Jyoti Sharma

We compare earthquake wavefield measurements from a ∼95 km urban dark fiber distributed acoustic sensing (DAS) array and a collocated 16 km aperture broadband seismic array in Dallas, Texas. Using data from a regional M 5.2 earthquake, we assess the ability of DAS to recover ground-motion amplitudes and wavefield direction of arrival (DOA). Surface-wave (Rayleigh) amplitude ratios between broadband and DAS range from 0.25 to 22.15, revealing significantly more heterogeneous DAS coupling than previously reported. Although broadband sensors provide consistent amplitude and high-quality DOA estimates across all phases (P, S, and Rayleigh), DAS data show large amplitude variability unrelated to fiber orientation, suggesting coupling and emplacement differences as dominant factors. Signal coherence in DAS is also highly variable, with some segments showing stronger correlations during pre-event noise than during the earthquake signal. Nonetheless, by applying hierarchical clustering to interchannel cross-correlations, we identify a 36-channel DAS subgroup and obtain a DOA estimate for the Rayleigh wave within 4.5° of that derived from the broadband array. These results highlight both the promise and limitations of using urban dark fiber DAS for earthquake monitoring and underscore the need for careful characterization of fiber coupling and geometry to enable reliable wavefield reconstruction.

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 (d_rms) is observed in daytime during the lockdown, in populated areas. However, data from station MOR reveals a drastic decrease in d_rms 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 (V_SV, V_SH, and V_Soigt) and radial anisotropy (ξ_oigt) 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 V_SV 4.44 km/s and V_SH 4.47 km/s, and another LVL below, of average V_SV 4.45 km/s and V_SH 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 ξ_oigt 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 V_S values, negative ξ_oigt 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 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.

As devastating earthquakes continue to inflict widespread destruction to life and property and hinder the development of urban areas, the technical and scientific problems of seismic hazard assessment and risk-related issues warrant urgent attention. In this regard, seismologists play a key role in defining the source parameters of earthquakes and estimating the associated site-specific ground motion amplification commonly termed site response. The relevant analyses are usually performed using either recorded ground motion for a handful of seismic events of moderate to large magnitude or by synthesizing stochastic processes based on the observational data. The hazard projection is aimed at a...