Physics-Based Planar Wavefront Model to Estimate Seismic Rocking Spectra Using Translational Spectra

Abstract

A new and simple planar wavefront model is presented for synthesizing the Fourier spectra of the seismic rocking ground motion (i.e., seismic ground rotation about a horizontal axis) at a station near the epicenter by using the corresponding translational motion data. As in the conventional models, the ground motion was assumed to result primarily from the incidence of planar wavefronts of body waves emitted by a point source buried in a homogeneous, elastic half-space. In contrast to the conventional models, however, the spatial variations of both the amplitudes and the incidence angles of the waves were considered. The spatial variation of the amplitudes was formulated by considering the far-field radiation patterns of body waves due to a kinematic shear dislocation point source. The proposed model can therefore accommodate information on the underlying focal mechanism and give the estimates of both in-plane and out-of-plane rocking spectra. It is shown through a numerical study that the proposed model leads to more accurate estimates of rocking spectra than the conventional models over a large range of frequencies, even when the input source information is unavailable.