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Exploring small-scale heterogeneities of the moon and the earth

Abstract : During their propagation, seismic waves are attenuated by two phenomena: on one hand, absorption caused by the anelastic properties of the materials, and on the other hand, scattering caused by the presence of small-scale heterogeneities in the medium. The aim of this thesis is to map the properties of scattering and absorption of seismic waves in two extreme geophysical contexts with very different spatial scales. The first part of this memoir is devoted to the stratification of heterogeneities in the Moon. We use a new diffusion model in spherical geometry to invert measurements of the time of arrival of the maximum of energy and the seismic coda decay on data from the Apollo missions. Our inversions provide evidence of a very sharp contrast of scattering properties between the highly attenuating megaregolith and the transparent deep lunar mantle. Attenuation is largerly dominated by scattering and suggests the presence of fractures down to about 100 km depth, into the mantle. A new method for estimating the depth of shallow moonquakes based on diffusive signals was developped and confirms the existence of active faults around 50 km deep. The second part of this thesis is devoted to the attenuation structure of Taiwan, a region with a wide variety of geological structures in the context of two subduction zones. We use the MLTWA (Multiple Lapse Time Window Analysis) -a method based on the ratio between the coherent and incoherent energy of the seismic signal- to image the lateral variations of attenuation. We worked first with the classical hypothesis of isotropic scattering in a half-space. Our results provide evidence for a globally high level of attenuation with sharp contrasts of scattering properties across small spatial scales, of the order of 10-20 km. Scattering is particularly strong in the basins of the west coast, southern Taiwan and the eastern Coastal Range associated with the collision with the Luzon volcanic arc. Absorption increases gradually eastwards and reaches a maximum below the volcanic arc. A posteriori examination of the fit between data and model shows unambiguously the limits of the hypothesis of isotropic scattering in a half-space for a number of stations located along the coasts. This leads us to explore the effects of anisotropic scattering in a guide for seismic waves representing the crust. Taking anisotropy into account significantly improves the fitness of the model to the data. In particular, at low frequency (1-2 Hz), our study shows the prevalence of backscattering. This result is compatible with the presence of sharp contrasts of impedance in the crust and suggests the strong presence of fluids in fault zones and volcanoes in Taiwan. The measurement of scattering anisotropy opens new perspectives for characterizing small-scale geophysical heterogeneities.
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Submitted on : Wednesday, December 5, 2018 - 11:36:06 AM
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  • HAL Id : tel-01945330, version 1

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Kévin Gillet. Exploring small-scale heterogeneities of the moon and the earth. Earth and Planetary Astrophysics [astro-ph.EP]. Université Paul Sabatier - Toulouse III, 2017. English. ⟨NNT : 2017TOU30310⟩. ⟨tel-01945330⟩

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