Figure 3. (a) Simplified geological map of the Kumaun Himalaya and adjacent regions along with the seismological stations, and (b) common conversion point (CCP) depth migrated RF image along the profile AB. Red and blue colors indicate positive and negative impedance contrast, respectively. A ramp in the MHT is identified at \(\sim\) 130 km profile distance and a low velocity zone (LVZ) is inferred beneath the Chiplakot crystalline belt. Abbreviations are: HFT - Himalayan Frontal Thrust, MBT - Main Boundary Thrust, SAT - South Almora Thrust, NAT - North Almora Thrust, MT/MCT-I - Munsiyari Thrust, and VT/MCT-II - Vaikrita Thrust, RT-Ramgarh Thrust, SCT-South Chiplakot Thrust, NCT-North Chiplakot Thrust, STD-South Tibetan Detachment, IGP - Indo-Gangetic Plain, CCB - Chiplakot Crystalline Belt, BK-Bajnath Klippe , and ASK - Askot Klippe. (From Hazarika 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.4), 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.5). 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.