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Free keywords:
Anisotropy, Shear-wave splitting, Upper-mantle flow, Vrancea seismic region, Intermediate-depth earthquakes
Abstract:
The Vrancea region, in the Southeastern Carpathians (Romania), represents a unique case among the seismic areas in the world taking into account the extreme concentration and persistence of seismicity and the tectonic stress field. Subduction in a post-collisional phase is still active in a narrow area located at the sharp bend of the mountain belt. Our goal is to show that the particular shape of the shear-wave splitting can be interpreted in the light of the decoupling and slab-retreat processes, which hypothetically induce a specific configuration of the upper-mantle flow. Shear-wave splitting of SKS phases shows a relatively coherent pattern outside the epicentral area, suggesting a prominent NE-SW anisotropy, in agreement with previous estimations performed in Central and Eastern Europe and following the trends of the deformation field as outlined by the GPS measurements. A clear change is pointed out inside the Vrancea area, where strike-parallel polarization is emphasized. Toward the NW (wedge side), the polarization turns to a strike-perpendicular direction in agreement to an upwelling asthenospheric flow in the back-arc region (i.e., polarization aligned to the local strike of the slab). These shear-wave splitting attributes are not consistent with conventional models of 2-D mantle flow near subduction zones, nor with a sub-vertical down-dipping flow driven by the sinking of the slab. They correlate well with lateral inhomogeneities outlined by the tomography image, heat flow, seismic-wave attenuation and thermal field. We suggest that the eastward slab retreat, and decoupling between the underlying asthenosphere and the slab itself, have induced strike-parallel mantle flow, likely favoring detachment of the slab along the arcuate mountain belt. These processes are directly related to the strong anisotropy observed in the SE Carpathians. The anisotropy and GPS data suggest a strong coupling of the surface and mantle processes.
