Understanding and modeling seismic noise generation and propagation

Abstract:

Seismic noise is the continuous oscillation of the Earth recorded worldwide, in response to the interaction amongst the atmosphere, the ocean and the solid Earth. Seismic noise spectrum is typically dominated by two main peaks at about 7 s and 14 s of period, called secondary and primary microseisms, respectively. Secondary microseisms are generated by the non-linear interaction of ocean gravity waves (Longuet-Higgins, 1950; Hasselmann, 1963), whereas primary microseisms are generated by the direct interaction between the ocean gravity waves and the seafloor, in coastal regions (Hasselmann, 1963; Ardhuin et al., 2015).

Noise source location and amplitude can be derived from a realistic ocean wave model and schematized as a pressure distribution along the ocean surface. We model the amplitude of the three-components of noise spectra 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 amount of noise Love waves, for which a different source mechanism is needed.

The ocean plays an important role on the amplification of seismic noise. The effect of the ocean on seismic noise is computed on Rayleigh waves using normal modes and on body waves by defining the wavefield as the superposition of plane waves. The ocean site effect varies strongly with period and ocean depth, although in different ways for body waves and for Rayleigh waves, amplifying different source regions at different periods.