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Observations of SKS splitting in the Central External Dinarides-Adria collision zone

Seismic anisotropy beneath the greater region of the Central External Dinarides is estimated from observations of SKS splitting. The area is located in the broad and complex Africa-Eurasia convergent plate boundary zone, where the Adriatic microplate interacts with the Dinarides. We analyzed recordings of 12 broadband stations located in the Croatian coastal region. Evidence of seismic anisotropy was found beneath all stations. Fast axis directions are fairly uniformly oriented approximately in the NE-SW to NNE-SSW direction, perpendicularly to the strike of the Dinarides. Average delay times range between 0.6 and 1.0 seconds. A counter-clockwise rotation in average fast axis directions was observed for the stations in the northern part with respect to the stations in the southern part of the studied area. Fast axis directions coincide with the assumed direction of asthenospheric flow through a slab-gap below Northern and Central External Dinarides, with the maximum tectonic stress orientation in the crust, and with fast directions of Pg and Sg- waves in the crust. These observations suggest that the detected SKS birefringence is primarily caused by the preferred lattice orientation of mantle minerals generated by the asthenospheric flow directed SW-NE to SSW-NNE, with a possible contribution from the crust.

anisotropy, upper mantle, shear wave splitting, Dinarides, Adria

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