High-resolution surface wave tomography beneath the Aegean-Anatolia region: constraints on upper-mantle structure

Abstract

SUMMARY This study provides new constraints on the upper‐mantle structure from western Greece to central Anatolia using seismic data of permanent broad‐band networks recently installed in Greece and Turkey and from a two‐year temporary array (SIMBAAD experiment). We used ∼200 seismic events recorded at 146 broad‐band stations with a typical interstation distance of 60–100 km across the study area. The high‐resolution 3‐D shear wave velocity model of the mantle is obtained by inversion of fundamental‐mode Rayleigh wave phase velocity maps for periods between 20 and 195 s. The tomography is based on ray tracing in heterogeneous media taking into account external propagation effects. The horizontal resolution is approximately 100 km, however small heterogeneities may suffer from some horizontal smearing and damping. The vertical resolution is approximately 100 km. The vertical smoothing is necessary to avoid unresolved spurious shear wave velocity oscillations in the upper mantle. The errors on shear wave velocities in our 3‐D model (0.02–0.1 km s−1) are significantly smaller than the amplitude of Vs variations (0.3–0.5 km s−1). In spite of the vertical and horizontal smoothing, our model shows details in the upper‐mantle structure never reached at regional scale in the area. The overall structure is characterized by a low‐velocity zone (80–200 km depth) reflecting a slow and warm asthenosphere underlying a thin lithosphere. The southwesternmost termination of the low‐velocity anomaly corresponds to the northward dipping Hellenic slab. The detailed shear velocity structure of the upper mantle beneath Anatolia appears to be far more geometrically complex than revealed in previous tomographic studies of the area. At depths larger than or equal to 160 km, velocities are overall high beneath Anatolia, partly due to the presence of dipping high‐velocity anomalies which we tentatively interpret as remnant slabs. The southernmost high‐velocity anomaly beneath Anatolia is separated from the eastern edge of the Hellenic slab by a major low‐velocity anomaly which we interpret as the trace of asthenospheric mantle material rising inside a vertical slab tear beneath southwestern Anatolia.

Arrival angle anomalies of Rayleigh waves observed at a broadband array: a systematic study based on earthquake data, full waveform simulations and noise correlations

“…Several groups have proposed surface wave tomography methods that take into account non-plane incoming waves (e.g. Friederich & Wielandt 1995;Bruneton et al 2002;Chevrot & Zhao 2007;Lin & Ritzwoller 2011;Salaün et al 2012). Li et al (2003) specifically assumed that the incoming wave field can be adequately approximated by a sum of two incident waves, an assumption which according to our results seems to be adequate at 20-40 s period in many cases.…”

“…In any case, all the methods that take into account propagation effects outside an array should be parametrized to allow for large deviations from the great-circle. In previous 3-D imaging (Bruneton et al 2004;Salaün et al 2012) we observed that the single main parameter that improved fit to data was the propagation direction, and that curvatures in the wave front remained a secondary effect.…”

“…Snieder 1986;Friederich 2003;Chen et al 2007;Fichtner et al 2013) or when the local structure is imaged below a large array (e.g. Friederich & Wielandt 1995;Bruneton et al 2002;Chevrot & Zhao 2007;Lin & Ritzwoller 2011;Salaün et al 2012).…”

The dynamics of laterally variable subductions: laboratory models applied to the Hellenides

“…A direct comparison with the Hellenic subduction zone at this stage is more hazardous since only one slab tear between the western edge of the Cyprus slab and the eastern edge of the Hellenic slab is clearly imaged by tomographic models (e.g. Salaün et al, 2012). The following stages instead more likely resemble the Hellenic subduction zone with the introduction of a lateral buoyancy variation, similar to the difference in slab buoyancy across the Kefalonia region, that diverts horizontal mantle flow from its previous symmetry.…”

Kinematic reconstructions and magmatic evolution illuminating crustal and mantle dynamics of the eastern Mediterranean region since the late Cretaceous

“…The width of this continuously subducting lithosphere was then probably reduced by several episodes of slab tearing, leading to an increase of its retreating rate. Since the middle Miocene, large-scale block rotations ( figure 3) and evolution of magmatism in the Cyclades and western Anatolia (see details in Section 5.5) support the existence of a slab tear below western Anatolia, as suggested by several tomographic models [de Boorder et al, 1998;Wortel and Spakman, 2000;Piromallo and Morelli, 2003;Jolivet et al, 2009Jolivet et al, , 2013Brun and Sokoutis, 2010;Salaün et al, 2012]. The reduced width of the slab thus could have induced its fast retreat and the fast rotation of the Hellenic trench from 15 to 8…”