SUMMARY 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.

Teleseismic receiver function analysis of seismograms recorded on a ∼700 km long profile of 17 broadband seismographs traversing the NW Himalaya shows a progressive northward deepening of the Indian Moho from ∼40 km beneath Delhi south of the Himalayan foredeep to ∼75 km beneath Taksha at the Karakoram Fault. Similar studies by Wittlinger et al. (2004) to the north of the Karakoram Fault show that the Moho continues to deepen to ∼90 km beneath western Tibet before shallowing substantially to 50–60 km at the Altyn Tagh Fault. The continuity of the Indian Moho imaged in the receiver functions reported here, along with those of Wittlinger et al. (2004), suggest that in this part of the Himalayan orogen the Indian plate may penetrate as far as the Bangong Suture, and possibly as far north as the Altyn Tagh.

SUMMARY 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.

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SUMMARY We present group velocity dispersion results from a study of regional fundamental mode Rayleigh waves propagating across the Indian region. 1‐D, path‐averaged dispersion measurements have been made for 1001 source–receiver paths and these combined to produce tomographic images between 15 and 45 s period. Because of the dense station coverage in peninsular India, these images have substantially higher lateral resolution for this region than is currently available from global and regional group velocity studies. Testing of the group velocity model shows that the average resolution across the region is about 7.5° for the periods used in this study. The tomographic maps demonstrate that while the Indian shield is characterized by high crustal and uppermost‐mantle group velocities, comparatively lower velocities exist beneath the Himalaya due to the thickened crust and beneath the Gangetic plains caused by the mollasse sediments and recent alluvium cover in the Himalayan foredeep. Northeastern India north of the Shillong Plateau also displays higher velocities, similar to the south Indian shield, indicative of colder crust beneath the region. The northern Bay of Bengal shows extremely low velocities due to the thick sediment blanket of the Bengal fan. Likewise, the Katawaz Basin in southern Pakistan shows lower velocities that resemble those seen in the Bay of Bengal. The geometry of the velocity contours south of the Katawaz Basin closely matches the prograding Indus fan in the Arabian Sea. Finally, the Tibetan Plateau has lower group velocities compared to the Indian shield at all periods as a result of the thick crust beneath southern Tibet.