Figure 2. Shear velocity structure of the upper mantle along two profiles, obtained from joint inversion of the surface wave dispersion  data and receiver functions. (From Jana et al., 2022) ( Reprinted from Gondwana Research, 111, Niptika Jana, Chandrani Singh, Arun Singh, Tuna Eken, Arun Kumar Dubey, Abhisek Dutta, Arun Kumar Gupta, Lithospheric architecture below the Eastern Ghats Mobile Belt and adjoining Archean cratons: Imprints of India-Antarctica collision tectonics, 209-222, Copyright (2022), with permission from Elsevier.)
    Das and Rai (2019) generated a 3-D shear velocity image of the crust through joint inversion of the P- RFs and Rayleigh wave group velocity dispersions, derived from cross correlation of ambient noise, to study the linkage and the boundary between the Dharwar craton and the Southern Granulite Terrain (SGT). The study delineated a 10–15 km thick high shear velocity layer in the lowermost crust of the southern part of the WDC continuing into the SGT up to 40 km beyond the E-W trending southern limb of the Palghat-Cauvery shear zone, suggesting that the Dharwar craton continues further south of the mapped orthopyroxene boundary to the Palghat shear zone.
    Vashishtha et al. (2022) used surface wave data from the April 25, 2015, Nepal earthquake (Mw 7.8) and its aftershocks, recorded at eleven stations in India to estimate group velocities of both Love and Rayleigh waves. All stations are at regional distances from the earthquake sources. It is observed that the group velocities for both Love and Rayleigh waves obtained from the mainshock data are lower than those obtained from aftershock data for stations located towards W and SW of the earthquake source region. Such a variation of group velocity obtained from mainshock and aftershock data for different stations may be due to source directivity for the mainshock affecting the source group time, which in turn affects the travel time of surface waves at different periods.

2.2 Himalaya

Kanna and Gupta (2020) studied the crustal structure of the Garhwal Himalaya along a linear profile, using regional travel times and receiver function analysis. Their receiver function modeling showed a prominent intra-crustal low velocity layer with a flat–ramp–flat geometry beneath the Main Central Thrust zone and a variation in the Moho depth from \(\sim\) 45 km beneath the Sub Himalaya to \(\sim\) 58 km to the south of the Tethys Himalaya. A similar study in the NW Himalaya and Ladakh-Karakoram indicates that the Moho depth increases from \(\sim\) 46 km below the Gangetic Plain to \(\sim\) 78 km at the southern flank of the Karakoram Fault. Several intra-crustal low-velocity layers were seen and the MHT is mapped as an LVL (Kanna and Gupta, 2021).
    Gupta S., et al. (2022) examined the P-wave velocity (Vp) and Vp/Vs variations using local earthquake arrival time measurements recorded over 41 seismic stations operated during November 2006 to June 2008 in the Kumaun–Garhwal Himalaya. In the 0–25 km depth range, the Vp and Vp/Vs varies between 4.8–6.8 km/s and 1.55–1.85, respectively and show a heterogeneous structure in the upper-mid crust. The seismic images exhibit signatures of unconsolidated sediments, close to the Main Frontal Thrust, and Klippes (Lansdowne and Almora) in the uppermost crust. In the upper-mid crust, the observed low Vp and high Vp/Vs (1.82–1.85) along with available conductivity values indicate saline-rich aqueous fluid and partial melt. Using the inferred crustal composition and constraints from earlier information, they proposed the bottom of these fluid zones as the top of the underthrusting India Plate, which shows a flat-ramp-flat geometry at 16–21 km depth. The earthquakes of moderate and smaller magnitude mostly occur in the fluid-rich zone above the mid-crustal ramp in the underthrusting Indian Plate. Madhusudhan et al. (2022) calculated the uppermost mantle seismic (Pn and Sn) velocities using 12 regional earthquakes recorded by 33 digital broadband seismological stations from the Gangetic plain to the Tethys Himalaya along a \(\sim\) 160 km profile and showed that the Moho dips in the north direction with an overall dip angle of 3.43\(^{\circ}\)- 4.2\(^{\circ}\) in the Eastern Kumaun Himalaya.
    Mandal et al. (2021c) imaged the lateral variations in the Moho depths and average crustal composition across the Kumaun–Garhwal Himalaya, through H-K stacking of 1400 radial P-RFs from 42 broadband stations. The modeled Moho depth and average crustal Vp/Vs values vary from 28.3 to 52.9 km and 1.59 to 2.13, respectively. They also mapped three NNE-SSW trending transverse crustal blocks in Uttarakhand Himalaya that extend down to the lithosphere-asthenosphere boundary. Local earthquake tomography of the Kumaun-Garhwal Himalaya (Mandal et al., 2022b; Gupta S., et al., 2022) shows a low velocity (Vp, Vs), high Vp/Vs mid-crustal layer, which was identified as the Main Himalayan Thrust associated with metamorphic fluid/partial melt.
    The Himalaya-Karakoram-Tibet region characterizes a unique setup of crustal and upper mantle structure related to present-day geodynamics. Kumar N., et al. (2019) performed surface wave tomography studies that reveal highly variable shear wave velocity structure indicating signatures of underthrusting of the Indian plate beneath Eurasia. The study reveals a NE-dipping Moho with its depth increasing from \(\sim\) 40 km beneath the frontal Himalaya to 70-80 km below the collision zone. Low near-surface seismic velocities indicate thick sediments (5-6 km) in the Indo-Gangetic plains. Broader low-velocity zone at mid-crustal depth beneath the southern parts of Tibet and Karakoram fault is due to the presence of partial melting and/or aqueous fluids.
    Kumar V., et al. (2022) used ambient noise cross-correlations from 530 seismological stations along with surface wave observations from 1,261 earthquakes to image the crust beneath the western Himalaya-Asia convergence zone encompassing western Himalaya-western Tibet-Ladakh-Karakoram-Pamir-Hindu Kush. The seismological data from the PASSCAL experiments, the Global Seismograph Network, experiments in Kyrgyzstan, Kazakhstan, Tajikistan, China, Nepal and western Tibet, French deployments in western Kunlun and Kazakhstan and Indian deployments in the western Himalaya were used in this study that resulted in 22,726 inter-station Rayleigh wave dispersion measurements in the period band of 5 to 60 s at a horizontal resolution of less than 0.5\(^{\circ}\)×0.5\(^{\circ}\). The 3-D shear wave velocity image revealed that the northern limit of the Indian crust extends beyond the Qiangtang block in western Tibet (77\(^{\circ}\)- 82\(^{\circ}\)E) and till the central Pamir farther west. The study suggests a continuation of LVZs across the Karakoram Fault at a depth below 20 km, indicating the fault’s upper crustal depth extent.
    The seismicity in the Kumaun Himalaya is concentrated in the Chiplakot Crystalline Belt (CCB). WIHG conducted a passive seismological study using receiver functions (RFs) along a profile (see Fig.1 of Hazarika et al., 2021). The H-K stacking analysis of RFs of teleseismic earthquakes reveals a significantly high value of the Poisson’s ratio (\(\sim\) 0.28) in the Dharchula region of the CCB which is coincident with the earthquake cluster (Hajra et al., 2019). The RF inversion and Common Conversion Point (CCP) stacking imaging reveals a variation of crustal thickness from \(\sim\)38 km in the Indo-Gangetic Plain to \(\sim\) 42 km near the Vaikrita Thrust (see Fig.6 of Hazarika et al., 2021). A ramp (\(\sim\) 20\(^{\circ}\)) structure on the MHT is revealed beneath the CCB. The spatial and depth distribution of seismicity pattern beneath the CCB and presence of steep dipping imbricate faults inferred from focal mechanism solutions suggest a Lesser Himalayan Duplex structure in the CCB above the MHT ramp (Hazarika et al., 2021; Hajra et al., 2021).
    Medved et al. (2022) obtained 3D models of the crust and uppermost mantle beneath the NW Himalaya down to a depth of 120 km by local earthquake seismic tomography using data of India Meteorological Department (IMD) complemented by the Global International Seismological Centre (ISC) Catalogue. Their results suggest that the Indian Plate not only underthrusts northwards below the Himalaya but also bends westwards as it gets closer to the Hindukush Region. A peculiar feature of the model is a high-velocity anomaly in the Kaurik Chango Rift, interpreted as a remnant of the oceanic crust, left after the closure of the Indo-Tethys Ocean. In the seismically active Delhi-Haridwar Ridge, a low-velocity upper crustal layer is possibly associated with sediments of the Indo-Gangetic Plain and with a large number of fault structures.
    Mir et al. (2021) estimated the shear wave velocity structure, together with Moho depths for the NW Himalaya, Hindu Kush and the Pamirs at a potential resolution of 0.5\(^{\circ}\)×0.5\(^{\circ}\) and at 1\(^{\circ }\)×1\(\)\(^{\circ }\)  in the surrounding area (Fig.3), by inverting fundamental mode Rayleigh wave group velocities calculated from regional earthquake (Δ \(\le\) 2500 km) data, and also from their joint inversion with teleseismic receiver functions at 38 out of the 59 broadband stations in the region. The results illuminate a) the deeper root zone structures of the main geomorphic features, b) a pervasive low velocity layer (Vs  \(\sim\)  3.1 km/s) at  \(\sim\) 30 km depth beneath the NW Himalaya. Another notable result is the distinctly shallower Moho beneath the Kashmir Himalaya apparently segmented by arc-normal shear zones that cross the rupture zones of the 1905 Kangra and the 2005 Kashmir earthquakes, in turn, marked by the current epoch seismicity.
    A high-resolution seismic image of the crust beneath the Arunachal Himalaya is documented by Singh A., et al. (2021), using RF analysis of data from 32 broadband seismic stations deployed in the Arunachal Himalaya during 2010-2016, along with data from the HIMNT, SIKKIM, Hi-CLIMB, and GANSSER networks. Their results reveal lateral variations in the crustal structure with the Moho depth varying from 40-60 km. They also observe a comparatively less complex crust, absence of a prominent mid-crustal ramp, a highly deformed layer running parallel to the Main Himalayan Thrust, and an intermittent anisotropic low velocity layer in the middle crust.
    In a recent study (Ravi Kumar et al., 2022), receiver function images of the detachment, mid-crustal ramp and the Moho of the underthrusting Indian plate along four profiles in the Arunachal Himalaya are documented (Fig.4). The results reveal a clear Moho signature in the depth range of 40 to 65 km, with the detachment mapped in the depth range of  \(\sim\) 10 to 20 km. A mid-crustal ramp can be traced in the higher Himalaya especially along one profile. Singh A., et al. (2021) imaged the crust beneath the Arunachal Himalaya using teleseismic receiver functions. A mechanically weak middle crust beneath Arunachal Himalaya, highly deformed layer parallel to MHT, and comparatively less complex crust beneath Arunachal than Nepal and Sikkim are some important observations that have been reported in this study.