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The Lonar impact crater in the Deccan Volcanic Province of India is an excellent analogue for impact-induced structures on the Moon and other terrestrial planets. We present a detailed architecture of the crater using a high-resolution 3-D seismic velocity image to a depth of 1.5 km through the inversion of high-frequency ambient noise data recorded over 20 broad-band seismographs operating around the crater. The ambient noise waveform is dominated by cultural noise in the 1–10 Hz band. The shear wave velocity (Vs) model is created from Rayleigh wave group velocity data with a horizontal resolution of 0.5–1 km in the period range of 0.1–1.2 s. A key feature of the model is a velocity reduction of 10–15 per cent below the crater compared to outside the ejecta zone. The low-velocity zone below the crater is nearly circular and extends to a depth of ∼500 m. This estimated crater's depth is consistent with global depth–diameter scaling relations for simple craters. The basement, with a Vs of more than 2.5 km s−1, lies beneath the Deccan basalt, which has a Vs of ∼2.4 km s−1. These results are consistent with laboratory-measured data from the Lonar crater and borehole data in the western Deccan trap. This study opens a new window for exploring impact craters and sub-basalt structures using high-frequency ambient noise tomography. [ABSTRACT FROM AUTHOR]

Ambient noise tomography is a powerful tool that has found increasing application in reservoir analysis and imaging. The Cachar fold belt in lower Assam, northeast India encompasses several wells under active hydrocarbon production, along with several dry wells. To overcome the lack of active seismic data over the entire fold belt, a passive seismic study was carried out to image the concealed three‐dimensional sub‐surface structures. The data were recorded from February to November 2011 by a network of 65 wideband seismometers spanning an area of about 40 × 60 km2. The data are crosscorrelated in the 2–5 s band, followed by phase‐weighted stacking to estimate noise correlation functions with surface wave signatures. The traveltimes picked from the frequency‐time analysis are utilized in a tomographic inversion for Rayleigh wave group velocities. The group velocity anomalies have a lateral resolution of ~ 3.5 × 5.5 km2 and variations of up to ±20%$\pm 20\%$ for each period. The group velocities are in turn inverted for S‐wave velocity distribution as a function of depth. The three‐dimensional S‐wave velocity tomograms reveal the tight anticlines and broad synclines, with high‐ and low‐velocity zones corresponding to structural highs and lows, respectively. The structural interpretation is supported for the part of the region with producing wells and covered by active seismic data, wherein the post‐stack time migrated seismic section shows anticlinal and synclinal features similar to those obtained from ambient noise tomography. The structures revealed by ambient noise tomography can help identify zones of interest to be targeted by active seismic surveys in the Cachar fold belt. [ABSTRACT FROM AUTHOR]

During 2014–16, a semi-permanent network of four 3-component broadband seismographs was operational in the Rajasthan craton and Aravalli mobile belt in the NW Indian shield. The reliable and accurate broadband data from 16 selected regional Indian earthquakes of Mw 5.5–7.8 from this seismic network enabled to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of northern India, covering the region below north India between the entire Himalaya (from the Pakistan Himalaya in west to the Burmese arc in the east) and Rajasthan (including Aravalli mobile belt). First, Rayleigh- (at 7–87s) and Love- (at 7–82s) wave group velocity dispersion curves were measured and then these curves were inverted to estimate the crustal and upper mantle structure below north India. It is observed that group velocities are of variable nature within the region. This could be attributed to the complex crust-mantle structure in the study region resulted from the magmatism episodes associated with the Proterozoic collision, 65 Ma Deccan volcanism and the Himalayan collision. The best model in the study region reveals a two-layered crust, with a 15-km thick upper-crust (UC) of average shear velocity (Vs) of 3.12 km/s and a 25-km thick lower-crust(LC) of average Vs of 3.44 km/sec. The modeling detects a drop in Vs (∼1-2%) at 79–120 km depths, underlying north India, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 79 km depth. A geothermal gradient extrapolated from the surface heat flow (∼74 mW/m2) shows that such a gradient would intercept CO2-bearing mantle peridotite solidus at 100 km depth, and thus could signal the presence of small amounts of partially melted magma below 100 km depth. Therefore, this 1–2% drop in Vs could be attributed to the presence of carbonatite melts in the upper mantle related to magmatic episode of 65 Ma Deccan plume activity as also suggested by existing geological and seismological evidence. [ABSTRACT FROM AUTHOR]

Abstract: Modeling of multimode surface wave group velocity dispersion data sampling the eastern and the western Ganga basins, reveals a three layer crust with an average V s of 3.7kms−1, draped by ∼2.5km foreland sediments. The Moho is at a depth of 43±2km and 41±2km beneath the eastern and the western Ganga basins respectively. Crustal V p/V s shows a felsic upper and middle crust beneath the eastern Ganga basin (1.70) compared to a more mafic western Ganga basin crust (1.77). Due to higher radiogenic heat production in felsic than mafic rocks, a lateral thermal heterogeneity will be present in the foreland basin crust. This heterogeneity had been previously observed in the north Indian Shield immediately south of the foreland basin and must also continue northward below the Himalaya. The high heat producing felsic crust, underthrust below the Himalayas could be an important cause for melting of midcrustal rocks and emplacement of leucogranites. This is a plausible explanation for abundance of leucogranites in the east-central Himalaya compared to the west. The uppermost mantle V s is also significantly lower beneath the eastern Ganga basin (4.30kms−1) compared to the west (4.44kms−1). [ABSTRACT FROM AUTHOR]

Surface wave data were initially collected from events of magnitude Ms ≥ 5.0 and shallow or moderate focal depth occurred between 1980 and 2002: 713 of them generated Rayleigh waves and 660 Love waves, which were recorded by 13 broadband digital stations in Eurasia and India. Up to 1,525 source-station Rayleigh waveforms and 1,464 Love wave trains have been processed by frequency-time analysis to obtain group velocities. After inverting the path-averaged group times by means of a damped least-squares approach, we have retrieved location-dependent group velocities on a 2° × 2°-sized grid and constructed Rayleigh- and Love-wave group velocity maps at periods 10.4–105.0 s. Resolution and covariance matrices and the rms group velocity misfit have been computed in order to check the quality of the results. Afterwards, depth-dependent SV- and SH-wave velocity models of the crust and upper mantle are obtained by inversion of local Rayleigh- and Love-wave group velocities using a differential damped least-squares method. The results provide: (a) Rayleigh- and Love-wave group velocities at various periods; (b) SV- and SH-wave differential velocity maps at different depths; (c) sharp images of the subducted lithosphere by velocity cross sections along prefixed profiles; (d) regionalized dispersion curves and velocity-depth models related to the main geological formations. The lithospheric root presents a depth that can be substantiated at ~140 km (Qiangtang Block) and exceptionally at ~180 km in some places (Lhasa Block), and which exhibits laterally varying fast velocity very close to that of some shields that even reaches ~4.8 km/s under the northern Lhasa Block and the Qiangtang Block. Slow-velocity anomalies of 7–10% or more beneath southern Tibet and the eastern edge of the Plateau support the idea of a mechanically weak middle-to-lower crust and the existence of crustal flow in Tibet. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of Rayleigh wave group velocity tomography in SouthIndia. South India is a stable continent comprising of Precambrian terrains such as the easternDharwar craton (EDC), the western Dharwar craton (WDC), the southern Granulite terrain(SGT), and the Cuddapah basin. The data used for analysis were recorded at 51 broadbandstations operated during 2009-2012 across this region. Based on the analysis a total of781 unique earthquake events with shallow to moderate depths were chosen. Theepicentral distance was from 5o-30o and magnitude Ms >= 4.5. This attributedto a total of 4873 source-station raypaths for which the dispersion curves werecalculated to obtain Rayleigh-wave group velocity maps for time periods from10-200 secs. Due to the high ray coverage over the southern terrain, we obtained ahigh resolution of 0.5o-1o across all time periods. The tomographic velocity mapsshow a thinner crust (∼35 Km) in the late Archean (∼2.7 Ga) EDC and relativelyhigher (∼40-50 Km) in adjoining terrains (mid-Archean and Proterozoic). Themid-Archean (∼3.36 Ga) WDC has a thicker crust (∼50 Km). Furthermore, the local groupvelocity dispersion curve obtained for each grid cell from 10-200 seconds wereinverted for shear wave velocity models. The shear wave velocity models gave aquantitative image of the crust as well as the lithospheric root. The mid-ArcheanWDC has a thicker lithosphere with lithosphere-asthenosphere(LAB) around anaverage of 160-180 Km. The southern root of the lithosphere of WDC continued tillnorthern province of SGT which showed higher LAB compared to the thickness in therest of SGT (∼140 Km). The LAB in late Archean EDC along with the Cuddapahprovince was found to be flat and thinner (∼140-160 Km) compared to WDC. Thecoastal areas showed a thin LAB (∼110-130 Km) except in the WDC boundary. Thesouthernmost tip showed a very thin LAB (100-110 Km). This observation suggests thatthe average crustal and lithospheric thickness of WDC is more compared to theEDC. This can infer that the WDC after it’s cratonization has remained mostlyinert, while EDC might have undergone tectonothermal changes in the Proterozoic. [ABSTRACT FROM AUTHOR]