Shear-Wave Velocity Tomography of the Lithosphere-Asthenosphere System beneath the Iranian Plateau

Abstract

We conducted a tomographic inversion of Rayleigh-wave dispersion to obtain 2D phase and group velocity tomographic images in the 10-100 s period range and shear-wave velocity structures for the Iranian plateau. For this purpose, the fundamental mode of Rayleigh waves, recorded along 1586 paths by 29 broadband stations, was identified by applying the frequency time analysis (FTAN) to each epicenter-station path which simultaneously satisfies the two-station method conditions. The fundamental modes identified by FTAN have been used to determine the path-average interstation phase and group velocities at selected periods. With this procedure, 243 group and phase velocity dispersion curves were processed to obtain tomographic maps by applying the Yanovskaya-Ditmar formulation for periods in the 10-100 s range. Averaged dispersion curves of phase and group velocities, which represent six rather homogeneous regions, are computed. Finally, we used a fully nonlinear inversion procedure to derive tomographic images of the elastic structure of the lithosphere and asthenosphere of the six main structural and seismotectonic features of the Iranian plateau. The dense path coverage in the Iranian plateau permits us to produce images that have substantially higher lateral resolution compared to images currently available from global and regional group velocity studies. Tomographic maps at high frequencies are well correlated with the upper crust structure, especially with sediment layers thickness. Estimated shear-wave velocity structures are well correlated with the shield-like lithosphere structure in Zagros. A low-velocity zone (LVZ) is not detected in Alborz or the south Caspian basin, which can imply thrusting of the oceanic crust of the southern Caspian Sea under the Alborz to the south. LVZs are derived for the region east of Iran, central Iran, and Kopeh Dagh.

The deep structure of the Iranian Plateau

“…The data of the fundamental mode Rayleigh-wave group velocities are extracted from the tomographic study by Rahimi et al (2014). Group velocities from teleseismic events recorded at 29 permanent broad-band stations distributed coverings Iran were measured for the fundamental mode Rayleigh waves, for the 10-100 sec period range.…”

“…More than 240 inter-station group velocity dispersion curves, covering all Iran Plateau, were inverted to prepare tomographic maps by applying the 2-D tomography method described by Yanovskaya and Ditmar (1990). The tests presented by Rahimi et al (2014) show that resolution length (the mean size of the averaging area) of the tomographic maps is mainly less than 100 km along the Iran profile. The available dispersion data in the region are discretized using a uniform 0.5 o ×0.5 o grid, therefore, in the joint inversion, for each station the fundamental mode Rayleigh wave dispersion is taken from the relevant tomographic 0.5 o ×0.5 o cell containing that station.…”

“…The drawbacks intrinsic in such problems can be minimized by considering reliable starting models obtained from other studies. The initial models used in this study are taken from Rahimi et al (2014) who has presented a set of S-wave velocity models for each tectonic area of Iran…”

“…In order to find the optimal parameterization for the joint inversion, we assign to each layer of the model a thickness equal to the IS used in the non-linear inversion of dispersion measurements reported by Rahimi et al (2014). The IS is dependent on several parameters, including the dispersion curve error (e.g.…”

“…1) down to 300 km depth, the maximum penetration depth of our data set. The depth resolution of the data set is determined by the partial derivatives (Urban et al, 1993) of the dispersion curves of the Rayleigh wave fundamental mode with respect to the shear wave velocity at different periods (Rahimi et al, 2014). The lithosphere thickness is marked in Figure 7 by white squares.…”