An analytical approach for estimating the spectral P/S ratio within ground motions

In the fields of strong ground motion predictions, site response, and geotechnical site characterization, P- and S-waves are typically de-coupled, assuming that near-surface body waves propagate vertically. This assumption, while often reasonable, may neglect secondary wave phases that result from inclined propagation and interlayer reflections. Therefore, an appropriate separation method that takes into account the contribution of both inclined P and S waves, their secondary phases (e.g. SP wave) and the physical parameters affecting them, is required. In this study, P and S separation involves the convolution of source, path and site definitions, where the site term is explicitly accounted for by a series of displacement reflection coefficients. Using ray-theory based modeling our analysis shows that the SP phase has a strong contribution to the total P wave. Plus, its arrival within the S time-window emphasizes the importance of separating P and S waves in the frequency domain rather than using time-windows. Also, we show that P waves dominate the ground motions at soft site classes at all distances above ~10 km, for all tested combinations of site thickness and source type. This may have significant implications for engineering applications, such as site-response analysis and ground motion predictions.