On the shaping factors of the secondary microseismic wavefield

Abstract:

Seismic noise in the period band 3-10 s is known as secondary microseism and it is generated at the ocean surface by the interaction of ocean gravity waves coming from nearly opposite directions. The seismic wavefield generated by a noise source is strongly affected by the source location and the propagation across 3D structures. In order to study the relative noise amplitudes recorded at the ocean bottom and on continental regions and investigate the seismic wavefield content, a simplified 2D model based on the spectral element method has been employed.

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 and a mode conversion from the first overtone to the fundamental mode of Rayleigh waves occurs at the ocean-continent boundary. The presence of a continental shelf at the ocean-continent boundary produces a negligible effect on land-recorded seismograms, whereas the source site effect, i.e. the source location with respect to the local ocean depth and sediment thickness, plays the major role. A source in shallow water mostly enhances the fundamental mode of Rayleigh waves, whereas a source in deep water mainly enhances the first overtone of Rayleigh waves. Land-recorded long period signals (T>6 s) are mostly due to deep water sources, whereas land-recorded short period signals (T<6 s) are due to sources in relatively shallow water, located close to the shelf break.