Sub‐Wavelength Seabed Stiffness Control of Seismic Amplitude Modulation in Seafloor DAS

Submarine distributed acoustic sensing cables record seafloor strain with striking spatial variability whose physical origin is not immediately obvious. By explicitly partitioning the recorded wavefield into ocean‐wave, Scholte‐wave, and teleseismic Rayleigh‐wave components, we show that these amplitude variations are not random but encode systematic modulation by seabed properties. Softer sediment intervals consistently amplify all wavefield components, whereas harder patches suppress them. Numerical simulations reproduce this behavior and reveal frequency‐ and mode‐dependent amplitude modulation controlled by surface‐wave sensitivity kernels confined to a shallow fraction of a wavelength. In this sense, the seabed acts as an elastic modulator governing how external energy, from ocean‐wave pressure to teleseismic arrivals, is converted into measurable strain on the cable. Amplitude information therefore provides a direct, spatially resolved proxy for near‐surface seabed stiffness. Additionally, undoing this seabed‐induced modulation is a necessary first step for quantitative analysis of other processes, such as wind forcing and microseism generation.