G. Mohan

A GIS based hydrogeomorphic approach for identification of site-specific artificial-recharge techniques in the Deccan Volcanic Province

The Deccan Volcanic Province (DVP) of India, as a whole, faces a severe shortage of water despite receiving a high annual rainfall, this is primarily due to excess runoff and lack of water conservation practices. In this study, an attempt is made to identify zones favourable for the application and adaptation of site-specific artificial-recharge techniques for augmentation of groundwater through a Geographical Information System (GIS) based hydrogeomorphic approach in the Bhatsa and Kalu river basins of Thane district, in western DVP. The criteria adopted for the GIS analysis were based on the hydrogeomorphological characteristics of both basins extracted from the IRS-1C LISS-III data supported by information on drainage pattern, DEM derived slope, lineament density, drainage density, and groundwater condition. The integrated study helps design a suitable groundwater management plan for a basaltic terrain.

Imprints of volcanism in the upper mantle beneath the NW Deccan volcanic province

In this study, we investigate the transition zone discontinuities beneath the northwestern Deccan volcanic province of India through analysis of ∼1000 high-quality receiver functions abstracted from three-component teleseismic waveforms from 428 earthquakes recorded by six broadband stations in the northwestern Deccan volcanic province. Our analysis reveals that the P 410 s and P 660 s time lags are delayed by ∼1 s relative to those predicted by the IASP91 model. A largely unperturbed mantle transition zone, revealed by the transition zone time lag ( t P660s – t P410s ) of 23.83 s, implies that the observed delays are primarily associated with reduced shear velocities in the upper mantle above the 410 km discontinuity. The velocity reduction is likely to be associated with lithospheric thinning coupled with compositional and reduced thermal variations in the shallow upper mantle. Our results contrast with the normal shield-like velocity structure imaged beneath the south-central Deccan volcanic province in an earlier receiver function study. For comparison, a revised composite receiver function plot for the south-central Deccan volcanic province was constructed by employing identical receiver function processing techniques on an updated high-quality data set of 1400 receiver functions from 11 stations, which reaffirms, with better precision, the earlier results. We propose that the relative differences in the lithospheric thicknesses beneath the northwestern and south-central parts of the Deccan volcanic province possibly governed the melting and flow patterns of the upwelling mantle material, resulting in the contrasting seismic signatures. The lithospheric architecture of the northwestern Deccan volcanic province, coupled with the reactivation of preexisting rift systems, appears to have facilitated the eruption of the Deccan basalts, for which source signatures are still retained in the upper mantle.

Shear velocity structure of the laterally heterogeneous crust and uppermost mantle beneath the Indian region

Abstract The shear velocity structure of the Indian lithosphere is mapped by inverting regionalized Rayleigh wave group velocities in time periods of 15–60 s. The regionalized maps are used to subdivide the Indian plate into several geologic units and determine the variation of velocity with depth in each unit. The Hedgehog Monte Carlo technique is used to obtain the shear wave velocity structure for each geologic unit, revealing distinct velocity variations in the lower crust and uppermost mantle. The Indian shield has a high-velocity (4.4–4.6 km/s) upper mantle which, however, is slower than other shields in the world. The central Indian platform comprised of Proterozoic basins and cratons is marked by a distinct low-velocity (4.0–4.2 km/s) upper mantle. Lower crustal velocities in the Indian lithosphere generally range between 3.8 and 4.0 km/s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities, respectively. A remarkable contrast is observed in upper mantle velocities between the northern and eastern convergence fronts of the Indian plate. The South Bruma region along the eastern subduction front of the Indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle. High velocities (≈4.8 km/s) are also observed in the upper mantle beneath the Ninetyeast ridge in the northeastern Indian Ocean.

Imaging of seismic scatterers beneath the Gauribidanur (GBA) array

Abstract A study has been conducted to image seismic scatterers beneath the Gauribidanur (GBA) array in the Precambrian shield of southern India. Short period digital data from teleseisms and regional events recorded over the 20-station L-shaped array were used to image seismic scatterers beneath the array employing a semblance technique. The results indicate a zone of dominant scattering encompassing the crust in a region west of GBA, coinciding with a large NS elongated granitic intrusion believed to be Precambrian suture zone between the east and west Dharwar craton.

Large-scale three-dimensional seismic tomography of the Zagros and Pamir-Hindukush regions

Abstract The nature of the deep seismic structure of the Zagros and Pamir-Hindukush regions is inferred from the three-dimensional P-wave velocity images obtained using the Aki, Christofferson and Husebye (ACH) inversion technique. We use teleseismic (25 ⩽ Δ ⩽ 90°) P-wave arrival times, recorded at 24 seismic stations during the period 1977–1986. A study of the azimuthal variation of residuals reveals that despite the crustal doubling, the Pamir-Hindukush region is characterized by faster arrivals (≈ − 1.0 s) of teleseismic P-waves. The Iranian plateau, on the other hand, exhibits delayed arrivals (0.4–1.0 s) from teleseisms. The results of the ACH inversion reveal contrasting P-wave velocity characteristics in the upper mantle of these deformed zones. In the depth interval 0–200 km, the Pamir-Hindukush region is characterized by 1–4% faster velocities relative to the Iranian plateau. Accounting for the anomalous crustal thickening in the Hindukush results in the subcrustal mantle (up to 200 km) exhibiting a significant higher velocity (≈ 2%), comparable to that of the Indian plate. However, in the depth interval 200–450 km, the Pamir-Hindukush region has 1–3% lower velocities relative to the Indian plate, while the upper mantle beneath the central Iranian plateau is 2–4% slower relative to the surrounding region. The high-velocity upper mantle till a depth of 200 km beneath the Hindukush region, reflects the possible presence of both the oceanic lithosphere and the crust and uppermost mantle of the Indian plate, which subducted beneath Eurasia. The low velocity observed beneath central Iran could be a reflection of the upper mantle thermal anomalies related to the Pliocene-Quaternary volcanism in this region.

Critical evaluation and geological correlation of teleseismic phase data from Indian seismological stations

We evaluated the quality of seismic phase data from Indian seismological stations through the analysis of teleseismic travel times reported during 1976–83 and infer that only WWSSN stations (NDI, SHL, POO, KOD) apart from GBA and HYB can be rated satisfactory while the majority of stations (more than 40) produce very poor quality data sets. Detailed analysis of teleseismic P-wave travel time residuals shows that while the average structure of the upper mantle beneath India has high velocity (negative residuals) there are marked lateral variations. In particular, three zones of anomalous positive residuals (low velocity) are observed: one beneath the north western part of the Deccan trap, the second covering the southernmost peninsula (granulite terrain) and a third rather localized one, to the north of Delhi coinciding with Delhi-Haridwar ridge. New Delhi exhibits strong negative residuals in the E-SE quadrant along with negative station anomaly, implying that it is underlain by an anomalous high velocity crust/upper mantle. The negative residuals observed over India, continue beneath the Himalaya till the south of Lhasa but change sign further northward, suggesting the northern limit of the Indian upper mantle structure.