High-resolution Rayleigh-wave velocity maps of central Europe from a dense ambient-noise data set

We present a new database of surface wave group and phase-velocity dispersion curves derived from seismic ambient noise, cross-correlating continuous seismic recordings from the Swiss Network, the German Regional Seismological Network (GRSN), the Italian national broad-band network operated by the Istituto Nazionale di Geosica e Vulcanologia (INGV). To increase the aperture of the station array, additional measurements from the Mediterranean Very Broad-band Seismographic Network (MedNet), the Austrian Central Institute for Meteorology and Geodynamics (ZAMG), the French, Bulgarian, Hungarian, Romanian and Greek stations obtained through Orfeus are also included. The ambient noise, we are using to assemble our database, was recorded at the above-mentioned stations between 2006 January and 2006 December. Correlating continuous signal recorded at pairs of stations, allows to extract coherent surface wave signal travelling between the two stations. Usually the ambient-noise cross-correlation technique allows to have informations at periods of 30 s or shorter. By expanding the database of noise correlations, we seek to increase the resolution of the central Europe crustal model. We invert the resulting data sets of group and phase velocities associated with 8-35 s Rayleigh waves, to determine 2-D group and phase-velocity maps of the European region. Inversions are conducted by means of a 2-D linearized tomographic inversion algorithm. The generally good agreement of our models with previous studies and good correlation of well-resolved velocity anomalies with geological features, such as sedimentary basins, crustal roots and mountain ranges, documents the effectiveness of our approac

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

Tomography of the Alpine region from observations of seismic ambient noise

We use correlations of the ambient seismic noise to study the crust in western Europe. Cross correlation of 1 year of noise recorded at 150 three components broadband stations yields more than 3 000 Rayleigh wave group velocity measurements. These measurements are used to construct Rayleigh group velocity maps of the Alpine region and surrounding area in the 5-80 s period band. In the 5-10 s period band, the seismic noise recorded in Europe is dominated by surface waves originating from the Northern Atlantic ocean. This anisotropy of the noise and the uneven station distribution affect the azimuthal distribution of the paths where we obtain reliable group velocity measurements. As a consequence our group velocity models have better resolution in the northeast direction than in the southwest direction. Finally we invert the resulting Rayleigh wave group velocity maps to determine the Moho depth. Our results are in good agreement with the result of the numerous active experiments in the Alps and provide a continuous image of the Alpine structur

Toward Forecasting Volcanic Eruptions using Seismic Noise

During inter-eruption periods, magma pressurization yields subtle changes of the elastic properties of volcanic edifices. We use the reproducibility properties of the ambient seismic noise recorded on the Piton de la Fournaise volcano to measure relative seismic velocity variations of less than 0.1 % with a temporal resolution of one day. Our results show that five studied volcanic eruptions were preceded by clearly detectable seismic velocity decreases within the zone of magma injection. These precursors reflect the edifice dilatation induced by magma pressurization and can be useful indicators to improve the forecasting of volcanic eruptions.Comment: Supplementary information: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguier_SI.pdf Supplementary video: http://www-lgit.obs.ujf-grenoble.fr/~fbrengui/brenguierMovieVolcano.av

Tomographie à partir de corrélations de bruit de fond sismique

Traditional tomographic methods are based on seismic waves emitted by earthquakes. This limits the resolution of the images of the Earth's interior since large areas are free from earthquakes. In this manuscript we test a new approach that requires only noiserecords. Indeed, it has been shown that the cross-correlation of white noise recorded at two stations A and B is the Green's function between A and B, provided the noise is generated everywhere in the medium. This allows us to measure the velocity of seismic waves between any pair of stations. To test this new idea, we first explain why noise correlations yields the Green's function of the medium. Then we study the origin of the noise in 5-40s period band. In the third part, we assess the precision of surface wave velocity measurements from the noise correlations. Chapter 7 and 8 are devoted to the tomography of California and the Alps. In the last chapter, we show that our results can be improved by correlating the coda of the noise correlations.Les méthodes tomographiques habituellement employées en sismologie nécessitent des enregistrements de séisme. Or de larges zones sur Terre sont asismiques : faute de séisme nous ne pouvons les étudier avec une bonne résolution. Nous testons ici une approche radicalement différente : il s'agit d'imager la croute terrestre à partir d'enregistrements de bruit de fond sismique. En effet, la corrélation entre deux points A et B d'un bruit blanc généré en tout point du milieu est la fonction de Green complète du milieu entre A et B. On pourrait ainsi mesurer la vitesse des ondes sismiques entre n'importe quel couple de points de la surface. Nous présentons tout d'abord la théorie formalisant le lien entre corrélation de bruit et fonction de Green. Nous étudions ensuite l'origine du bruit de fond sismique sur la bande de période 5-40s afin de voir si il respecte ou non les exigences de la théorie. Après avoir évalué la précision des mesures de la vitesse des ondes de surface réalisées grace aux corrélations de bruit, nous étudions les Alpes. Enfin nous montrons que nos résultats peuvent etre amélioré enrecorrélant les corrélations

Towards improving ambient noise tomography using simultaneously curvelet denoising filters and SEM simulations of seismic ambient noise

International audienceThe aim of this article is to investigate a possible way to improve ambient noise tomography by expanding the data base of useful cross-correlation measurements. We show that the curvelet transform makes it possible to compute synthetic noise correlations by simulating directly seismic ambient noise using the spectral element method. These synthetic correlations can in turn be used to identify surface waves and overtones even on correlations having a signal to noise ratio much lower than one. We hope that the possibility to compute SEM synthetic correlations and to extract more information from correlations will be useful to improve ambient noise tomography and monitoring