FN/FP is about 1 above a transition frequency. The results may reflect small-scale isotropic source terms in the earthquake rupture zones that homogenize the high-frequency radiation in different directions at near-fault sites. The FN/FP ratios at low frequencies and transition frequencies above which FN ~ FP vary among the 11 earthquakes, and have no clear correlation with the moment magnitudes. The lack of correlation may signify a characteristic rupture length scale (e.g., process zone size) controlling the isotropic radiation and/or wave propagation and other effects that mask the source effects.