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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]

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 use broad-band teleseismic data recorded at eight sites along a north–south profile from Karimganj (24.84°N, 92.34°E), south of the eastern Shillong Plateau, to Bomdilla (27.27°N, 92.41°E) in the eastern Lesser Himalaya, to determine the seismic characteristics of the crust in northeastern India. We also analyse data from the Chinese Digital Seismic Network station at Lhasa and INDEPTHII stations located on the southern Tibetan Plateau north of our profile, to extend the seismic images of the crust further northwards. Although the northeastern Indian stations and the Tibetan stations do not lie along a linear profile across the Himalaya, the well-recognized uniformity of the Himalaya along strike make this comparison of the two profiles meaningful. Receiver functions calculated from these data show that the crust is thinnest (∼35–38 km) beneath the Shillong Plateau. Receiver functions at Cherrapunji, on the southern edge of the Shillong Plateau, have a strong azimuthal dependence. Those from northern backazimuth events show that the Moho beneath the southernmost Shillong Plateau is at a depth of∼38 km while receiver functions from southern backazimuth events indicate that the Moho beneath the northernmost Bengal Basin is at a depth of∼44 km. Receiver functions from sites on the Brahmaputra Valley demonstrate that the Moho is deeper by∼5–7 km than below the Shillong Plateau, a result which agrees with the hypothesis that the Shillong Plateau is supported by shearing stress on two steep faults that cut through the crust. Further north of the eastern Himalayan foredeep, the Moho dips gently northwards, reaching a depth of∼48 km beneath Bomdilla in the Lesser Himalaya, and 88 km below Lhasa in Tibet. Using the crustal velocity models obtained from receiver function inversions, we redetermined focal depths of well-recorded earthquakes across this part of the Indo-Tibetan collision zone and find all of these to occur within the crust. Hence we find no evidence for bimodal depth distribution of earthquakes beneath this region of northeastern India. [ABSTRACT FROM AUTHOR]

Observations of relative motion in a geodetic network in Ladakh, India, and across southern Tibet indicate slow shear on the Karakorum fault, rapid east-west extension across the whole of southern Tibet, and constant arc-normal convergence between India and southern Tibet along the Himalayan arc. Measurements of ten campaign-style and six permanent sites with global positioning system (GPS) precise geodesy provide these bounds on the style and rates of the large-scale deformation in the Tibet-Himalaya region. Divergence between sites at Leh, Ladakh, India, and Shiquanhe, western Tibet, as well as slow relative motion among sites within the Ladakh network, limit right-lateral slip parallel to the Karakorum fault to only 3.4 ± 5 mm/yr. This low rate concurs with a recent estimate of 3–4 mm/yr for Late Holocene time, but disagrees with the much higher rate of 30-35 mm/yr that has been used to argue for plate-like behavior of the Tibetan Plateau. Convergence between Ladakh and the Indian subcontinent at 18.8 ± 3 mm/yr at 224° ± 17° (1σ) differs little from estimates of convergence across the central segment of the Himalaya. Finally, lengthening of the baseline between Leh, Ladakh, and Lhasa (in southeastern Tibet) at 17.8 ± 1 mm/yr or between Leh and Bayi (farther to the southeast) at 18 ± 3 mm/yr, is consistent with an extrapolation of rates of east-west extension of the Tibetan Plateau based both on shorter GPS baselines (e.g., Lhasa-Simikot) and on diverging slip vectors of earthquakes in the Himalaya. We interpret these results to indicate that Tibet behaves more like a fluid than like a plate. [ABSTRACT FROM AUTHOR]