Including Radiation-Pattern Effects in Ground-Motion Models for Taiwan

Most ground-motion models (GMMs) parameterize the earthquake source by magnitude, style of faulting, and source depth. GMMs are isotropic in terms of the source scaling for these three parameters. The radiation pattern, which leads to azimuthally varying source effects, is not included in most GMMs. As a result, any systematic radiation-pattern effects are treated as aleatory variability in the path terms in the GMMs, which does not make physical sense. We incorporate the far-field radiation pattern into GMMs for pseudospectral acceleration and Fourier amplitude spectra for the Taiwan region. A key issue is how to combine the radiation pattern from the SH and SV components for predicting the RotD50 amplitudes of ground motions used in GMMs. A suite of point-source simulations was generated for a range of focal mechanisms and site azimuths. Empirical models for the phase differences between the radial and transverse components were developed to constrain the between-component phase differences in simulations. The results show that the vector sum of the SH and SV far-field radiation patterns, FS, has a higher correlation with the RotD50 amplitudes of the ground motion than the arithmetic mean or the geometric mean. The radiation-pattern term is modeled by S0(M)+S1(M)ln(Fs+S2), in which S0(M) and S1(M) are linear functions of magnitude between M 4 and 6; they equal 0 for M > 6. The magnitude dependence reflects that, for large magnitudes and short distances, ground motions are more affected by the slip distribution and rupture timing than by the average radiation pattern from the subevents along the rupture, but the radiation pattern has a significant effect on the long-period ground-motion amplitude for small-magnitude events. Including radiation patterns into GMMs will improve estimates of 3D path effects from small-magnitude events by removing radiation-pattern effects from combined radiation-pattern and path terms used in current GMMs.