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Probing Alpine geodynamics with state-of-the-art seismic arrays and tomography – AlpArray-FR

Probing Alpine geodynamics in 3D with state-of-the-art seismic arrays and tomography

While most concepts that underlie current studies of mountain belts and convergence dynamics were born in the Alps, this belt has not been given the attention it deserves using recent geophysical techniques. The ambition of the European AlpArray collaborative project is to re-evaluate the deep structures and processes that occur beneath the Alps by exploiting cutting-edge developments in geophysical imaging.

The international AlpArray projec: re-evaluation of the deep structures and processes that occur beneath the Alps by exploiting cutting-edge developments in geophysical imaging

Millions of Europeans live on land formed by the Alpine orogeny and are affected by its topography, geology and associated natural hazards. The Alps have been intensely studied by geologists for more than a century, and they provide a unique natural laboratory to advance our understanding of orogenesis and its relationship to present and past mantle dynamics. While the complex history of the Alpine mountain belt is largely constrained by geological studies, there is still a huge gap between this detailed geological information and the comparatively limited amount of geophysical data on the structure and fabric of the lithosphere-asthenosphere system beneath the belt. The ambition of AlpArray, a European collaborative project implicating ~45 institutes in 18 countries, is to fill that gap. This is being achieved by the deployment of a dense and homogeneous seismic experiment at an unprecedented scale in Europe, the stations operating simultaneously for two to three years for the land array and one year for the offshore-onshore array. Deciphering the particularly complex and fully 3-D geometry of the orogen not only requires high-quality and dense seismic data, but also adapting and further developing new seismic imaging methods. The AlpArray dataset will provide a unique opportunity to test the applicability of these methods, including for example the extraction of seismic body waves propagating in the crust (previously carried out in shield areas only) and full waveform inversion (so far developed mainly for oil exploration purposes). Another challenge is the integration of geological and geophysical data in a common 4D model to decipher the present-day geometry of the orogeny and to restore its past structure and dynamics, which will require close collaboration between geologists and geophysicists.

The AlpArray team has deployed a dense and homogeneous onshore-offshore seismic antenna in the broad Alpine region. It includes more than 270 temporary stations that complement the ~350 permanent stations. The stations will be operating simultaneously for at least two years for the land array (2016-2019) and 8 months for the offshore array (2017-2018). AlpArray is the largest European collaborative geophysical experiment ever, creating a large collaborative network of European Earth scientists who contribute to the experiment through national funding and equipment. Most data are distributed in quasi-real time to all participants through the EIDA european data portal. Deciphering the particularly complex and fully 3-D geometry of the orogen not only requires high-quality and dense seismic data, but also adapting and further developing new seismic imaging methods. The AlpArray dataset will provide a unique opportunity to test the applicability of these methods, including for example the probabilistic joint inversion of different geophysical observables and full waveform inversion. Finally, we will integrate geological and geophysical data in a common 4D model to decipher the present-day geometry of the orogeny and to restore its past structure and dynamics. This requires upscaling of geological observations and downscaling (or higher resolution) of tomographic models of the crust and mantle, and close collaboration between geologists and geophysicists. The French part of the project, AlpArray-FR, will, in addition to the global objectives of the AlpArray project address specific challenges associated with unravelling the structure and history of the Western Alps. Seismic imaging of the most arcuate part of the alpine arc is even more challenging due to its 3-D structure, the presence of the Ivrea high velocity – high density anomaly at mid crustal depths and the thick low velocity sediments of the Po plain which mask the deeper mantle structure.

With its 274 temporary seismic stations (onshore and offshore) that complement 352 permanent stations (in July 2017), the AlpArray seismic antenna is by far the most important seismic network ever installed in Europe. This was achieved in an unprecedented transnational effort joining 36 laboratories and institutes of 11 different countries (Hetenyi et al., 2018). Eighty new stations have been installed in Eastern France in the framework of the AlpArray-FR project, out of which 24 have or will become permanent. Data of 2/3 of the temporary AlpArray network are available to project participants in quasi-real time (2-3 days delay) on the portal of the European integrated data archive EIDA. Most stations of the AlpAray seismic network provide data of excellent quality.
Using ambient noise tomography and 1300 seismic stations, Lu et al. (2018) have computed the best 3-D S-wave velocity available to-date for the European crust and mantle. It has a high resolution of 30 km in the upper crust and 90 km at Moho depth. The probabilistic scheme used in the inversion for Vs provides useful error estimates on the parameters, including Moho depth. In the Western Alps that are of specific interest for the AlpArray-FR project, their model displays a 8-km Moho jump beneath the Belledonne external crystalline massif, that was undetected by the ECORS-CROP deep reflection seismic profile. If the existence of this jump is confirmed by the upcoming Cifalps-2 experiment, it may be interpreted as a lithospheric fault associated with the transpressional late tectonic deformation of the Northwestern Alps.

Y. Lu has developped a wave-equation tomography methodology to compute a 3-D Vs model directly from noise correlation waveforms. A publication will soon be submitted. D. Soergel (ISTerre) is using C3 (correlations of codas of correlations) to measure azimuthal anisotropy of S-wave velocity and wave attenuation in the Alpine crust and upper mantle. C. Alder (LGLTPE) inverts group velocity data of Rayleigh and Love waves for radial anisotropy. She also uses noise correlations that include AlpArray data.
In autumn 2018, the postdoc funded by ANR will start working in IPGS on the joint inversion of surface wave dispersion, body wave traveltimes and receiver functions using a new probabilistic inversion scheme.
Geoazur looks for a PhD candidate to apply their full-waveform inversion scheme to teleseismic P-wave arrivals recorded by the AlpArray network.
A. Renouard (IPGS) started her PhD 1 year ago on local earthquake location and tomography in the northeast of France. She uses AlpArray data in addition to permanent network data. The goal is to delineate potential seismic faults and their focal mechanisms.
The «Alps and Southeast basin« project was selected by the RGF program (France geological frame of reference). From 2018 to 2022, it will fund Master and PhD theses, in particular on geological interpretations of geophysical data from AlpArray.
In collaboration with Chinese and Italian colleagues, ISTerre will install a linear array of 56 seismic stations in the NW Alps (Mâcon to Ligurian Alps across Mont-Blanc and Gran Paradiso) for 18 months starting in September 2018. The objective is to complement the AlpArray dataset and get a high-resolution (or the order of 10 km) seismic tomography of the lithosphere in the NW Alps, in particular beneath the subduction complex of the Gran Paradiso.

2 publications in international peer-reviewed journals:
- Hete´nyi, G., and co-authors (incl. W. Crawford, J-X. Dessa, C. Doubre, A. Paul), 2018. The AlpArray Seismic Network – a large-scale European experiment to image the Alpine orogeny, Surveys in Geophysics, doi:10.1007/s10712-018-9472-4.
- Lu, Y., L. Stehly, A. Paul, and AlpArray Working Group, 2018. High-resolution surface wave tomography of the European crust and uppermost mantle from ambient seismic noise, Geophys. J. Int., 214,1136-1150.
5 posters or oral communications in international meetings:
- Lu, Y., L. Stehly, A. Paul and AlpArray Working Team, Surface wave tomography of Europe from ambient seismic noise (poster), Geophysical Research Abstracts Vol. 19, EGU2017- 8807-1, 2017.
- Kopp, H., W. Crawford, A. Paul, et al. AlpArray offshore : Preliminary results of the Ligurian Sea OBS network and refraction lines (poster), Geophysical Research Abstracts, Vol. 20, EGU2018-15896, EGU General Assembly 2018.
- Lu, Y., A. Paul, L. Stehly, and AlpArray Working Group. Along strike changes in the crustal structure of the Alps documented by a new ambient-noise tomography (poster), Geophysical Research Abstracts Vol. 20, EGU2018-18935, 2018, EGU General Assembly 2018.
- Lu, Y., L. Stehly, A. Paul and AlpArray Working Group, 2018. A new high-resolution and probabilistic shear-wave velocity model of the European crust and uppermost mantle derived from ambient noise tomography (oral), Geophysical Research Abstracts Vol. 20, EGU2018-6692-2, 2018, EGU General Assembly 2018.
- Renouard A., M. Grunberg, C. Doubre et al.. Building a precise seismic catalog in the intraplate northwestern European region from the AlpArray experiment (poster), Geophysical Research Abstracts Vol. 20, EGU2018-15328, 2018, EGU General Assembly 2018.

Millions of Europeans live on land formed by the Alpine orogeny and are affected by its topography, geology and associated natural hazards. The Alps have been intensely studied by geologists for more than a century, and they provide a unique natural laboratory to advance our understanding of orogenesis and its relationship to present and past mantle dynamics. While most concepts that underlie current studies of mountain belts and convergence dynamics were born in the Alps, this belt has not been given the attention it deserves using recent geophysical techniques. The ambition of the European AlpArray collaborative project is to re-evaluate the deep structures and processes that occur beneath the Alps by exploiting cutting-edge developments in geophysical imaging. AlpArray will provide unified and homogeneous seismological land-sea coverage of the greater Alpine area at an unprecedented scale and station density within Europe (~40 km average station spacing). AlpArray is timely, feasible and practically possible, as the scientific and technical obstacles have now all been overcome. The number of permanent seismic stations has risen significantly over the last decade, making the dense backbone with the number of available mobile broadband seismic pools feasible. The distributed European data distribution system EIDA is now operational and efficient. Finally, the seismological high-resolution imaging methods are not only available, but also practically feasible through the increase in computational capacity. With all these elements assembled, the enthusiasm for joining forces to install and operate AlpArray has been immediate across all of the ~15 countries involved.
The AlpArray-FR project is the French component of AlpArray. The French team will deploy and operate a temporary network of 80 land and 9 seafloor seismometers, located in Eastern France and the Mediterranean, in total providing more than 20% of the total data collected by AlpArray. In addition to participating in the collective research effort on the entire Alpine region, the AlpArray-FR team will focus particularly on the westernmost part of the Alps and adjoining areas, with special interest in the junction and interaction between the Alps and the Apennines – hence the inclusion of sea based observations in the Ligurian Sea. Despite negligible present-day horizontal shortening, the Western Alps and their junction with the Mediterranean Sea are among the most seismically active areas in Western Europe. AlpArray will improve the threshold of detection of this enigmatic seismicity at the scale of the Western Alps and their forelands including the highly populated Alpine valleys, the Rhône valley and the Mediterranean border. A third area of research is focussed on developing and adapting new methods of seismic imaging to the particularly complex and challenging target of the Alpine lithosphere-asthenosphere system, ranging from exploring the limits of seismic noise correlation techniques, and the combined inversion of complementary geophysical observables (e.g. seismic and gravity data) to full-waveform inversion.
Funding has already been secured in most partner countries and funding of this project by ANR this year would put the whole AlpArray project in a safer position. All equipment necessary for AlpArray-FR is already available through the national research facility RESIF (associated with the RESIF national investment project, ‘PIA EquipEx’). AlpArray-FR is moreover an excellent opportunity to add scientific value to RESIF. The dense seismic profiles necessary for the calibration of the 3D images are operated through a Chinese-French-Italian cooperation. AlpArray-FR includes 3 partners in charge of field operations and research, and 5 additional research partners. The research activities will take place during and for at least 5 years after the end of the present project and will be largely funded through other national, regional and local sources.

Project coordination

Anne PAUL (Institut des Sciences de la Terre)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

ISTEP Institut des Sciences de la Terre de Paris
ISTO Institut des Sciences de la Terre d'Orléans
Chrono-Environnement Chrono-Environnement
IPGP Institut de Physique du Globe de Paris
LGL-TPE Laboratoire de Géologie de Lyon, Terre, Planètes, Environnement
IPGS Institut de Physique du Globe de Strasbourg
GéoAzur GéoAzur
ISTerre Institut des Sciences de la Terre

Help of the ANR 620,000 euros
Beginning and duration of the scientific project: September 2015 - 48 Months

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