Effect of the Ocean-Continent Boundary on the Secondary Microseism Wavefield and Love Wave Generation.

Secondary Microseisms are generated by ocean gravity wave interactions close to the ocean surface in the frequency band 0.1-1 Hz. These gravity wave interactions produce pressure fluctuations close to the ocean surface, from which P-waves are emitted. In a 1D Earth model, the resulting seismic wavefield consists of P, SV and Rayleigh waves but no Love wave can be generated. Nevertheless, there are several observations of Love waves in the secondary microseism frequency band. We investigate the effect of ocean-continental slope on the seismic wavefield to test how Love waves could be generated.We model synthetic seismograms using the Spectral Element Method in cartesian 3D models.We study two different cases : 1) the source is located close to the ocean surface above the ocean-continental slope in shallow water environment, and 2) the source is located close to ocean surface but far away from ocean-continental slope in deep water environment. Receivers are both located on the sea floor and on land in the two cases. We analyze the polarization of the seismic signals in the horizontal plane in different frequency bands in order to detect Love waves. We show that in some frequency band, the observed elliptical polarization is due to the mode conversions, between higher mode Rayleigh waves and fundamental mode Rayleigh waves. After carefully selecting the frequency band, we can detect Love waves and evaluate the Rayleigh to Love wave ratio.