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Abstract: We use data from 24 broadband seismographs located south of the Eastern Himalayan plate boundary system to investigate the crustal structure beneath Northeast India. P wave receiver function analysis reveals felsic continental crust beneath the Brahmaputra Valley, Shillong Plateau and Mikir Hills, and mafic thinned passive margin transitional crust (basement layer) beneath the Bengal Basin. Within the continental crust, the central Shillong Plateau and Mikir Hills have the thinnest crust (30 ± 2 km) with similar velocity structure, suggesting a unified origin and uplift history. North of the plateau and Mikir Hills the crustal thickness increases sharply by 8–10 km and is modeled by ∼30∘ north dipping Moho flexure. South of the plateau, across the ∼1 km topographic relief of the Dawki Fault, the crustal thickness increases abruptly by 12–13 km and is modeled by downfaulting of the plateau crust, overlain by 13–14 km thick sedimentary layer/rocks of the Bengal Basin. Farther south, beneath central Bengal Basin, the basement layer is thinner (20–22 km) and has higher Vs (∼4.1 km s−1) indicating a transitional crystalline crust, overlain by the thickest sedimentary layer/rocks (18–20 km). Our models suggest that the uplift of the Shillong Plateau occurred by thrust faulting on the reactivated Dawki Fault, a continent margin paleorift fault, and subsequent back thrusting on the south dipping Oldham Fault, in response to flexural loading of the Eastern Himalaya. Our estimated Dawki Fault offset combined with timing of surface uplift of the plateau reveals a reasonable match between long‐term uplift and convergence rate across the Dawki Fault with present‐day GPS velocities. [ABSTRACT FROM AUTHOR]

The 3-D shear velocity structure beneath South India's Dharwar Craton determined from fundamental mode Rayleigh waves phase velocities reveals the existence of anomalously high velocity materials in the depth range of 50–100 km. Tomographic analysis of seismograms recorded on a network of 35 broad-band seismographs shows the uppermost mantle shear wave speeds to be as high as 4.9 km s–1 in the northwestern Dharwar Craton, decreasing both towards the south and the east. Below ∼100 km, the shear wave speed beneath the Dharwar Craton is close to the global average shear wave speed at these depths. Limitations of usable Rayleigh phase periods, however, have restricted the analysis to depths of 120 km, precluding the delineation of the lithosphere–asthenosphere boundary in this region. However, pressure–temperature analysis of xenoliths in the region suggests a lithospheric thickness of at least ∼185 km during the mid-Proterozoic period. The investigations were motivated by a search for seismic indicators in the shallow mantle beneath the distinctly different parts of the Dharwar Craton otherwise distinguished by their lithologies, ages and crustal structure. Since the ages of cratonic crust and of the associated mantle lithosphere around the globe have been found to be broadly similar and their compositions bimodal in time, any distinguishing features of the various parts of the Dharwar shallow mantle could thus shed light on the craton formation process responsible for stabilizing the craton during the Meso- and Neo-Archean. [ABSTRACT FROM PUBLISHER]

We report an optimum ID velocity model of the Kumaon-Garhwal Himalaya, based on analysis of --5000 seismograms generated at 50 digital broadband seismic stations in the region during April 2005 and June 2008. These include data from 385 local earthquakes with azimuth gaps of < 180°, each of which had a mini-mum of seven P- and five S-phase readings. This velocity model was then used to map the locations of 1150 earthquakes (1 < M < 5) that occurred in the region. A majority (~75%) of these earthquakes occurred in the upper 20 km of the crust, largely above the Main Himalayan Thrust, and forms an ~10-km-wide band along the surface trace of the Munsiari Thrust. However, we also find another parallel band of earthquakes, about 70 km to its southwest through the Tehri dam in the Lesser Himalaya, and a significant number (~15%) both in the Tethys Himalaya and within the flexed crust of the underthrusting Indian plate, beneath the Ganga basin. Furthermore, unlike the reported absence of seismicity in the lower crust of the Nepal Himalaya, and its reappearance in the shallow mantle, the Kumaon-Garhwal Himalayan crust is found to be seismically active throughout, with no event reliably recorded in the shallow mantle of this region during the epoch of our seismic recording. [ABSTRACT FROM AUTHOR]

Analysis of data from nine, temporary broadband seismic stations operated across West Bengal and Sikkim, along with publicly available data from seismographs in the surrounding region, provides the first image of the descending Indian Plate beneath the Darjeeling-Sikkim Himalaya. The down-going Indian crust is imaged by receiver function common conversion point stacking using data from 32 sites in combination with more detailed analyses from simultaneous modelling of receiver function data and Rayleigh wave group velocity dispersion at 13 stations. Compared to locations farther south on the Indian Shield our southernmost station shows evidence for thickened crust beneath the Mahanadi Rift basin with a possible mafic basal layer. North of the Mahanadi Rift the Indian Moho is ∼38 km deep below the Archean terranes of the northeast part of the Indian Shield. The Moho then dips gently northward beneath the Himalayan foreland basin reaching a depth of 44-48 km below the Himalayan foothills. Below Sikkim the Moho continues to deepen but there are indications of secondary structures in the receiver function image and modelling results suggesting some imbrication of the crust as it flexes downward. The crust thickens further beneath the Greater Himalaya and southern Tibet reaching depths of ∼65-70 km below the Southern Tibet Detachment (STD). Below the Lhasa Terrane north of the STD a double discontinuity exists with interfaces at 55-60 km and ∼80 km depth. There is a significant reduction in the average shear wave velocity of the crystalline crust between sites to the south of and on the Himalayan foreland basin and sites in the Himalaya and to the north. Below the Himalaya and southern Tibet the P-to- S conversion ( Ps) has a lower amplitude compared to that observed at sites on the undeformed Indian Shield. This decrease in amplitude of the Moho Ps phase could arise from a lower impedance contrast across the crust-mantle boundary or from scattering due to deformation of the crust and Moho. A coherent negative arrival beneath the Darjeeling-Sikkim Himalaya indicate the presence of a low velocity zone (LVZ), possibly associated with the Main Himalayan Thrust. This LVZ can be traced beneath the Darjeeling-Sikkim Himalaya but disappears beneath the Greater Himalaya. [ABSTRACT FROM AUTHOR]

We present the results of a study designed to investigate and compare the seismic characteristics of the once contiguous terranes of eastern Gondwanaland, now incorporated in five separated continental masses, which, during the Neoproterozoic (∼600 Ma) Pan-African orogeny, suffered a high degree of thermal stress and deformation. Receiver functions and surface wave data from stations located in East Antarctica, Sri Lanka, the southern-Indian high-grade terranes, Madagascar and the Tanzania–Mozambique belt, were used to determine the shear-wave velocity structure, Moho depth and values of the respective crustal segments. This study provides an additional dimension to the otherwise well-documented characteristic petrology of their surface exposures and other geological signatures such as their extensive granulitization and gem formation during the Pan-African event. Analysis of the receiver functions and surface wave data for these seismic stations located on their present day widely distributed continental fragments have been made. It is observed that with the exception of KOD (at Kodaikanal hill), situated on the southern Indian granulites having the thickest crust (∼43.5 km), most of the Pan-African granulitic terranes have a crustal thicknesses of , with a transition to higher velocity at mid-crustal depths, and that their bulk composition is felsic. Average crustal values and thicknesses , for four stations (SYO, PALK, TRV and ABPO), now located in East Antarctica, Sri Lanka, India and Madagascar, respectively, show remarkable similarity, indicating that the Pan-African orogeny was extensive enough to reorder the crustal structure of a wide region with a broadly similar stamp. [ABSTRACT FROM AUTHOR]

The effect of the 26 December 2004 Sumatra-Andaman earthquake on the Indian continent has been estimated from the analysis of GPS data from permanent and campaign GPS sites in the Indian continent. Co-seismic displacements at these sites have been determined for 11 permanent GPS stations of the national network, five campaign sites in southern India, and four campaign sites in Andaman and Nicobar Islands. The results indicate co-seismic eastward displacements of 12-20 mm in southern India almost directly west of Andaman, 1.8-6 mm in Central India and insignificant displacement in the Himalayas. Permanent sites in north-east India which lie almost towards the northward extension of the rupture plane show smaller co-seismic displacements ranging from 5 to 10 mm southward. Four campaign sites in the Andaman and Nicobar Islands show large horizontal co-seismic displacements of 1.6-6.49 m WSW and SW. Vertical displacement varies from an uplift of 0.6 m in north Andaman to 1.1 m subsidence at Car Nicobar. The observed GPS displacements are modelled using coulomb 2.6, and the slip on the four segments of the rupture plane (450 km×175 km; 250 km×140 km; 250 km×100 km; 150 km×100 km) that best fits both the far and near field displacements is estimated to be predominantly 12 m reverse in the southernmost segment, which slowly translates to an oblique slip of 7 m in the northernmost segment of the rupture plane. The seismic moment of the Sumatra-Andaman earthquake for the above rupture plane and slip is M o = 5.21×1022 Nm, which corresponds to a moment magnitude of M w = 9.1. [ABSTRACT FROM AUTHOR]

We present a new algorithm for 1D magnetotelluric (MT) data inversion. It inverts a normalized impedance response function derived from the classical Cagniard impedance function. The scheme transforms the nonlinear problem of estimating layer resistivities and thicknesses into a linear problem of estimating the coefficients of power series of the new response function. This is achieved by working with a model where each layer has a thickness of constant penetration. The first coefficient of the series provides top-layer resistivity, which, in conjunction with the constant penetration parameter, then provides the layer thickness. The scheme employs a recurrence relation developed between the coefficients of the power series of two successive layers. This relation is used to continue downward and estimate the remaining layer resistivities and thicknesses. The scheme has been tested on a synthetic model and on three well-studied data sets relating to deep, intermediate, and shallow exploration. [ABSTRACT FROM AUTHOR]

We use seismograms from regional earthquakes recorded on digital seismographs in peninsular India to determine the frequency-dependent Q of Lg for the Indian platform. We measure Lg attenuation by determining the decay of spectral amplitudes with distance. The available data suggest some spatial variation in attenuation but a much denser ray-path coverage would be required to validate such observations. We, therefore, combine all the measurements of overlapping regions that span both the shield and intervening terranes to obtain an average value of attenuation for the Indian platform: Lg Q = 665 ± 10 with the frequency exponent n = 0.67 ± 0.03. This average value of Lg attenuation for the Indian platform is similar to the average for other stable regions of the globe. [ABSTRACT FROM AUTHOR]

We investigate the upper mantle shear-wave speed structure beneath the south Indian shield by measuring and modeling fundamental mode Rayleigh wave phase-velocity dispersion. Observed phase velocities for the south Indian shield closely match those observed for the Canadian shield. We constrain the south Indian crust using published receiver function results and invert the dispersion data for upper mantle shear-wave structure. The ∼155-km-thick seismic lithosphere of the south Indian shield is composed of a 35 km-thick, two-layer crust and a ∼120-km-thick, high-velocity upper mantle lid. Beneath the Moho the average Sn wave speed is ∼4.7 km sec-1. Both Sn travel times data and the dispersion data suggest a positive sub-Moho shear-wave speed gradient. Beneath the seismic lithosphere there is a low-velocity layer where the shear-wave speed drops to ∼4.4 km sec-1. [ABSTRACT FROM AUTHOR]