Modelling the ocean site effect on seismic noise body waves

Type Article
Date 2014-05
Language English
Author(s) Gualtieri Lucia1, 2, Stutzmann Eleonore1, Farra V.1, Capdeville Y.3, Schimmel M.4, Ardhuin FabriceORCID5, Morelli A.6
Affiliation(s) 1 : Inst Phys Globe Paris, Sorbonne Paris Cite, CNRS, UMR 7154, Paris, France.
2 : Univ Bologna, Dipartimento Fis & Astron, Settore Geofis, Bologna, Italy.
3 : Univ Nantes, Lab Planetol & Geodynam Nantes, CNRS, UMR 6112, F-44035 Nantes, France.
4 : CSIC, Inst Earth Sci Jaume Almera, E-08028 Barcelona, Spain.
5 : IFREMER, Lab Oceanog Spatiale, F-29280 Plouzane, France.
6 : Ist Nazl Geofis & Vulcanol, Bologna, Italy.
Source Geophysical Journal International (0956-540X) (Oxford Univ Press), 2014-05 , Vol. 197 , N. 2 , P. 1096-1106
DOI 10.1093/gji/ggu042
WOS© Times Cited 47
Keyword(s) Body waves, Site effects, Theoretical seismology
Abstract Secondary microseismic noise is generated by non-linear interactions between ocean waves at the ocean surface. We present here the theory for computing the site effect of the ocean layer upon body waves generated by noise sources distributed along the ocean surface. By defining the wavefield as the superposition of plane waves, we show that the ocean site effect can be described as the constructive interference of multiply reflected P waves in the ocean that are then converted to either P or SV waves at the ocean-crust interface. We observe that the site effect varies strongly with period and ocean depth, although in a different way for body waves than for Rayleigh waves. We also show that the ocean site effect is stronger for P waves than for S waves. We validate our computation by comparing the theoretical noise body wave sources with the sources inferred from beamforming analysis of the three seismogram components recorded by the Southern California Seismic Network. We use rotated traces for the beamforming analysis, and we show that we clearly detect P waves generated by ocean gravity wave interactions along the track of typhoon Ioke (2006 September). We do not detect the corresponding SV waves, and we demonstrate that this is because their amplitude is too weak.
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