Coda-Q in the 2.5s -20s period band from seismic noise - Application to the greater Alpine area

International audienceCoda-Q is used to estimate the attenuation and scattering properties of the Earth (Aki & Chouet 1975). So far focus has been on earthquake data at frequencies above 1 Hz, as the high noise level in the first and second microseismic peak, and possibly lower scattering coefficient, hinder stable measurements at lower frequencies. In this work, we measure and map coda-Q in the period bands 2.5 s-5 s, 5 s-10 s and 10 s-20 s in the greater Alpine region using noise cross-correlations between station pairs, based on data from permanent seismic stations and from the temporary AlpArray experiment. The observed coda-Q for short interstation distances is independent of azimuth so there is no indication of influence of the directivity of the incoming noise field on our measurements. In the 2.5 s-5 s and 5 s-10 s period bands, our measurements are self-consistent, and we observe stable geographic patterns of low and high coda-Q in the period bands 2.5 s-5 s and 5 s-10 s. In the period band 10 s-20 s, the dispersion of our measurements increases and geographic patterns become speculative. The coda-Q maps show that major features are observed with high resolution, with a very good geographical resolution of for example low coda-Q in the Po Plain. There is a sharp contrast between the Po Plain and the Alps and Apennines where coda-Q is high, with the exception a small area in the Swiss Alps which may be contaminated by the low coda-Q of the Po Plain. The coda of the correlations is too short to make independent measurements at different times within the coda, so we cannot distinguish between intrinsic and scattering Q. Measurements on more severely selected datasets and longer timeseries result in identical geographical patterns but lower numerical values. Therefore, high coda-Q values may be overestimated, but the geographic distribution between high and low coda-Q areas is respected. Our results demonstrate that noise correlations are a promising tool for extending coda-Q measurements to frequencies lower than those analysed with earthquake data

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

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.Nous avons adapté des méthodes utilisées sur les enregistrements de séismes aux corrélations de bruit dans les Alpes, en tirant parti des données du projet AlpArray et des stations permanentes en Europe. D'abord nous montrons qu'il est possible de mesurer le facteur de qualité de la coda sur des corrélations de bruit. Nous parvenons ainsi à obtenir le facteur de qualité de la coda entre 2.5 s et 5 s et 5 s et 10 s, des bandes de périodes inaccessibles pour les mesures de coda de séismes. Le facteur de qualité de la coda est indépendant de l'azimut pour des distances interstation inférieures à 200 km et dépend du nombre de jours utilisé pour calculer la corrélation, les mesures se stabilisant à partir de 200 jours de corrélation. À partir de 10 secondes de période, les mesures perdent leur cohérence géographique et à partir de 20 seconde de période, il n'y a plus de coda visible. Nous obtenons des valeurs de facteur de qualité faibles dans la plaine du Pô et des valeurs plus élevées dans la chaine Alpine. Ensuite nous avons corrélé la coda des corrélations pour explorer les possibilités en termes de répartition et de choix des sources de coda. Nous parvenons à améliorer significativement la qualité des corrélations de coda de corrélations par un choix judicieux des sources, cependant le choix des sources de coda dépend de la bande de période utilisée. Ayant confirmé avec les corrélations de coda de corrélations l'absence de fort biais azimutal de vitesse de groupe des corrélations de bruit, nous adaptons une méthode de beamforming utilisée sur des téléséismes pour imager l'anisotropie azimutale avec des petits réseaux et l'appliquons aux Alpes pour obtenir des cartes d'anisotropie azimutale des ondes de Rayleigh entre 15 secondes et 70 secondes. Les mesures de vitesse de phase montrent une anisotropie visible, mais également des biais ponctuels lié au hétérogénéités présentes dans la chaîne alpine, que nous parvenons à quantifier. Les résultats aux périodes supérieures à 40 secondes sont globalement en accord avec les travaux existants, mais à plus courte période ils mettent en évidence une anisotropie faible dans les Alpes elle-mêmes, la présence d'une zone de forte anisotropie au nord-ouest des Alpes ainsi qu'une anisotropie perpendiculaire à la chaîne dans les Apennins, toutes probablement situées dans la croûte

Extension of imaging methods by seismic noise cross-correlations : attenuation and anisotropy under AlpArray

Nous avons adapté des méthodes utilisées sur les enregistrements de séismes aux corrélations de bruit dans les Alpes, en tirant parti des données du projet AlpArray et des stations permanentes en Europe. D'abord nous montrons qu'il est possible de mesurer le facteur de qualité de la coda sur des corrélations de bruit. Nous parvenons ainsi à obtenir le facteur de qualité de la coda entre 2.5 s et 5 s et 5 s et 10 s, des bandes de périodes inaccessibles pour les mesures de coda de séismes. Le facteur de qualité de la coda est indépendant de l'azimut pour des distances interstation inférieures à 200 km et dépend du nombre de jours utilisé pour calculer la corrélation, les mesures se stabilisant à partir de 200 jours de corrélation. À partir de 10 secondes de période, les mesures perdent leur cohérence géographique et à partir de 20 seconde de période, il n'y a plus de coda visible. Nous obtenons des valeurs de facteur de qualité faibles dans la plaine du Pô et des valeurs plus élevées dans la chaine Alpine. Ensuite nous avons corrélé la coda des corrélations pour explorer les possibilités en termes de répartition et de choix des sources de coda. Nous parvenons à améliorer significativement la qualité des corrélations de coda de corrélations par un choix judicieux des sources, cependant le choix des sources de coda dépend de la bande de période utilisée. Ayant confirmé avec les corrélations de coda de corrélations l'absence de fort biais azimutal de vitesse de groupe des corrélations de bruit, nous adaptons une méthode de beamforming utilisée sur des téléséismes pour imager l'anisotropie azimutale avec des petits réseaux et l'appliquons aux Alpes pour obtenir des cartes d'anisotropie azimutale des ondes de Rayleigh entre 15 secondes et 70 secondes. Les mesures de vitesse de phase montrent une anisotropie visible, mais également des biais ponctuels lié au hétérogénéités présentes dans la chaîne alpine, que nous parvenons à quantifier. Les résultats aux périodes supérieures à 40 secondes sont globalement en accord avec les travaux existants, mais à plus courte période ils mettent en évidence une anisotropie faible dans les Alpes elle-mêmes, la présence d'une zone de forte anisotropie au nord-ouest des Alpes ainsi qu'une anisotropie perpendiculaire à la chaîne dans les Apennins, toutes probablement situées dans la croûte.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

Seismic Evidence for Craton Formation by Underplating and Development of the MLD

International audienceInconsistencies between observations from long and short period seismic waves and geochemical data mean craton formation and evolution remains enigmatic. Specifically, internal layering and radial anisotropy are poorly constrained. Here, we show that these inconsistencies can be reconciled by inverting cratonic Rayleigh and Love surface wave dispersion curves for shear-wave velocity and radial anisotropy using a flexible Bayesian scheme. This approach requires no explicit vertical smoothing and only adds anisotropy to layers where required by the data. We show that all cratonic lithospheres are comprised of a positively radially anisotropic upper layer, best explained by Archean underplating, and an isotropic layer beneath, indicative of two-stage formation. Within the positively radially anisotropic upper layer, we find a variable amplitude low velocity zone within 9 of 12 cratons studied, that is well correlated with observed Mid-Lithospheric Discontinuities (MLDs). The MLD is best explained by metasomatism after craton formation

Bayesian analysis of azimuthal anisotropy in the Alpine lithosphere from beamforming of ambient noise cross-correlations

International audienceSurface waves extracted from ambient noise cross-correlations can be used to study depth variations of azimuthal anisotropy in the crust and upper mantle, complementing XKS splitting observations. In this work, we propose a novel approach based on beamforming to estimate azimuthal anisotropy of Rayleigh wave phase velocities extracted from ambient noise cross-correlations. This allows us to identify and remove measurements biased by wavefront deformation due to 3D heterogeneities, and to properly estimate uncertainties associated with observed phase velocities. In a second step, phase velocities measured at different periods can be inverted at depth with a transdimensional Bayesian algorithm where the presence or absence of anisotropy at different depths is a free variable. This yields a comprehensive probabilistic solution that can be exploited in different ways, in particular by projecting it onto a lower dimensional space, appropriate for interpretation. For example, we show the probability distribution of the integrated anisotropy over a given depth range (e.g. upper crust, lower crust). We apply this approach to recent data acquired across the AlpArray network and surrounding permanent stations. We show that only the upper crust has a large-scale coherent azimuthal anisotropy at the scale of the Alps with fast axis directions parallel to the Alpine arc, while such large-scale patterns are absent in the lower crust and uppermost mantle. This suggests that the recent Alpine history has only overridden the anisotropic signature in the upper crust, and that the deeper layers carry the imprint of older processes. In the uppermost mantle, fast directions of anisotropy are oriented broadly north-south, which is different from results from XKS-splitting measurements or long-period surface waves. Our results therefore suggest that XKS observations are related to deeper layers, the asthenosphere and/or subduction slabs. The area north-west of the Alps shows strong anisotropy in the lower crust and uppermost mantle with a fast axis in the north-east direction that could be related to Variscan deformation

Bayesian analysis of azimuthal anisotropy in the Alpine lithosphere from beamforming of ambient noise cross-correlations

International audienceSurface waves extracted from ambient noise cross-correlations can be used to study depth variations of azimuthal anisotropy in the crust and upper mantle, complementing XKS splitting observations. In this work, we propose a novel approach based on beamforming to estimate azimuthal anisotropy of Rayleigh wave phase velocities extracted from ambient noise cross-correlations. This allows us to identify and remove measurements biased by wave front deformation due to 3-D heterogeneities, and to properly estimate uncertainties associated with observed phase velocities. In a second step, phase velocities measured at different periods can be inverted at depth with a transdimensional Bayesian algorithm where the presence or absence of anisotropy at different depths is a free variable. This yields a comprehensive probabilistic solution that can be exploited in different ways, in particular by projecting it onto a lower dimensional space, appropriate for interpretation. For example, we show the probability distribution of the integrated anisotropy over a given depth range (e.g. upper crust, lower crust). We apply this approach to recent data acquired across the AlpArray network and surrounding permanent stations. We show that only the upper crust has a large-scale coherent azimuthal anisotropy at the scale of the Alps with fast axis directions parallel to the Alpine arc, while such large-scale patterns are absent in the lower crust and uppermost mantle. This suggests that the recent Alpine history has only overridden the anisotropic signature in the upper crust, and that the deeper layers carry the imprint of older processes. In the uppermost mantle, fast directions of anisotropy are oriented broadly north-south, which is different from results from XKS-splitting measurements or long-period surface waves. Our results therefore suggest that XKS observations are related to deeper layers, the asthenosphere and/or subduction slabs. The area northwest of the Alps shows strong anisotropy in the lower crust and uppermost mantle with a fast axis in the northeast direction that could be related to Variscan deformation

Coda-Q in the 2.5s -20s period band from seismic noise - Application to the greater Alpine area

International audienceCoda-Q is used to estimate the attenuation and scattering properties of the Earth (Aki & Chouet 1975). So far focus has been on earthquake data at frequencies above 1 Hz, as the high noise level in the first and second microseismic peak, and possibly lower scattering coefficient, hinder stable measurements at lower frequencies. In this work, we measure and map coda-Q in the period bands 2.5 s-5 s, 5 s-10 s and 10 s-20 s in the greater Alpine region using noise cross-correlations between station pairs, based on data from permanent seismic stations and from the temporary AlpArray experiment. The observed coda-Q for short interstation distances is independent of azimuth so there is no indication of influence of the directivity of the incoming noise field on our measurements. In the 2.5 s-5 s and 5 s-10 s period bands, our measurements are self-consistent, and we observe stable geographic patterns of low and high coda-Q in the period bands 2.5 s-5 s and 5 s-10 s. In the period band 10 s-20 s, the dispersion of our measurements increases and geographic patterns become speculative. The coda-Q maps show that major features are observed with high resolution, with a very good geographical resolution of for example low coda-Q in the Po Plain. There is a sharp contrast between the Po Plain and the Alps and Apennines where coda-Q is high, with the exception a small area in the Swiss Alps which may be contaminated by the low coda-Q of the Po Plain. The coda of the correlations is too short to make independent measurements at different times within the coda, so we cannot distinguish between intrinsic and scattering Q. Measurements on more severely selected datasets and longer timeseries result in identical geographical patterns but lower numerical values. Therefore, high coda-Q values may be overestimated, but the geographic distribution between high and low coda-Q areas is respected. Our results demonstrate that noise correlations are a promising tool for extending coda-Q measurements to frequencies lower than those analysed with earthquake data

Nouvelle vision de la structure lithosphérique des Alpes par tomographie de bruit ambiant

National audienceAlors que les Alpes sont la chaîne de montagne la plus étudiée par les géologues depuis plus d'un siècle, la connaissance de leur structure lithosphérique restait parcellaire faute d'un réseau sismologique dense et homogène. Seule l'imagerie sismologique permet en effet de "voir" les structures profondes, d'infirmer ou confirmer les modèles géologiques. Le réseau sismologique temporaire AlpArray et sa composante française AlpArray-FR (lettre d'information Résif n°18) ont fourni les données nécessaires à l'implémentation de nouvelles méthodes de tomographie de bruit ambiant et leur application aux Alpes. Quatre thèses ont exploité les enregistrements continus de près de 1000 stations temporaires et permanentes d'Europe Occidentale, dont AlpArray

Nouvelle vision de la structure lithosphérique des Alpes par tomographie de bruit ambiant

National audienceAlors que les Alpes sont la chaîne de montagne la plus étudiée par les géologues depuis plus d'un siècle, la connaissance de leur structure lithosphérique restait parcellaire faute d'un réseau sismologique dense et homogène. Seule l'imagerie sismologique permet en effet de "voir" les structures profondes, d'infirmer ou confirmer les modèles géologiques. Le réseau sismologique temporaire AlpArray et sa composante française AlpArray-FR (lettre d'information Résif n°18) ont fourni les données nécessaires à l'implémentation de nouvelles méthodes de tomographie de bruit ambiant et leur application aux Alpes. Quatre thèses ont exploité les enregistrements continus de près de 1000 stations temporaires et permanentes d'Europe Occidentale, dont AlpArray

Nouvelle vision de la structure lithosphérique des Alpes par tomographie de bruit ambiant

National audienceAlors que les Alpes sont la chaîne de montagne la plus étudiée par les géologues depuis plus d'un siècle, la connaissance de leur structure lithosphérique restait parcellaire faute d'un réseau sismologique dense et homogène. Seule l'imagerie sismologique permet en effet de "voir" les structures profondes, d'infirmer ou confirmer les modèles géologiques. Le réseau sismologique temporaire AlpArray et sa composante française AlpArray-FR (lettre d'information Résif n°18) ont fourni les données nécessaires à l'implémentation de nouvelles méthodes de tomographie de bruit ambiant et leur application aux Alpes. Quatre thèses ont exploité les enregistrements continus de près de 1000 stations temporaires et permanentes d'Europe Occidentale, dont AlpArray