Crustal structure of the Alps as seen by attenuation tomography

•Improved mapping of absorption quality factor.•Large contrast of Q between different geological unit.•Correlation between attenuation and surface geology at low frequency.•Sharp NS dichotomy in attenuation structure at high frequency.•Attenuation reveals structures not seen by other geophysical met...

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Veröffentlicht in:Earth and planetary science letters 2016-04, Vol.439, p.71-80
Hauptverfasser: Mayor, Jessie, Calvet, Marie, Margerin, Ludovic, Vanderhaeghe, Olivier, Traversa, Paola
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Sprache:eng
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Zusammenfassung:•Improved mapping of absorption quality factor.•Large contrast of Q between different geological unit.•Correlation between attenuation and surface geology at low frequency.•Sharp NS dichotomy in attenuation structure at high frequency.•Attenuation reveals structures not seen by other geophysical methods. We develop a simple tomographic approach exploiting the decay rate of coda waves to map the absorption properties of the crust in a region delimited approximately by the Rhine Graben to the North, the Apennines to the South, the Massif Central to the West and the Dinarides to the East. Our dataset comprises 40 000 coda records of about 2000 weak to moderate crustal earthquakes, with magnitude ranging from 2.8 to 6 and recorded by broad-band, accelerometric and short-period stations. After proper choice of a coda window minimizing the effects of variable epicentral distances, we measure the coda quality factor Qc in five non-overlapping frequency windows covering the 1–32 Hz band for all available source station pairs. These measurements are subsequently converted into maps of absorption quality factor (Qi) using a linearized, approximate relation between Qc and Qi. In practice the following procedure is applied in each frequency band: (1) we divide the target region into 40×40 km cells; (2) for each source-station pair, we assign the measured Qc value to each pixel intercepted by the direct ray path; (3) the results are averaged over all paths and subsequently smoothed with a 3×3 pixels moving window. Our approach is consistent with the high sensitivity of Qc to the value of Qi between source and station. Our tomographic approach reveals strong lateral variations of absorption with length scales ranging from 100 km to 1000 km. At low frequency (∼1 Hz), the correlation with the surface geology is clear, Cenozoic and Mesozoic sedimentary basins (resp. crystalline massifs) being recognized as high (resp. low)-absorption regions. Furthermore the Qi map delineates finer geological features such as the Ivrea Body, the Rhône Valley, or felsic intrusions in the central Alps. At high-frequency (>16 Hz), only the thickest Cenozoic sedimentary deposits show up as high-attenuation regions and a north/south dichotomy is apparent in the absorption structure. The limit between low-attenuation regions to the North and high-attenuation region to the South correlates geographically with the location of the Periadriatic Lineament (PL), a major late-alpine crustal- to lithosp
ISSN:0012-821X
1385-013X
DOI:10.1016/j.epsl.2016.01.025