Mtechprojects

M.Tech Projects

2009

1. "INVESTIGATION OF SEISMIC ANISOTROPY UNDERNEATH THE INDIAN SHIELD USING DIRECT S-PHASE"

by Dipankar Siakia

Abstract:

Shear wave splitting measurements are performed at 40 broadband seismic stations from Indian Shield using direct S-wave from deep and intermediate depth events. Unlike to the core refracted phases direct S-waves are not vertically polarized and hence sensitive to both radial and azimuthal anisotropy and contain the influence of anisotropy lying anywhere in the ray path from source to receiver. To avoid the source side contamination events occurring below 410km, olivine-spinel transition zone are generally considered. Anisotropic contribution from transition zone and mid mantle are inferred from larger delay times which cannot be explained with only mantle contribution. From our measurement of splitting parameters we have not found any such unusual delay time which requires contribution from mid mantle or transition zone. Only two measurements with > 2 s splitting time are from two relatively shallow events and may have possible source side contamination. Fast polarization directions in the northern part are found to have two distinct trend NE and NS, predominantly NE which is the absolute plate motion direction of the Indian plate.

- For the award of the degree of Master of Technology from Tezpur University

2008

2. "SEISMICITY AND MOMENT TENSOR INVERSION STUDIES IN SIKKIM HIMALAYA"

by Pinki Hazarika

Abstract

In this dissertation work a study of seismicity and focal mechanisms in Sikkim Himalaya have been carried out. A temporary broadband seismic network has been operated in Sikkim Himalaya by the National Geophysical Research Institute, Hyderabad since 2004. A total number of 355 local earthquakes were well located for the period 2006-2007 in this work. The seismicity map shows that seismicity is not uniformly distributed in the study region. Most of the earthquakes were located in between Main Boundary Thrust (MBT) and Main Central Thrust (MCT). Two cross sections in N-S and E-W directions cuttings across MBT and MCT have been studied. The regions north of MBT and MCT are found to be seismically active. The seismic activity is continuous from surface to lower crust. Some earthquakes are located having focal depth more than 50 km in northern Sikkim. The b-value, i.e. slope of the frequency magnitude relation, is found 0.64, which is less than the normal value of 1. Waveform modelling has been done using moment tensor inversion technique. The tectonic setting is very complex in Sikkim Himalaya and due to that previous earth models are not fit well while doing moment tensor inversion. For getting well fitted earth model grid search and Monte Carlo methods have been attempted. The focal mechanisms yield from this study gives both strike-slip and reverse mechanism.

- For the award of the degree of Master of Technology from Tezpur University

3. "ANISOTROPHY OF THE STABLE AND DEFORMING REGIONS OF THE INDIAN CRUST FROM SPLITTING OF P-TO-S CONVERTED PHASES"

by Saifur Rehman

Abstract

On the basis of regional orientation of motion and seismicity, the Indian subcontinent is divided into four provinces. These are Mid-continent stress province, the southern shield, the Bengal basin, and the Assam wedge. The geology of the Indian subcontinent is quite interesting. Huge variations in mineralogy are easily found in the subcontinent. Indian shield is a Precambrian craton, formed at early to late Archean age and demarcated by rift zones containing Proterozoic to and/or Phanerozoic sediments. Crustal seismic anisotropic characteristics of Indian subcontinent remained elusive in view of the paucity of work on crustal anisotropy. This study is mainly related to understand the seismic anisotropic characteristics of the crust of Indian plate. Moho converted phase Ps from 167 events recorded at 27 broadband seismic stations located on various geological units of the Indian plate were analyzed and results were presented. At most of the stations in the Indian shield the fast axis azimuths strongly correlate with the fast axis azimuths measured on analysis of SKS/SKKS waveforms. Most of the stations from our study region, we have found the delay time between the fast and slow axis about 0.3 second which was expected. However, at stations SMLA we have found the fast axis azimuth parallel to the strike of mountain belts. Among the two stations from north east India, station TEZ show fast axis azimuth well correlate with stress patterns of the regions and station SHL show fast axis direction close to the mountain strike direction. The delay time for these two stations is 0.29s. Station CAL located on Bengal Basin show fast axis azimuth close to N-S direction.

- For the award of the degree of Master of Technology from Tezpur University

2007

"SEPARATION OF MULTIPLES FROM THE RECEIVER FUNCTION DATA BY RADON TRANSFORM"

by Datir Harish Baban

Abstract

Presence of additional mantle layering in the shallow depth regimes, between 100-300km is increasingly being reported from study of P-S conversions. However, two different problems curse the results concerned to these depth domains. One is related directly to the quality and ambiguity related to their detection itself by P-S conversions while the other concerns with the origin of these boundaries. It is important to note that for the intervening regions of 100-300km, the time of arrivals of primary P-S converted waves from interfaces in the depth range 150-300km are indeed often obscured by multiple energy arising due to shallower structure (e.g. crustal multiples), leading to their misidentification and doubtful results. This problem further accentuates in detection of interfaces associated with negative velocity contrasts due to presence of low velocity layers (LVL; e.g. lower boundaries of subducting slabs, plume affected regions). Recognizing that primary conversions and multiples can be treated as forward and back-scattered energy fields, intuitively, transformation of the receiver function recordings (a time series) into an appropriate domain where these two energy fields can get distinctly separated seems viable. This can be explored applying/designing mathematical transforms that affect such a separation. After transformation, the undesirable multiple energy response can be suppressed (muted) and the desirable primary converted energy (signals) arising from velocity interfaces can be inverse-transformed into time-domain. This enables unambiguous identification and depth localization of the interface/velocity boundary responsible for generating the observed primary conversions. Investigations in this direction are in progress exploring the utility of Radon Transform. The utility is often used in Seismic Data Processing. The primary focus of this dissertation work is to separate the conversions from multiple energy.

For the award of the degree of Master of Technology from IIT Roorkee