The equilibrium oceanic microseism spectrum

Abstract

Microseisms are seismoacoustic waves excited by nonlinear interactions of ocean waves and are evident in the band 0.1 to 5 Hz in measurements of pressure or displacement spectra from the deep seafloor. A remarkable uniformity in microseism amplitudes is observed at the seafloor, despite many orders of magnitude variation in the amplitude of ocean waves overhead. This paper looks at how a balance is established between the excitation of microseisms under large source regions and the dissipation of this energy within a waveguide formed by the ocean, crust, and upper mantle. The excitation functions and dissipation of modes within ocean models are calculated to determine the ‘‘equilibrium microseism spectrum’’ of displacement or pressure for which local excitation is balanced by local dissipation. Efficient propagation of energy within the ocean waveguide also acts to make deep ocean microseism observations homogeneous.