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 (MW 5.0), a foreshock (MW 4.1), and an aftershock (MW 3.8). The ratios of SV-wave to SH-wave radiation pattern coefficients (SV/SHs) 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/SHs 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.