Shallow Seismic Structure of the Seafloor in the Cascadia Basin from Observations of Scholte Waves by Ocean-Bottom Seismographs

The seafloor plays an important role in the propagation ofseafloor noise because its low shear velocity forms a strongwaveguide and the high shear velocity gradient facilitatesconversion processes.In 2001 (JASA), O. A. Godin and D. M.F. Chapman studiedpropagation of interface (Scholte) waves in models with ashear speed profile with a power-law depth dependence.They analyzed of four datasets from shallow-watersites, which they fit well with two-parameter models.Furthermore, they show that the mode wavefunctions are self-similar.Data from the deep seafloor from seafloor sources observedby Ocean-Bottom Seismographs frequently exhibit afundamental mode ending in an Airy phase with a frequencyof a few Hertz. This is, of course, incompatiblewith self-similarity. Adjusting the power-law shear velocityprofile in the upper 5 meters of the seafloor however,can produce an Airy phase and thus match the observed data.In the case of data from the east flank of the Gorda Ridge,the basic power-law is cs(z) = c0 zν with c0 =30.7(z in meters and ν = 0.6) and fits the fundamentalmode Scholte dispersion relation, which is linear in slowness-frequency spacein the 0.5-2.0 Hz range. Increasing the velocityand reducing the depth gradient in the upper 5 meters of the modeproduces the observed Airy Phase.These data were generated by a few tens of kilograms of explosiveand were observedby ocean-bottom seismographs at ranges of a few hundred meters.