P and S deep velocity structure of the Hellenic area obtained by robust nonlinear inversion of travel times

We derived a new model for the P and S velocity structure of the Lithosphere in the subduction area of the Hellenic arc and the Aegean Sea from the inversion of travel times of local events. The inversion technique applied is nonlinear, since three-dimensional ray tracing is incorporated. At the same time, an appropriate preconditioning of the final linearized system is used in order to reduce ray density effects on the results. The P-S coherency is controlled by an additional damping of the V-P/V-S ratio. The study focuses mainly on the structure of the Hellenic subduction in the southern Aegean region. Interesting features and details of the subducted slab can be recognized in the final tomographic images. On the western part, the shallower section of the slab is dipping at a very small angle (similar or equal to 10 degrees). This changes to approximately 25 degrees for the deeper part of the subduction, resulting in a prominent kink at a depth of about 70 km, which is in accordance with the general characteristics of the associated Benioff zone. The smaller eastern part of the slab is not resolved with the same detail, but a clearly steeper subduction zone can be recognized. Moreover, detailed information about the crustal thickness variations are inferred from the velocity structure and correlate very well with the existing results from refraction experiments. Finally, the results confirm the recent suggestion concerning the existence of a low-velocity crustal layer at shallow depths (similar or equal to 10 degrees-15 km) in the accretionary prism of the Alpine belt.