Theoretical Modeling of Secondary Microseisms Considering Source and Receiver Site Structures

The continuous Earth’s seismic wave field is predominantly generated in the oceans through nonlinear interactions of ocean surface gravity waves. Using pressure sources close to the ocean surface derived from state-of-the-art ocean-wave model, we simulate secondary microseim surface waves.  Previous modeling approaches accounted only for an ocean layer of variable thickness overlying a homogeneous half-space at each source location.  Here, we incorporate spatially varying 1-D velocity models from CRUST1.0 at both source and receiver locations. Within this framework, we derive analytical expressions for source and receiver site coefficients that depend solely on local velocity model. Our results show that ocean-bottom sediments can strongly modulate the excitation and amplification of SM Rayleigh waves —by up to a factor 100— consistent with the observations. We further demonstrate that our modeling is valid for frequencies below 0.2 Hz; at higher frequencies, the contribution of Scholte modes must be taken into account to avoid misidentifications of surface wave modes. Finally, we show that this new modeling approach accurately reproduces SM amplitudes recorded both at the ocean bottom and on land.