Interacting Seismic Waveguides: Multimode Surface Waves and Leaking Modes

Recent developments in seismic recording provide dense sampling of the seismic wavefield that allows the extraction of higher surface wave modes as well as the fundamental, and in some circumstances also P-dominated modes. The character of the modal branches and their dispersion with frequency and phase speed depends on interactions between different aspects of the seismic structure and between wavetypes. The influence of P waves becomes quite strong in the very near surface when there are strong wavespeed gradients. An effective tool for understanding the nature of the modal interactions due to structure is provided by the seismic response in frequency-phase speed space, the kernel for seismogram calculation. Such displays for all three-components of the surface response extract the full modal response, including leaking modes effects, for Rayleigh and Love waves and an indication of the way that the modes are excited.