Shear velocity structure in the Aegean area obtained by inversion of Rayleigh waves

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SUMMARY

The purpose of this work is to derive a 3-D tomographic image of the shear wave velocity structure of the crust—uppermost mantle in the Aegean area using the group velocities of Rayleigh wave fundamental mode. The database consists of 185 regional earthquakes recorded at broad-band stations that were installed for a period of 6 month in the Aegean area within the framework of a large-scale experiment. In a previous work (Karagianni et al. 2002), an averaged group velocity has been determined using the method of frequency time analysis (FTAN) for each epicentre–station ray path and the data were used in order to determine the local group velocities for different periods over the area covered by the seismic ray paths. Taking into account additional resolution results obtained for the local group velocities, a grid of 0.5° was adopted for the Aegean area and a local dispersion curve was defined for each gridpoint. More than 80 local dispersion curves were finally inverted using a non-linear inversion approach, deriving the corresponding 1-D shear velocity models. The interpolation of these models resulted in a 3-D S-wave tomographic image of the crust and uppermost mantle in the broader Aegean area. As expected, as a result of the complex tectonic setting of the Aegean area, strong lateral variations of the S-wave velocities of the crust and uppermost mantle of the studied area are found. In the southern Aegean sea, as well as in a large part of the central Aegean sea a thin crust of approximately 20–22 km is observed, whereas the remaining Aegean sea area exhibits a crustal thickness less than 28–30 km. On the contrary, a crustal thickness of 40–46 km is observed in western Greece along the Hellenides mountain range, whereas in the eastern continental Greece the crust has a typical thickness of approximately 30–34 km.

For shallow depths (<10 km) low S-wave velocities are observed under the sedimentary basins of the north Aegean sea, the Gulf of Thermaikos (Axios basin) and western Greece. At depths ranging from 10 to 20 km, low S-wave velocities are mainly found in western Greece under Peloponnesus as well as in Rhodes. This low-velocity zone seems to extend along the Hellenic arc and can be correlated to the Hellenides mountain range and the Alpine orogenesis, in agreement with previous P-wave tomographic results. In the southern Aegean sea very low S-wave velocities (3.6–4.0 km s−1) are observed at depths of approximately 30–40 km just below the Moho discontinuity, while in the rest of the inner Aegean sea and continental Greece the uppermost mantle is characterized by velocities around 4.3–4.4 km s−1. This low-velocity zone in the southern Aegean sea can be associated with the high temperatures and the presence of significant percentage partial melt in the mantle wedge of the southern Aegean subduction zone, in agreement with previous studies.

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