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Extension des méthodes d'imagerie par corrélation de bruit sismique : atténuation et anisotropie sous AlpArray

Abstract : We have adapted methods used on earthquake recordings to the noise cross-correlations in the Alps, taking advantage of data from the AlpArray project and permanent stations in Europe. First we show that it is possible to measure the coda quality factor on noise cross-correlations. We thus manage to obtain the coda quality factor between 2.5 s and 5 s and 5 s and 10 s, period bands that are inaccessible for earthquake coda measurements. The coda quality factor is independent of azimuth for interstation distances less than 200 km and depends on the number of days used to calculate the correlation, with measurements stabilizing after 200 days of correlation. From 10 seconds of period on, the measurements lose their geographical coherence and from 20 seconds of period on, there is no more visible coda. We obtain low quality factor values in the Po plain and higher values in the Alpine chain. Then we correlated the coda of the cross-correlations to explore the possibilities in terms of distribution and choice of coda sources. We manage to significantly improve the quality of the correlations of the coda of correlations by a judicious choice of sources, however the choice of coda sources depends on the period band used. Having confirmed with the correlations of coda of correlations the absence of strong azimuthal group velocity bias of the noise correlations, we adapt a beamforming method used on teleseisms to image the azimuthal anisotropy with small arrays and apply it to the Alps to obtain maps of the azimuthal anisotropy of Rayleigh waves between 15 seconds and 70 seconds. Phase velocity measurements show a visible anisotropy, but also point biases related to the heterogeneities present in the Alpine chain, which we manage to quantify. The results for periods longer than 40 seconds are generally in agreement with existing work, but for shorter periods they show a weak anisotropy in the Alps themselves, the presence of a zone of strong anisotropy in the north-western part of the Alps as well as an anisotropy perpendicular to the chain in the Apennines, all probably located in the crust.
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https://tel.archives-ouvertes.fr/tel-03185939
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Submitted on : Tuesday, March 30, 2021 - 5:13:09 PM
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Dorian Soergel. Extension des méthodes d'imagerie par corrélation de bruit sismique : atténuation et anisotropie sous AlpArray. Sciences de la Terre. Université Grenoble Alpes [2020-..], 2020. Français. ⟨NNT : 2020GRALU031⟩. ⟨tel-03185939⟩

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