Contract nr. ANR-06-BLAN-0317

SIMBAAD project Seismic Imaging of the Mantle Beneath the Aegean-Anatolian Domain

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Scientific background and objectives Description of project, methodology Expected results Map of seismic stations Scientific background and objectives The Aegea-Anatolia system is a good place to test competing hypotheses on how the surface kinematics is related to mantle flow, firstly because we have a fair idea of motions in the region and of the internal deformation, which vary widely over the region, and secondly because the past deformation is reasonably well known and differs significantly from the present deformation. Several hypotheses have been proposed to explain the dynamics of this system. Most of them involve lateral heterogeneities in the mantle (e.g. slab break-off). However, fair-resolution data on mantle structure and deformation remain scarce. Seismic tomography (using either body waves or surface waves) is a powerful method to investigate the structures of the upper mantle. Seismic anisotropy related to the lattice-preferred-orientation of anisotropic crystals (especially olivine) is considered as a good proxy for past and present mantle flow. Description of project, methodology We have installed a temporary array of ~34 seismic stations over continental Greece, the Aegean, S-Bulgaria and Western Turkey (see map). It was equipped with broadband velocimeters (STS-2, 120s natural frequency, and CMG-3ESP, 90s natural frequency). It was designed to fill-in the gaps between permanent broadband stations, reducing the interstation spacing to ~60 km in continental areas and ~80 km in the Aegean. In order to get enough reliable data with good azimuthal and distance coverage, we ran the network for 2 years (April 2007 - May 2009). In addition, we installed 2 N-S transects of 23 stations each with intermediate-period sensors (20 to 60s natural frequency) in Western Anatolia (see map). Each transect was run for 1 year. The array was installed and maintained with the support of KOERI (Kandilli Observatory and Earthquake Research Institute, Istanbul) in Turkey, the Geophysical Laboratory of AUTH (Aristotle University of Thessaloniki) in Greece and the Geophysical Institute of the Bulgarian Academy of Science (Sofia) in Bulgaria. Portable seismic stations mostly belong to the French national pool of mobile seismic instruments Sismob, plus a few stations from LGIT (Grenoble), AUTH (Thessaloniki) and Laboratoire de Géologie de l'ENS (Paris). An important deliverable of the Simbaad project is the dataset of continuous seismic records, which includes data of the temporary experiments (80 stations), but also data of permanent broadband networks of Greece and Turkey (~80 stations). These records are now being analyzed to image the velocity structure of the crust and upper mantle beneath the Aegean-Anatolia domain at a resolution of a few tens of kilometers. We are using all seismological techniques appropriate for such a study: body-wave and surface-wave tomography, tomography from ambient noise correlations, receiver functions, SKS splitting and surface-wave anisotropy.   Expected results We expect to obtain a 3-D picture of the velocity structure beneath Greece and Western Anatolia to a depth of ~300 km. The resolution should be of several tens to a hundred of km, that is 2-3 times better then in the global or regional tomography models that were available at the start of the project. This velocity structure will give constraints on the evolution of the slab beneath Anatolia and the Aegean. It will also give constraints on mantle heterogeneities and flow across a geodynamical system with strong spatial variations. Results of the Simbaad experiment will be compared with existing data, especially with structural geology, and they will used as input to numerical and analogue experiments to model the dynamics of the system.