Dr. Lucia Gualtieri, Postdoctoral Research Fellow at Lamont-Doherty Earth Observatory of Columbia University, presents "Numerical modeling of seismic noise amplitude and propagation" at the MIT Earth Resources Laboratory on November 13, 2015.

"Independently from the earthquake activity, the Earth is never truly at rest. The interaction amongst the atmosphere, the ocean and the solid Earth causes a continuous signal recorded everywhere on Earth and called “seismic noise”. A typical seismic noise spectrum is characterized by two main peaks – the secondary (T~7 s) and the primary (T~14 s) microseism -- and by a long period signal called “hum” of the Earth (about T>20 s). Secondary microseisms are generated by the non-linear interaction of ocean gravity waves (Longuet-Higgins, 1950; Hasselmann, 1963), whereas primary microseisms and hum are generated in coastal regions by the direct interaction of ocean gravity waves with the seafloor (Hasselmann, 1963; Ardhuin et al., 2015).
Noise source location and amplitude can be retrieved from a realistic ocean wave model and schematized as a distribution of vertical single forces. Thus, the amplitude of the three-components of noise spectrum can be modeled by using normal mode summation. The fundamental mode of Rayleigh waves is the dominant signal on land-records and allows us to well estimate the amplitude of the vertical component of noise spectrum in various environments. The discrepancy between real and synthetic spectra on the horizontal components enables instead to estimate the energy amount associated with noise Love waves, for which a different source mechanism is needed.
The ocean depth below each source region plays a resonance effect on seismic noise. This effect – called “ocean site effect” – modulates seismic noise amplitude. The computation of the ocean site effect by normal modes allows the identification of the geographical regions where the seismic noise generation is enhanced. The ocean site effect varies strongly with period and ocean depth.
Finally, the effect of the ocean-continent boundary on the propagation of seismic noise is studied using the 2D spectral-element method. The seismic wavefield recorded on the vertical component seismograms below the seafloor is mainly composed by the fundamental mode and the first overtone of Rayleigh waves. A mode conversion from the first overtone to the fundamental mode of Rayleigh waves occurs both gradually moving from deep to shallow water and at the ocean-continent boundary. The source location with respect to the local ocean depth and sediment thickness, plays a major effect on land-recorded seismograms."