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Modélisation et simulation d'ondes T : modes de Rayleigh et effets 3D

Abstract : When an earthquake occurs below the ocean, seismic waves convert at the crust/ocean or solid/fluid interface into low-frequency acoustic waves, known as T-waves, which can propagate in the water column over very long distances.These waves are of great importance for monitoring the submarine seismic and volcanic activity, as they fill a gap of information in the land-based seismological records. The localization of the acoustic source can be deduced by trilateration from T-wave arrival-times at several hydrophones and an acoustic magnitude can be derived from the received levels.Beyond that, the information obtained from T-wave signals remains incomplete since the mechanisms for generating Twaves, the way they propagate and the importance of 3D-effects in the process are poorly known.To investigate these issues, this thesis uses a spectral finite element code (SPECFEM) able to perform full wave-form simulations of both seismic waves in the crust and acoustic waves in the ocean. The analytical model developed in this thesis describes propagating T-waves as Rayleigh modes; this model also predicts the spectrum of PN- and SN-waves, which are precursors of T-waves in records from high magnitude events. In a 2D configuration with a flat bottom and a uniform ocean, the theoretical (analytical solution) and simulated (SPECFEM) T-wave modes and PN- and SN-wave spectra show a very good agreement. The numerical model is thus applied with confidence to configurations for which an analytical model cannot be simply derived: a crust/ocean interface with a seamount, with or without a SOFAR channel in the ocean, and a flat seabed with a short-wavelength roughness. These simulations highlight the conditions necessary for the generation of energetic T-waves and confirm the predominant role of Rayleigh modes in their propagation. The outputs from the model with a rough sea-bottom compare well with hydroacoustic records from a major earthquake occurring below an abyssal plain. A 3D version of SPECFEM is finally used to investigate the importance of 3D-effects in the generation of T-waves. At a distance, different amplitudes and arrival-times are obtained depending on whether a seismic event occurs below a seamount or below a ridge. Hence, source localizations from arrival-times can be biased by 3D topographic effects in the vicinity of the epicenter region.
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Jean Lecoulant. Modélisation et simulation d'ondes T : modes de Rayleigh et effets 3D. Sciences de la Terre. Université de Bretagne occidentale - Brest, 2019. Français. ⟨NNT : 2019BRES0046⟩. ⟨tel-02953022⟩

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