Figure 4. 3D migrated P-RF images along the Arunachal Himalaya profiles along with the elevation within ±50 km. Focal mechanisms of moderate magnitude earthquakes and local seismicity are superimposed on the 3D migrated images (circles). D: Detachment and M: Moho are marked based on P-RF images. (From Ravi Kumar et al., 2022) (Reprinted from Journal of Asian Earth Sciences, 236, Ravi Kumar Mangalampalli, Padma Rao Bommoju, Mahesh Perugu, Vempati Venkatesh, Scattered wave imaging of the Main Himalayan Thrust and mid-crustal ramp beneath the Arunachal Himalaya and its relation to seismicity, 105335, Copyright (2022), with permission from Elsevier.)
    Crustal thickness in the NE India including the eastern Himalaya is investigated using surface wave tomography and the RF studies (Kundu et al., 2020; Kumar A., et al., 2021). The results reveal that the Bengal Basin comprises thick sediments (up to \(\sim\) 20 km) with thickness increasing from west to east. The Moho depth increases from \(\sim\) 40 km in the Shillong plateau and Brahmaputra valley to \(\sim\) 70 km beneath the Higher Himalaya and southern Tibet. The crustal thickness and Poisson’s ratios reveal thickening of the crust from \(\sim\) 46 km beneath the Brahmaputra valley in the west to \(\sim\) 55 km in the western part of the Lohit Plutonic Complex (LPC). Similar analysis carried out in the Siang Window of the NE Himalaya reveals a variation of crustal thickness from \(\sim\) 38 km in the Brahmaputra valley (Pasighat) to \(\sim\) 53 km at the northern boundary of the window (Gelling area). The estimated Poisson’s ratio in the Brahmaputra valley is low (0.23), suggesting a felsic composition of the crust. It is intermediate in the Mishmi Thrust zone (0.249–0.261) and in some parts of the LPC. A high Poisson’s ratio (0.277–0.293) is obtained for the Tidding Tuting Suture Zone (TTSZ) and western part of the LPC, indicating the presence of aqueous fluid/partial melt in the crust (Kundu et al., 2020).
    Shukla et al. (2022) investigated the crustal configuration beneath northeast India based on receiver function analysis of teleseismic earthquakes recorded by 19 broadband seismological stations using H-k stacking method. The study reveals a large variation in crustal thickness and Poisson’s ratio which are correlated with the complex geology and tectonics of the region. The crust is observed to be thinner (36.5–41.6 km) beneath Bengal Basin, Shillong Plateau, and the Brahmaputra valley compared to the Indo-Burma Ranges (IBR) ( \(\sim\) 40–54 km) and Arunachal Higher Himalaya (TAWA station, \(\sim\) 45 km) and Sikkim Himalaya (GTK station, \(\sim\) 46.5 km). A large variation of Poisson’s ratio is observed in the region ( \(\sim\) 0.230–0.306).
    On the occasion of the Diamond Jubilee Year of CSIR-NGRI in 2021, Manglik et al. (2021) reviewed the geophysical studies carried out by the Institute since 1961 for the crustal and upper mantle structure of the Himalaya.

2.3 Indo-Burmese Arc

Saikia et al. (2020) investigated the mantle transition zone (MTZ) structure beneath the eastern Himalaya, southern Tibet, Assam valley, Burmese arc and Bengal basin regions using receiver functions of 327 stations. A depression in the 410 and 660 km discontinuities is observed beneath the Bengal basin and to the east of the eastern Himalayan syntaxis. The 410 is elevated by ∼10 km along the Himalayan collision front, while it deviates in the range of \(\pm\)5 km beneath most parts of Tibet and the Himalayan Foredeep. The 410 and 660 km discontinuities are uplifted by nearly 10 km beneath the Arunachal Himalaya. They observe a thick (>20 km) transition zone beneath the Burmese Arc and close to the Tengchong volcano.
    Dubey et al. (2022) presented 3-D P- and S-wave velocity perturbation maps of the upper-mantle beneath eastern Himalaya and Burmese subduction zones. Tomograms revealed that the subducting Indian lithospheric plate extends up to Bangong-Nujiang Suture Zone, overturns and descends steeply beyond 200 km below the Himalayan arc. A southward plunging detached slab can be traced beyond 600 km. Results reveal no evidence for the detachment of a S-E deflecting Indian lithospheric slab below the Burmese arc.
    Surface wave tomography reveals thick sediments represented by low shear wave velocity down to \(\sim\) 21 km depth in the eastern Bengal Basin beneath the southern Indo-Burma Range (IBR) (Kumar A., et al., 2021; Chanu et al., 2022). Tomography images also report subduction of Indian plate beneath the Burmese arc with signatures of a medium of high shear wave velocity below \(\sim\) 50 km to \(\sim\) 75 km depth.

2.4 Indian Ocean and Bay of Bengal

Rao et al. (2020) investigated the mantle transition zone (MTZ) structure beneath the Indian Ocean Geoid Low (IOGL) region using P-RFs. 3‐D time-to-depth migration of P-RFs reveals a thin MTZ primarily due to an elevation of the 660 km discontinuity. This is suggestive of anomalously hot temperatures in the mid-mantle beneath the IOGL region, possibly sourced from the African Large Low Shear Velocity Province (LLSVP). The seismic structure of the D′′ layer beneath the Indian Ocean is investigated by modeling the ScS-S and PcP-P differential travel time residuals (Rao and Ravi Kumar, 2022b). Modeling of the residuals using a grid search approach revealed velocity perturbations in the range of -3.06% to 5.72% for the shear and -4.81% to 5.47% for compressional waves in the D′′, which were positive below the Indian Ocean Geoid Low (IOGL) and negative below the adjoining region (Fig.5). The results reveal presence of high velocity material atop the Core Mantle Boundary (CMB) beneath the IOGL.
    Paul and Ravi Kumar (2022) identified salient features of the mantle beneath the Indian Ocean and Ross Sea, by analyzing 8 global tomographic models. Their study indicates low velocity anomalies of dVs \(\sim\) 1.1% in the \(\sim\) 400–680 km depth range and inconsistent high velocity anomalies of dVs \(\ge\) 1% at depths below 1600 km beneath both Indian Ocean and Ross Sea. A consistent low velocity structure throughout the mantle beneath the southwestern Indian Ocean and east Africa is associated with a plume from the African LLSVP. Forward modeling of the Geoid indicated that the E-W extent of the IOGL, influenced by upper mantle anomalies, could be precisely predicted, however, the N-S extent is underestimated since the lower mantle anomalies are inconsistent.