The Earth’s ‘hum’ is driven by ocean waves over the continental shelves

摘要: The Earth is regularly emitting a low frequency 'hum' equivalent to the energy released from magnitude 5.75 earthquake. Since its discovery source of this 'noise' has been projected be some combination interactions between atmosphere, oceans and solid Earth, but exactly what produces it remained something mystery. Now Spahr Webb presents calculations that suggest seismic generated by interaction ocean waves with shallow continental shelves, rather than atmospheric turbulence, probable hum. Observations show normal modes at frequencies near 10 mHz are excited nearly constant level in absence large earthquakes1. This background excitation called ‘hum’ Earth2, maximum earthquake3. Its origin debated, most studies attributing forcing analogous solar oscillations turbulence2,4,5,6,7. Some reports also predicted turbulence might excite planetary Mars detectable levels4. Recent observations on however, predominant lies under oceans8,9,10. Here I very weak source, instead interacting over shelves drive hum Earth. Ocean couple into through quadratic nonlinearity surface boundary condition, which couples pairs slowly propagating similar high phase velocity component approximately double frequency. process generate well known ‘microseism peak’ dominates spectrum 140 mHz (refs 11, 12), frequencies, mechanism differs significantly depth dependence. A calculation coupling reproduces observed. Measurements temporal correlation wave data data9,10 have confirmed waves, driving