The microtremor measurements are carried out in and around the Gorakhpur city (Uttar Pradesh), India, overlain by alluvium at about 150 sites to understand the local site conditions. Horizontal-to-vertical spectral ratio (HVSR) conBrms that the majority of sites have a predominant frequency of ${\sim}$0.45 Hz, which may suggest the prevalence of thick soft sediments in the area. Conspicuously, a number of multiple peaks in HVSR curves at few sites may reflect the presence of different interfaces with significant impedance contrasts. Maximum amplification is observed of 4.0–5.3 to the NW–SE of the city, whilst few sites in the city are found to be associated with different values of peak amplification factor that varied between 2.0 and 4.0. It is also observed that the ground vulnerability index ($K_g$) in Gorakhpur city has values higher than 10.0 at most of the sites. Assimilation of 1-D velocity model for the city clearly shows that low shear wave velocity (${\sim}$300 m/s) down to the depth of ${\sim}$35 m, suggesting thick piles of sediments that may correspond to Cuvial river system in the area, whilst the peak frequency of about 0.45 Hz may correspond to the Quaternary–Tertiary sediment boundary that may exist at deeper layers (${\sim}$1000 m). The inference of this study may be used as inputs for earthquake risk management by reducing the severity of earthquake shaking through design of earthquake risk resilient structures.

The present-day crustal structure of tectono-magmatic regions is the product of dynamic interactions of crust and mantle materials. The Narmada–Tapti region is a mosaic of tectono-magmatic signatures and is characterized by active seismicity, deep-seated faults, shear zones, and high heat flow, suggesting it to be a zone of crustal weakness. The availability of ample seismic and magnetotelluric datasets and inherent complexity drew our attention to image the crustal structure in the third dimension using high-resolution gravity data. The derived 3D crustal density model shows that the Deccan trap extends from 200–1700 m partly below the 90–150 m thick Quaternary sediment exposed in some pockets. The sub-trappean Mesozoic sediment is present at a depth of 250–2400 m followed by the basement. Our 3D model further shows that the high gravity values in residual anomalies are due to high-density magmatic intrusions between 1.5 and 9 km depth. The gravity high in regional anomaly is modelled with a broad dome-shaped high-density (3.02 g/cm3) underplated layer between 14 and 38 km depth. The spatial correlation of delineated high-density lower crustal body with the high-velocity and high conductivity zones mapped by earlier workers in this region indicates the possible presence of mantle magma intrusion in the realm of Deccan volcanism. Analysis of isostatic residual anomaly indicates that the region beneath Narmada–Tapti is not in local isostatic equilibrium. Analysis of the isostatic residual anomaly, root depth, and crustal thickness from the 3D model further ascertains the modification of the crust due to the interaction of mantle plume material. The gravity effect of residual geoid up to 50 km corroborates the high-density magmatic material distribution at two different places, i.e., one at Navsari near the west coast and the other is Junapani near Khandwa. The region has signatures of upliftment and together with the crustal-scale basic magmatic intrusion, satisfies both high gravity anomalies and positive residual geoid undulation. The residual geoid undulations are bounded by major tectonic faults and together with the magmatic underplate at the crustal base indicate that these faults were activated during the Deccan magmatism.

$\bf{Highlights}$

$\bullet$ Narmada-Tapti region has a weak crustal architecture with crustal and sub-crustal magmatic intrusions, dyke swarms, atypical geophysical signatures, and crustal upliftment.

$\bullet$ 2½D crustal density modelling along available seismic sections using high-resolution gravity data in Narmada-Tapti region.

$\bullet$ Three-dimensional crustal-scale density structure with multistage magmatic intrusion in the Narmada-Tapti region, central India.

$\bullet$ Positive Bouguer and isostatic anomalies and geoid undulation over the Narmada-Tapti region provide extra arguments for densification of the crust through multistage magmatic intrusions caused by the Deccan magmatism.

$\bullet$ About 250–2400 m thick Mesozoic sediments delineated at a depth of about 500–3000 m illustrates the potential for hydrocarbon exploration in the Narmada-Tapti region.

The Singhbhum Craton, Singhbhum Mobile Belt along its northern, eastern, and western edges, and Chotanagpur Gneissic Complex farther north are all parts of the Precambrian eastern Indian shield. Modern isotope dates and associated geological evidence suggest that these crustal units may be one cratonic block that developed sequentially between 3.55 and 1.00 Ga. The region has always been the focus of numerous geoscientific studies due to its complex evolutionary history and abundant mineralisation. We used the terrestrial gravity data from the Gravity Map Series of India and the EGM2008 global gravity dataset in the Bay of Bengal to model the 3D Moho geometry of the eastern Indian shield and the adjoining Bay of Bengal by inverting the gravity data. The Bouguer gravity data were filtered at several levels before applying the Parker–Oldenburg iterative inversion procedure. The Moho depth measurement is then computed by presuming a constant density contrast. The effects of sediments were eliminated from gravity data by collecting thickness and density details of the sediment from a worldwide sedimentary thickness map CRUST1.0 and applying a correction comparable to the Bouguer correction that uses the density difference of 0.24 g/cm$^3$. Spectral analysis is used to fix a reference depth level and the low-frequency range associated with Moho deflection in the Bouguer anomaly filtered for sedimentary overburden. We subsequently executed the gravity inversion of a basic two-layer structure having aconstant density difference of 0.40 g/cm$^3$ across the Moho fixed at an average depth of 35 km. The gravity inversion analysis shows that the Moho depth within the Bay of Bengal is between 18 and 24 km. In the continent, the Moho depth varies from 34 km near the coastline to 38 km towards the Singhbhum Cratonand Chhotanagpur Gneiss Complex. In the northern portion of the region, the Moho depth increases to over 40 km underneath the convergence of the Mahanadi–Damodar Gondwana basins and the Ganga foreland basin.

TheSaurashtraHorst, south of the Kachchh RiftBasin, is a significant tectonic block in the northwestern Deccan Volcanic Province of India. The Saurashtra Horst belongs to the stable continental region and typically experiences shallow intraplate earthquakes with depths up to 20 km.We analysed spectra of P- and S-waves using 182 broadband seismograms from 40 small earthquakes (M3.5–5.1). It has been observed that there is a shift in Pwave corner frequency for most earthquakes to the higher side compared to that of S-corner frequency in both regions.The seismicmomentsmeasured fromprimary waves, i.e.,$M_0$(P), vary from 7.7 ${\times}$ 10$^{12}$ to 5.88 ${\times}$ 10$^{16}$ Nm, and from secondary waves, i.e., $M_0$(S) range from 1.52 ${\times}$ 10$^{13}$ to 1.88 ${\times}$ 10$^{17}$ N-m for the southern part of Saurashtra.The corresponding variation of seismicmoments for the events in northern Saurashtra is 3.08 9 1012 to 4.50 ${\times}$ 10$^{14}$ N-m for P-waves and 7.01 ${\times}$ 10$^{12}$ to 2.34 ${\times}$ 10$^{15}$ N-m for S-waves. The estimated stress drop values lie in the range of 0.02–20.08 bars and 0.01–9.09 bars for the earthquakes in Saurashtra’s southern and northern parts, respectively.The small earthquakes have been found to be self-similar in the southern part, while there is a breakdown of self-similarity for small earthquakes in the northern region. This indicates the diverse stress regimes in the Saurashtra Horst. A scaling relation ($M_0$f$^3$$_c$ = 2:19 ${\times}$ 10$^{15}$ N-m=s$^3$) for the southern part of Saurashtra is obtained during the linear regression investigation among the established seismicmoment, i.e.,$M_0$, and corner frequency, i.e., $f_c$. The structural heterogeneities corroborate the diversity in source parameters for two different parts of SHin the regions. The variability of stress drops for the best-located events shows that the higher stress drop values correspond to the source zone having lower $V_p$/$V_s$ and vice versa.