Theoretical Modeling of Secondary Microseisms Considering Source and Receiver Site Structures, With a Focus on Ocean-Bottom Sediment Effects

Opposite-direction oceanic wave interactions at the ocean surface generate microseisms between 0.1 and $\sim$0.5 Hz, known as secondary microseisms (SM). SM recordings aid in imaging Earth's crust, but they also impede monitoring seismic signals due to tectonic activities. Thus, quantification of SM energy would benefit research in both areas. Previous studies on modeling SM energy have primarily focused on ocean modulation of SM, neglecting lateral variations in ocean and crustal structures between SM sources and seismic stations. In this study, we theoretically define source and receiver site coefficients which only depend on the local velocity model. Using these coefficients, we demonstrate how ocean-bottom sediments modulate the excitation and amplification of SM Rayleigh waves. A notable finding is that ocean-bottom sediments can amplify SM energy by a factor of 100, also supported by our field observation. We incorporate these modulation effects into modeling SM power spectral densities. Thanks to these theoretical improvements, our modeling matches field observations from both ocean-bottom seismometers and permanent land stations. This study potentially aids research on ambient seismic noise, ocean waves, and ocean-bottom seismic monitoring.