The Indo-Burman arc is the boundary between the India and Burma plates, north of the Sumatra–Andaman subduction zone. The existence of active subduction in the Indo-Burman arc is a debatable issue because the Indian plate converges very obliquely beneath the Burma plate. Recent GPS measurements in Bangladesh, Myanmar, and northeast India indicate 13–17 mm/y of plate convergence along a shallow dipping megathrust while most of the strike-slip motion occurs on several steep faults, consistent with patterns of strain partitioning at subduction zones. A short period of instrumentally recorded seismicity and sparse historical records are insufficient to assess the possibility of great earthquakes at the Indo-Burman megathrust. Using the advantage of the Block-and-Fault Dynamics model allowing simultaneous simulation of slow tectonic motions and earthquakes, we test the hypothesis whether the India-Burma detachment is locked and able to produce great earthquakes, or it slips aseismically? We have shown that the model of locked detachment is preferred because it more adequately reproduces observed tectonic velocities. The integral characteristics of synthetic seismicity, the earthquake size distribution, and the rate of seismic activity are consistent with those derived from observations. Our results suggest that the megathrust is locked and can generate great M8+ earthquakes. The estimated average return period of great events exceeds one thousand years. Earthquakes of this size pose a great threat to NE India, Bangladesh and Myanmar, the most densely populated areas of the world.

We image the lateral variations in the Moho depths and average crustal composition across the Kumaon–Garhwal (KG) Himalaya, through the H–K stacking of 1400 radial PRFs from 42 three-component broadband stations. The modelled Moho depth, average crustal Vp/Vs, and Poisson’s ratio estimates vary from 28.3 to 52.9 km, 1.59 to 2.13 and 0.17 to 0.36, respectively, in the KG Himalaya. We map three NS to NNE trending transverse zones of significant thinning of mafic crust, which are interspaced by zones of thickening of felsic crust. These mapped transverse zones bend toward the north to form a NE dipping zone of maximum changes in Moho depths, below the region between Munsiari and Vaikrita thrusts. The 1991 M_w6.6 Uttarakashi and 1999 M_w6.4 Chamoli earthquakes have occurred on the main Himalayan thrust (MHT), lying just above the mapped zone of maximum changes in Moho depths. Modelled large values of average crustal Vp/Vs (> 1.85) could be attributed to the high fluid (metamorphic fluids) pressure associated with the mid-crustal MHT. Additionally, the serpentinization of the lowermost crust resulted from the continent–continent Himalayan collision process could also contribute to the increase of the average crustal Vp/Vs ratio in the region.

Spatial-temporal patterns of aftershocks of the 2001 Mw7.7 Bhuj earthquake during 2001–2008 reveal a northward spatial migration of seismic activity in the Kachchh seismic zone, which could be related with the loading stresses caused by the continued occurrences of aftershocks on the north Wagad fault (NWF), the causative fault of the 2001-mainshock. Aiming at explaining the observed northward migration of activity, we modelled the Coulomb failure stress change (DCFS) produced by the 2001-mainshock, the 2006 Mw5.6 Gedi fault (GF) and the 2007 Mw4.5 Allah bund fault (ABF) events on optimally oriented plane. A strong correlation between occurrences of earthquakes and regions of increased DCFS is obtained on the associated three faults i.e. NWF, ABF and GF. Predicted DCFS on the GF increased by 0.9 MPa at 3 km depth, where the 7^th March 2006 Mw5.6 event occurred, whereas predicted DCFS on the ABF increased by 0.07 MPa at 30 km depth, where the 15^th December 2007 Mw4.5 event occurred. Focal mechanism solutions of three events on the ABF have been estimated using the iterative inversion of broadband data from 5–10 stations, which are also constrained by the first P-motion data from 8–12 stations. These focal mechanism solutions for the ABF events reveal a dominant reverse movement with a strike-slip component along a preferred northwest or northeast dipping plane (∼50–70°). Focal mechanisms of the events on all the three fault zones reveal an N-S oriented P- axis or maximum principal stress in the region, which agrees with the prevailing N-S compression over the Indian plate. It is apparent that the northward migration of the static stress changes from the NWF, resulting from the occurrence 2001 Bhuj mainshock, might have caused the occurrence of the events on the GF and ABF during 2006–08.

The earthquake hazard associated with the Main Himalayan Thrust (MHT) is a critical issue for India and its neighbouring countries in the north. We used data from a dense seismic network in Uttarakhand, India, to model the lateral variations in the depths of MHT (2–6% drop in V_sat 12–21 km depths), Moho (a sharp increase in V_s(by ~ 0.5–0.7 km/s) at 39–50 km depths) and lithosphere (a marked decrease in V_s(~ 1–3%) at 136–178 km depths), across the Himalayan collisional front. Our joint inversion of radial PRFs and group velocity dispersion data of Rayleigh waves detects three NNE trending transverse lithospheric blocks segmenting the lithosphere in Uttarakhand Himalaya, which spatially correlate well with the northward extension of the Delhi -Haridwar Indian basement ridge, an inferred tectonic boundary and great boundary fault, respectively. Our radial receiver function imaging detects highly deformed and segmented crustal and lithospheric structures associated with three mapped transverse lithospheric blocks, suggesting a reduction in rupture lengths of future earthquakes, thereby, reducing earthquake hazards in Uttarakhand.