Pressure and electric field fluctuations are suitable for detecting seismic disturbances on the seafloor in the frequency range of 0.02 to 2 Hz. The background noise has two maxima in this range. From 0.01 to 0.03 Hz the pressure spectrum decreases because hydrodynamic filtering increasingly removes the direct influence of long surface gravity waves. The electric spectrum decreases because of increasing absorption of ionospheric disturbances. From 0.09 to 0.11 Hz both spectra increase sharply to a series of peaks where the non-linear interference of opposed surface wave trains generates microseisms. At 0.09 Hz the inferred vertical ground motion on the seafloor approaches that of quiet continential sites.