Seafloor topography, ocean infragravity waves, and background Love and Rayleigh waves

We propose that background Love and Rayleigh waves in a frequency range 5-20 mHz are generated primarily by ocean infragravity waves in the same frequency range by a linear coupling process with seafloor topography. Wavelengths of infragravity waves in this frequency range are on the order of 10 to 40 km in the deep ocean. The seafloor topography with wavelengths of this order is dominated by abyssal hills, which are the most widespread physiographic forms on Earth, covering as much as 85% of the Pacific floor. Interaction of infragravity waves in the deep ocean with these hills generates a random distribution of point-like tangential forces on the seafloor which may be large enough to excite Love and Rayleigh waves simultaneously. We quantify this idea by using the known statistical property of hills distribution in the Pacific and by noting that heights of abyssal hills are an order of magnitude smaller than depths of the deep ocean, so that the topography-related phase velocity change can be neglected. The model is reasonably consistent with the Love to Rayleigh wave amplitude ratio reported at 10-20 mHz and the observed background Rayleigh wave spectrum with a characteristic plateau around 8 mHz. Contribution of topographic coupling in shallow, coastal seas is not included in our simple model but should be important, especially at frequencies above 20 mHz.