Evidence of low velocity layers at the top and bottom of the Mantle Transition Zone (MTZ) below the Uttarakhand Himalaya, India.

The Mantle Transition Zone (MTZ) beneath the Uttarakhand Himalaya has been modelled using Common Conversion Point (CCP) stacking and depth-migration of radial P-receiver functions. In the Uttarakhand Himalaya region, the depths of the 410-km discontinuity (d410) and the 660-km discontinuity (d660) are estimated to be approximately 406 ± 8 km and 659 ± 10 km, respectively. Additionally, the thickness of the mantle transition zone (MTZ) is modelled to be 255 ± 7 km. The average arrival times for d410 and d660 conversions are (44.47 ± 1.33) s and (71.08 ± 1.29) s, respectively, indicating an undisturbed slightly deeper d410 and a deformed noticeably deeper d660 in the area. The model identifies the characteristics of the d410 and d660 mantle discontinuities beneath the Lesser Himalayan region, revealing a thickening of the MTZ towards northeast, which could be due to gradual cooling or thickening of the Indian lithosphere towards its northern limit. We simulate a low-velocity layer (perhaps partially molten) above the d410 discontinuity at depths of 350 to 385 km, indicating the presence of a hydrated MTZ beneath the area. We also interpret a negative phase at d660 as a low-velocity layer between 590 and 640 km depths, which could be attributed to the accumulation of old subducted oceanic materials or increased water content at the bottom of the MTZ. Our results suggest the presence of residues from paleo-subducted lithospheric slabs in and below the mantle transition zone underlying the Uttarakhand Himalayas.

Evidence of structural segmentation of the Uttarakhand Himalaya and its implications for earthquake hazard.

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 Vs at 12-21 km depths), Moho (a sharp increase in Vs (by ~ 0.5-0.7 km/s) at 39-50 km depths) and lithosphere (a marked decrease in Vs(~ 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.

Evidence of secular variation in Archean crust formation in the Eastern Indian Shield.

Understanding the dominant crustal accretion model in any Archean craton is the key to understanding the dominant geodynamic process responsible for early crust formation during the Hadean (> 4.0 Ga) and Archaean (4.0-2.5 Ga). The continental crust has been proposed to have formed through either horizontal/vertical accretion related to subduction or mantle plume tectonic processes. Here, the Moho depths and average crustal Vp/Vs ratios are modelled at 16 broadband stations in the Eastern Indian Shield (EIS) through HK stacking of radial P-receiver functions (PRFs). These modelled parameters are used to test both plume and subduction models, which might have played a key role in the crustal accretion of the EIS throughout the Archean. We observe a correlation between crustal age and composition within the ellipsoidal Paleoarchean cratonic domain in the Singhbhum-Odisha-Craton (SOC), which reveals an increase in age from the younger granitoid core of the SOC (with thinning of felsic crust) to the surrounding older greenstone belts (with thickening of felsic crust). A thinner mafic crust resulting from multiple magmatic events characterizes the neighbouring Meso-Proterozoic Chotanagpur Granitic Gneissic terrain (CGGT). The Common Conversion Point (CCP) image of radial PRFs reveals northward subduction of the Paleoarchean SOC below the Meso-Proterozoic CGGT.

Modelling of crustal composition and Moho depths and their Implications toward seismogenesis in the Kumaon-Garhwal Himalaya.

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 Mw6.6 Uttarakashi and 1999 Mw6.4 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.

Modelling the seismic potential of the Indo-Burman megathrust.

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.

Detection and potential early warning of catastrophic flow events with regional seismic networks.

Early warning is a critical potential tool for mitigating the impacts of large mass wasting and flood events, a major hazard in the Himalaya. We used data from a dense seismic network in Uttarakhand, India, to detect and track a fatal rockslide to mass flow to flood cascade and examine the potential for regional networks to provide early warning for extreme flow events. Detection limits of the 7 February 2021 event depend on the nature of the active process and on the anthropogenic and environmental seismic noise levels at each station. With the existing network, a seismic monitoring system could have detected all event phases from up to 100 kilometers and provided downstream warnings within minutes of event initiation.