Crustal structure of the Alps as seen by attenuation tomography

Jessie Mayor (1) , Marie Calvet (1) , Ludovic Margerin (1) , Olivier Vanderhaeghe (2) , Paola Traversa (3)

1 IRAP - Institut de recherche en astrophysique et planétologie
2 GET - Géosciences Environnement Toulouse
3 EDF - EDF

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 Q_c 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 (Q_i) using a linearized, approximate relation between Q_c and Q_i. 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 Q_c 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 Q_c to the value of Q_i 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 Q_i 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 lithospheric-scale structure. Furthermore, the attenuation structure seems to prolong the PL to the West along a line marked by large historical earthquakes. The Apennines orogenic belts exhibit a distinct frequency behavior, with high attenuation at low-frequency and low-attenuation at high-frequency. Low-frequency absorption may likely be explained by the relatively thick cover of Cenozoic sedimentary materials, as well as by shallow geothermal activity. We hypothesize that the frequency dependence of the attenuation structure, in particular in the Apennines, is caused by a change of the wavefield composition which accentuates the sensitivity of the coda to the deeper parts of the medium as the frequency increases.

Mots clés: seismic attenuation; coda waves; crustal structure; Alps; tomography

Domaines: Planète et Univers [physics]

Jessie Mayor, Marie Calvet, Ludovic Margerin, Olivier Vanderhaeghe, Paola Traversa. Crustal structure of the Alps as seen by attenuation tomography. Earth and Planetary Science Letters, 2016, 439, pp.71-80. ⟨10.1016/j.epsl.2016.01.025⟩. ⟨insu-03675420⟩