The interaction of the Indian continental lithosphere with the Reunion plume resulted in massive outpouring of basalts manifested in the Deccan Volcanic Province. Although it is postulated that preexisting weak rift zones facilitated the eruption, deeper signatures of the plume remain contentious. In this study, we investigate the shear wave velocity (Vs) structure of the crust and upper mantle by inverting regionalized group velocity dispersion data in the period range of 6 to 100 s, down to 220 km depth, using the genetic algorithm technique. We used 1,286 dispersion curves derived from waveforms of 77 regional earthquakes recorded at 38 broadband stations. Our results reveal distinct intracrustal layers, Moho, mantle lid, and asthenospheric low‐velocity layer. Signatures of magmatic underplating are recognized in terms of high Vs and a thick crust beneath the Kutch seismic zone and Western Ghats. The lithospheric thickness varies from 80 to 124 km, being thinnest at the junction of Cambay and Narmada rifts, which could be the source zone of volcanic eruption. A thin lithosphere has also been observed beneath the Kutch seismic zone. A low‐velocity zone at depths ~80 km can be related with upwarping of the asthenosphere and/or presence of partial melts. A predominant low‐velocity zone beneath the Cambay, Saurashtra, and adjoining regions may be due to the effect of a residual thermal anomaly. A thin lithosphere beneath northwestern Deccan Volcanic Province could be the result of weakening due to plume‐lithosphere interaction, which facilitated volcanism through preexisting rift zones.

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A study of source, path, and site characteristics was conducted for the Uttarakhand Himalaya region using accelerogram data from 15 earthquakes (M L ≥ 3:5). These earthquakes were recorded at the 16-station accelerograph network operated by the Indian Institute of Technology, Roorkee, during 2005-2011. The average seismic moment (M 0 ) of the studied earthquakes ranges between 1:20 × 10 22 and 1:02 × 10 24 dyn·cm, and the average moment magnitude (M w ) is between 4.0 and 5.3. The estimated corner frequency (f c ) varies from 1.1 to 3.3 Hz, radius of rupture (r d ) from 0.5 to 1.4 km, and stress drop (Δσ) from 6 to 172 bars, indicating continuous seismic energy release in the Uttarakhand region. The interdependence between estimated source parameters is shown by determining scaling laws for the studied region. The constant Q 0 of the shear-wave quality factor (Q S Q 0 f n ) varies between 40 and 300 and exponent n varies between 0.85 and 1.5, providing an average relation of Q S 174f 1:27 . However, least-square fitting of the observed data set in the frequency range 0.1-20 Hz gives Q S as 159f 1:16 . The value of Q S demonstrates that the region is heterogeneous, seismically active, and attenuative. The high-frequency spectral falloff factor (γ) varies from 1.3 to 2.1 and the upper crustal attenuation factor (κ) from 0.023 to 0.07 s at different sites, with an average of 0.044 s. The site response characteristics are estimated by horizontal-to-vertical spectral ratio (HVSR) and generalized inversion (GINV) techniques; results obtained from both the techniques show 1:1 correspondence. The site amplification factor varies between 2.3 and 9.4 using HVSR and between 2.6 and 10.9 using GINV among different stations. The predominant frequency ranges from 1.3 to 8.3 Hz with HVSR and from 1.3 to and 9.0 Hz with GINV.

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The Covid-19 pandemic created havoc and forced lockdowns in almost all the countries worldwide, to inhibit social spreading. In India as well, as a precautionary measure, complete and partial lockdowns were announced in phases during March 25 to May 31, 2020. The restricted human activities led to a drastic reduction in seismic background noise in the high frequency range of 1–20 Hz, representative of cultural noise. In this study, we analyse the effect of anthropogenic activity on the Earth vibrations, utilizing ambient noise recorded at twelve broadband seismographs installed in different environmental and geological conditions in Gujarat. We find that the lockdowns caused 1–19 dB decrease in seismic noise levels. The impact of restricted anthropogenic activities is predominantly visible during the daytime in urban areas, in the vicinity of industries and/or highways. A 27–79% reduction in seismic noise ground displacement (drms) is observed in daytime during the lockdown, in populated areas. However, data from station MOR reveals a drastic decrease in drms amplitude both during the day (79%) and night times (87%) since factories in this area operate round the clock. The noise at stations located in remote areas and that due to microseisms, shows negligible variation.

The Deccan volcanic province (DVP) witnessed a massive outpouring of flood basalts of ∼2 million km3 volume, at ∼65 Ma, in less than a Myr. The volcanic eruption is concomitant with crustal extension, lithospheric thinning and magma influx beneath the major rift systems namely Cambay, Narmada, and Kutch. In this study, we investigate the anisotropic and isotropic variations within the crust and upper mantle beneath the northwestern DVP by estimating the shear wave velocity (VSV, VSH, and VSoigt) and radial anisotropy (ξoigt) models using the Surface Wave Tomography technique. A joint inversion of the regionalized Rayleigh and Love wave group velocities is performed, using the genetic algorithm approach. Our results reveal different intracrustal layers, lid, and a low‐velocity zone (LVZ). This LVZ comprises of a uniform asthenospheric low‐velocity layer (LVL) of average VSV 4.44 km/s and VSH 4.47 km/s, and another LVL below, of average VSV 4.45 km/s and VSH 4.41 km/s. Furthermore, the LVZ represents a negative anomaly with reference to different global models (AK135, STW105, PREM, and S2.9EA). A negative ξoigt is observed in the LVZ, indicating dominance of vertical flow. This could be related to presence of partials melts, volatile materials and/or a thermal anomaly. We also identified the Moho (∼34–40 km) and lithosphere‐asthenosphere boundary (∼84–123 km). The low VS values, negative ξoigt and a thin lithosphere (∼84 km) in the vicinity of Gulf of Cambay affirm the presence of a plume head beneath it, in concurrence with the hypothesis of Indian Plate‐Reunion plume interaction.

The Gujarat region, situated in the westernmost part of India, experienced a deadly intraplate 2001 Mw 7.6 Bhuj earthquake. In the aftermath of the disaster, the Institute of Seismological Research established the Gujarat (India) seismic network in 2006. The network is being operated in online and offline modes, whereas, seismicity monitoring is being done in near-real-time, using data received from the online seismic stations. The Coronavirus disease-19 lockdown provided an opportunity to assess the network reliability in a difficult and challenging scenario. The positive aspect of the lockdown is reflected in signal-to-noise ratio, which improved significantly at all the sites during the lockdown, with more prominent being at sites located on top of the Quaternary sediments due to the absence of high-frequency anthropogenic noise. A sharp fall in the seismic background noise is noticed at most of the stations during the lockdown period, with respect to the prelockdown period. We used the lockdown data to identify other natural sources of noise, besides anthropogenic. The lockdown helped in solving the enigma of seismicity in certain pockets, which turned out to be related to quarry blasts.

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