Empirical Frequency-Dependent Radiation Pattern of the 1998 Miyagiken-Nanbu Earthquake in Japan

Abstract

It has been pointed out that radiation patterns for earthquakes with double-couple focal mechanisms are not observed at high frequencies. This is due to the source process heterogeneities and the scattering on the small-scale heterogeneities of the medium with increasing frequency. To quantify the frequency-dependent radiation pattern, I examine radial-to-transverse spectral ratios (R/Ts) of S waves recorded at 12 stations during the 1998 Miyagiken-Nanbu earthquake ( M W 5.0), a foreshock ( M W 4.1), and an aftershock ( M W 3.8). The ratios of SV -wave to SH -wave radiation pattern coefficients ( SV / SH s) are slightly different from R/Ts for S waves synthesized using the 1D site-specific flat-layered model. The observed R/Ts are similar to the calculated R/Ts rather than the SV / SH s at frequencies less than 3 Hz and become unity at frequencies higher than 6 Hz. The boundary frequencies of 3 and 6 Hz do not depend on event magnitudes and the limited hypocentral distance (13-27 km). Considering these observed results, an empirical frequency-dependent radiation-pattern model is proposed in order to synthesize horizontal components of strong motions for moderate to large earthquakes by the stochastic and empirical Green's function method. Manuscript received 15 April 2001.