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S U M M A R YWe analysed local earthquake waveforms recorded on a broad-band seismic network in northwestern Himalayas to compute the intrinsic and scattered attenuation parameters from coda waves. Similar to other tectonically active and heterogeneous regions, attenuation-frequency relation for western Himalaya iswhere Q c is the coda Q parameter. Intrinsic (Q −1 i ) and scattering (Q −1 s ) attenuations was separated using Q c and direct S-wave Q data (Q d ). It is observed that estimated Q −1 c is close to Q −1 i and both of them are much larger than Q −1 s suggesting that coda decay is predominantly caused by intrinsic attenuation. At higher frequencies, both the attenuation parameters Q c and, Q d are similar indicating that coda is predominantly composed of back-scattered S waves at these frequencies.

We have evolved 3‐D seismic velocity structures in northeast India region and its adjoining areas to understand the geodynamic processes of Indian lithosphere that gently underthrusts under the Himalayas and steeply subducts below the Indo‐Burma Ranges. The region is tectonically buttressed between the Himalayan arc to the north and the Indo‐Burmese arc to the east. The tomographic image shows heterogeneous structure of lithosphere depicting different tectonic blocks. Though our results are limited to shallower depth (0–90 km), it matches well with the deeper continuation of lithospheric structure obtained in an earlier study. We observe low‐velocity structure all along the Eastern Himalayas down to ~70 km depth, which may be attributed to deeper roots/thicker crust developed by underthrusting of Indian plate. Parallel to this low‐velocity zone lies a high‐velocity zone in foredeep region, represents the Indian lithosphere. The underthrusting Indian lithosphere under the Himalayas as well as below the Indo‐Burma Ranges is well reflected as a high‐velocity dipping structure. The buckled up part of bending Indian plate in study region, the Shillong Plateau‐Mikir Hills tectonic block, is marked as a high‐velocity structure at shallower depth. The Eastern Himalayan Syntaxis, tectonic block where the two arcs meet, is identified as a high‐velocity structure. The Bengal Basin, tectonic block to the south of Shillong Plateau, shows low velocity due to its thicker sediments. Based on the tomographic image, a schematic model is presented to elucidate the structure and geodynamics of Indian lithosphere in study region.

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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...