S U M M A R YMore than 340 earthquakes recorded by the Institute of Geophysics, University of Tehran (IGUT) short period stations from 1996 to 2004 were analysed to estimate the S-coda attenuation in the Alborz region, the northern part of the Alpine-Himalayan orogen in western Asia, and in central Iran, which is the foreland of this orogen. The coda quality factor, Q c , was estimated using the single backscattering model in frequency bands of 1-25 Hz. In this research, lateral and depth variation of Q c in the Alborz region and central Iran are studied. It is observed that in the Alborz region there is absence of significant lateral variation in Q c. The average frequency relation for this region is Q c = 79 ± 2f 1.07±0.08 . Two anomalous high-attenuation areas in central Iran are recognized around the stations LAS and RAZ. The average frequency relation for central Iran excluding the values of these two stations is Q c = 94 ± 2f 0.97±0.12 . To investigate the attenuation variation with depth, Q c value was calculated for 14 lapse times (25, 30, 35, . . . 90s) for two data sets having epicentral distance range R < 100 km (data set 1) and 100 < R < 200 km (data set 2) in each area. It is observed that Q c increases with depth. However, the rate of increase of Q c with depth is not uniform in our study area. Beneath central Iran the rate of increase of Q c is greater at depths less than 100 km compared to that at larger depths indicating the existence of a high attenuation anomalous structure under the lithosphere of central Iran. In addition, below ∼180 km, the Q c value does not vary much with depth under both study areas, indicating the presence of a transparent mantle under them.The attenuation of short-period S waves, expressed as the inverse of the quality factor (Q −1 ), helps in understanding the physical laws related to the propagation of the elastic energy of an earthquake through the lithosphere. Seismic waves in the Earth attenuate with distance at rates greater than predicted by geometrical spreading. The contributing factors are intrinsic attenuation due to the medium anelasticity, and scattering attenuation associated with the inhomogeneities. Knowledge of the relative contributions of scattering (Q s −1 ) and intrinsic (Q i −1 ) attenuation is important for appropriate subsurface material identification, tectonic interpretations and quantification of the ground motion (e.g. Hoshiba 1993; Akinci et al. 1995;Del Pezzo et al. 1995;Bianco et al. 1999Bianco et al. , 2002. Attenuation inferred from the decay rate of the coda (Aki & Chouet 1975;Singh & Herrmann 1983;Sato & Fehler 1998) is a combination of scattering and intrinsic attenuations. The intrinsic attenuation is associated with small-scale crystal dislocations, friction, and movement of interstitial fluids. The scattering attenuation, associated with an elastic process of redistributing wave energy by reflection, refraction and conversion at irregularities in the medium, is often characterized by an exponential attenuation quality factor, Q s...

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The quality factor of coda (Q c ) waves has been estimated by using single backscattering and single isotropic scattering models. The earthquakes used were recorded by three permanent and one temporary network located in the central and eastern Alborz, Iran. The database was composed of 746 local earthquakes with local magnitude from 1.1 to 5.7. The estimated Q c has been found to be similar for lapse times greater than twice the S-wave travel time (2t S ) for both methods.The estimated Q c for central frequencies < 1:0 Hz shows less frequency dependency compared with the higher frequencies. By using a Q 0 f n relation, the average frequency dependence of Q c for the whole area has been estimated as 59f 1:03 , 69f 0:97 , 78f 0:97 , 105f 0:93 , 123f 0:89 , 159f 0:79 , and 203f 0:68 for central lapse times 30, 40, 50, 65, 85, 110, and 155 s, respectively. We found the value of Q 0 decreases at an average depth of 78 km, which may be the result of high dissipating media at this depth. The average Q c values, estimated for central and eastern Alborz and their frequencydependent relationships are similar to those of tectonically active regions.

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.

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Complex interaction of rheologically contrasting layers within the lithosphere during the collision of continental plates leads to active faulting, which represents a serious hazard to the population and infrastructure. One of the collision scenarios presumes the existence of a middle-lower crustal channel composed of subducted silicic upper crustal rocks, which is thought to exist in the Tibetan-Himalayan system. Based on the results of seismic tomography, we argue that a similar mechanism of crustal channeling takes place beneath the Zagros mountain system in southwestern Iran. The 3D seismic velocity model reveals an inverted crustal architecture of the collision zone, in which the low-velocity felsic (granitic and sedimentary) upper crustal rocks of the Arabian plate form a seismically inactive lower crustal channel below the higher-velocity mafic (basaltic) middle-upper crustal layer of the Iranian crust. Based on existing numerical models, we suggest that the formation of the felsic channel is likely governed by separation (delamination) of the weak felsic upper crust of the subducting Arabian lithosphere and its ductile underplating under rheologically stronger upper-middle crust of the Iranian plate.

To obtain the shear velocity structure across North-West of Iran and surrounding areas to a depth of 160 km, we performed a namely Hedgehog nonlinear inversion on Rayleigh wave group velocity dispersion curves in the period range from 7 to 60 s. The distributed dispersion curves are the results of our surface wave dispersion tomography using the data of 280 local and regional seismic events, recorded by the medium-and broad-band seismic stations in the region. We outline different crust and upper mantle structures for the study area based on calculated group and shear velocities. Our results reveal relatively low velocities at the shorter periods (7-10 s) in the presence of sedimentary basins (e.g., South Caspian Basin) and for eastern Anatolia and relatively high velocities along the Sanandaj-Sirjan Metamorphic zone, Alborz, Talesh, and the Lesser Caucasus Mountains. By depth inversion of group velocities, we observed 14-km-thick sediments in South Caspian Basin and Kura Depression. Based on our maps at 20 s, we outline different crustal models for the region and highlight the differences between South Caspian Basin and NW Iran, on one side, and the similarities between the South Caspian Basin and Kura Depression that extend beneath Talesh, Alborz, and Lesser Caucasus, on the other. Comparing the shear velocity of lower crust in South Caspian Basin and Kura Depression with that of NW Iran proves different origination of lower crust in the basin, probably oceanic source, because of its significant higher shear velocity rather than NW Iran. In Talesh, we observe indications of an under-thrusting of the lower crust of SCB beneath NW Iran while the middle crust is locked. The analysis of group velocities at longer periods (≥ 35 s) and obtained shear velocity models allows us to outline different lithospheric structures and crustal depth in the region. The high group velocities in Talesh, South Caspian Sea, and Lesser Caucasus on one side and Zagros Folding and Thrust Belt on the other, beside the result of shear velocity models, suggest the presence of a stable and thick mantle lid that seems to be thin or absent in the eastern Anatolia and much of NW Iran. The shallowest Moho

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