![]() ![]() Comparing real observations with regional 3D acoustic/seismic simulations, we illustrate how continental margins influence the energy of Love waves observed on land. In this work, after highlighting the role that 3D path effects at continental margins play in apparent source locations of surface waves generated in deep water, we focus on the origin of secondary microseism Love waves and understanding better their relationship with Rayleigh waves. Surface waves propagation effects related to crustal heterogeneities and wave refraction at continental margins need to be taken into account to properly understand those differences 21. Although the source locations derived from ocean wave models show broad agreement with locations determined from land-based seismic observations, some differences are observed 9, 11, 13. Ocean wave models 24 have been used successfully to reproduce the secondary microseism energy recorded on land, outlining source variability with frequency and bathymetry, as well as seasonal variations 25, 26, 27. Although, both cases are most likely occurring 21, secondary microseims noise sources observed from land appear to be dominant in coastal waters and continental shelf regions, where land arrays’ beampower seems to correlate strongly with wave height 22, 23. Furthermore, source regions for the secondary microseisms observed on land are highly debated, between sources located in deep water 19 and sources located in near-coastal or continental shelf regions 20. Love wave energy is equal to or dominant over Rayleigh waves for primary microseisms, but this is not the case for secondary microseisms with observed Rayleigh to Love wave energy ratios ranging from 0.4 to 1.2 (refs. Bathymetry variations may also lead to P to SH conversions through scattering 14. ![]() These have been interpreted as scattering and energy transfers from Rayleigh to Love waves controlled by sedimentary basin boundaries 9, 12, 13. However some differences exist 12, 13, including broader ranges of back azimuths observed for Love waves compared to Rayleigh waves. This mechanism is valid in shallow water for primary microseisms, but not for secondary microseisms that represent the strongest noise level in the seismic noise spectrum observed on land.Ĭommon source locations have been observed for both Love and Rayleigh waves associated with secondary microseisms 9, 10, 11, suggesting that there may be a causal relationship. The occurrence of Love waves in recorded primary microseism signals has been explained by the direct interaction between propagating ocean waves and sea-bottom topography gradients 6, 7, 8. In contrast, secondary microseisms derive from second-order pressure variations, resulting from the interaction of opposing ocean wave fronts 5, leading to vertical pressurization of the ocean floor in arbitrarily deep water. Generated in the ocean, primary microseisms originate from the direct action of propagating ocean gravity waves in shallow water. There are two energetic spectral windows in the microseism wavefield, the primary (dominant periods 10–20 s) and secondary (dominant periods 3–10 s). Hence, they yield rich spatio-temporal information about ocean-land coupling in deep water.Ī better understanding of ocean generated seismic noise sources and associated wavefields is crucial for a variety of applications from seismic imagery 1 to subsurface monitoring 2 in addition to ocean wave climate and storm activity studies 3, 4. We conclude that, in contrast to Rayleigh waves, microseism Love waves observed on land do not directly relate to the ocean wave climate but are significantly modulated by continental margin morphologies, with a first order effect from sedimentary basins. We show that while Rayleigh to Love wave conversions occur along the microseism path, Love waves predominantly originate from steep subsurface geological interfaces and bathymetry, directly below the ocean source that couples to the solid Earth. Here, using terrestrial seismic arrays and 3D synthetic acoustic-elastic simulations combined with ocean wave hindcast data, we demonstrate that, observed from land, our general understanding of Rayleigh and Love wave microseism sources is significantly impacted by 3D propagation path effects. While the origin of associated Rayleigh waves is well understood, there is currently no quantified explanation for the existence of Love waves in the most energetic region of the microseism spectrum (3–10 s). Wind driven ocean wave-wave interactions produce continuous Earth vibrations at the seafloor called secondary microseisms. ![]()
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