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New Delhi, India

Saikia S.,Ministry of Earth science | Chopra S.,Ministry of Earth science | Baruah S.,NEIST formerly RRL Jorhat | Baidya P.R.,Seismology Division | Singh U.K.,Indian School of Mines
Geomatics, Natural Hazards and Risk | Year: 2015

Over 450 receiver functions from 8 broadband stations located in the Indo-Gangetic plain and Northwest Himalayan region are analyzed to examine the crustal properties across the contiguous region. We identified the P-to-S phase beneath each station and estimated the crustal thickness from time delay of this phase with respect to the direct P arrival. With the help of the slant stacking technique, we determined bulk crustal chemical properties and validated our estimate of crustal thickness. The Moho was encountered in the Indo-Gangetic plain at an average depth of 33 km and thickened towards the Northwest Himalaya with the Moho depth varying from 37 to 52 km. The thickest crust matched the highest topography, which is strong evidence of the occurrence of a crustal root of the mountain range. The time domain iterative linearized inversion technique is used to invert radial receiver functions to wave velocity structures for both tectonic regimes. From the forward modelling, we found mid-crustal low-velocity layers at different patches at a depth of 10–30 km in the Northwest Himalaya region. The presence of melts may be inferred in the mid crust with high values of Poisson ratio (σ ≥ 0.260) for the stations in the Northwest Himalaya. Towards south in the Indo-Gangetic alluvium plain, we estimated a medium to higher value of Poisson ratio (0.240 ≤ σ ≤ 0.290), but velocity modelling implies absence of an intracrustal low-velocity zone around the region. © 2015 Taylor & Francis Source

Qadri S.M.T.,University of Sargodha | Sajjad S.H.,University of Sargodha | Sheikh R.A.,University of Punjab | Rehman K.,University of Peshawar | And 3 more authors.
Natural Hazards | Year: 2015

Earthquakes are one among the highly dangerous geological hazards. The damage from earthquakes is greatly influenced by the local geological conditions. The history of Pakistan is full of severe earthquakes that caused the mass destruction at national scale. Major earthquake-affected areas in Pakistan are mainly concentrated in the north and western sections of the country. Site response analysis is an initial step towards seismic risk assessment. The present study aims at local site effects by acquiring data along 105 sites of Rawalpindi–Islamabad, the twin cities of federal capital of Pakistan, by applying H/V spectral ratio method. Out of the data acquired for 105 sites, data for 88 sites were considered reliable after observing reliability conditions determined by SESAME guidelines devised in 2004. Fundamental frequency f0 of soft sediments, amplitude A0 of corresponding H/V spectral ratios, thickness of soft sediments residing on the bedrock and soil vulnerability index Kg over 88 reliable data sites within the study area were estimated and analysed. The results show that the fundamental frequency of the sediments reflects variation and lies within a wide range between 0.6 and 14.4 Hz. Similarly, amplification factor A0 up to 5.5 was observed at a site. Overburden thickness of soft sediments H also indicates high variation and has been calculated in the range of 1.7–316.5 m. As far as Kg is concerned, it is also highly variable and is lying in the range of 0.30–62.7. These results will help in designing a policy to mitigate the impact of seismic hazard in the study area. © 2015, Springer Science+Business Media Dordrecht. Source

Prajapati S.,Institute of Seismological Research Raisan | Chauhan M.,Institute of Seismological Research Raisan | Gupta A.K.,Institute of Seismological Research Raisan | Pradhan R.,Institute of Seismological Research Raisan | And 2 more authors.
Bulletin of the Seismological Society of America | Year: 2011

The lithospheric velocity structure of the lower Indus basin has been evaluated through inversion of fundamental modes of both Love and Rayleigh wave group velocities from the broadband records of a seismic network maintained by the Institute of Seismological Research, Gujarat, India. We have considered three clusters of wave paths A, B, and C that mainly cross the lower Indus basin from south to north; the wave paths of A mainly cross the continental shelf, and the wave paths of B and C pass through the lower Indus basin. The measured group velocities correspond to periods of 5 to 90 s for Rayleigh waves, and 5 to 115 s for Love waves. These data sets resolve the structure of the lithosphere through a nonlinear inversion based on a genetic algorithm with a wide solution space. The mean and standard deviation (S.D.) of the 70 accepted solutions for each of these three clusters provide the 2D structure for the lower Indus basin from south to north. The sediment consists of two layers with total thickness from 5.7 to 6.6 km increasing northward. The crustal thickness also increases northward from 32.9 (cluster A) to 39.7 km (cluster C) in the lower Indus region. The S-wave velocity below the crust varies from 4.55 to 4:59 km/s, which is close to the corresponding velocity of 4:60 km/s of the Indian shield region to the east of the Aravalli range. The thicknesses of the lithosphere, as well as the velocities of the uppermost mantle of the lower Indus plain, are similar to that of the Indian shield. Source

Kumar A.,Indian Institute of Science | Mitra S.,Indian Institute of Science | Suresh G.,Seismology Division
Tectonics | Year: 2015

The eastern Himalayan and Indo-Burman plate boundary systems are distinct from the rest of the India-Eurasia continental collision, due to oblique convergence across two orthogonal plate boundaries resulting in a zone of distributed deformation both within and away from the plate boundary. To understand the seismotectonics of this region, we model the source mechanism of 44 earthquakes using waveform inversion and combine them with source mechanism of 30 previously studied earthquakes. Depth distribution of these earthquakes reveal that the entire crust beneath northeast India is seismogenic. From spatial distribution and source mechanism it is evident that the N20'E convergence between India and Tibet is accommodated by N-S convergence and E-W subduction. The N-S convergence is accommodated through (a) shallow thrust earthquakes within the eastern Himalayan wedge, (b) lower crustal thrust earthquakes along the northern edge of Shillong Plateau, (c) lower crustal dextral strike-slip earthquakes in the Kopili fault zone, and (d) sinistral strike-slip earthquakes within the Bengal Basin crust. The E-W subduction results in shallow thrust earthquakes to intermediate depth strike-slip earthquakes and deep focus thrust earthquakes underneath the Indo-Burman convergence zone. Orientation of the fault plane and slip vectors point to downdip extension and along arc compression of the subducted Indian plate in response to slab pull forces and buckling at depth. Earthquake slip vectors are in good agreement with the GPS velocity vectors across northeast India and conforms to the clockwise rotating "microplates" model. ©2015. American Geophysical Union. All Rights Reserved. Source

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