Unexplained spectral peaks in Earth tremor

Mulargia F.
Source: Geophys. J. Int., Vol. 216, pagg  515–520
Year: 2019
DOI: 10.1093/gji/ggy420

Independent multi-year analyses of Earth tremor have suggested a continuous excitation of Earth normalmodes by ocean storms, but also a number of unexplained spectral peaks extraneous to them, mostly in the 0.2–2 mHz frequency band.We reassess the worldwide existence of such peaks by stacking themultitaper high-resolution spectra of all stations of the International Geodynamics and Earth Tide Service superconducting gravimeter network with at least 30 months of uninterrupted record, analysing a global epoch of 656 months. The analysis, beyond showing the predominance of 0Sn, n = 0, .., 12 Earth spheroidal modes, confirms the existence of unexplained spectral peaks which (1) cannot be ascribed to instrumental noise, (2) occur at frequencies extraneous to Earth normal modes, (3) have a statistical significance comparable to them and (4) appear incompatible with any natural or anthropic terrestrial source. While at odds with the hypothetical Earth ‘tune in’ on a continuum detectable gravitational wavefield, the peaks appear to be compatible in terms of amplitude, frequency and—according to cosmological constraints—expected number, with the independently calculated gravitational wave monochromatic emission of a few binary systems consisting of a star with mass ∼1/10 of the sun captured in close orbit by the supermassive black hole at the centre of our galaxy.

Key Points:

  • Tides and planetary waves;
  • Fourier analysis;
  • Surface waves and free oscillations;
  • Seismic noise.