S. Kalita

Clockwise rotation of the Brahmaputra Valley relative to India: Tectonic convergence in the eastern Himalaya, Naga Hills, and Shillong Plateau

GPS data reveal that the Brahmaputra Valley has broken from the Indian Plate and rotates clockwise relative to India about a point a few hundred kilometers west of the Shillong Plateau. The GPS velocity vectors define two distinct blocks separated by the Kopili fault upon which 2–3 mm/yr of dextral slip is observed: the Shillong block between longitudes 89 and 93°E rotating clockwise at 1.15°/Myr and the Assam block from 93.5°E to 97°E rotating at ≈1.13°/Myr. These two blocks are more than 120 km wide in a north-south sense, but they extend locally a similar distance beneath the Himalaya and Tibet. A result of these rotations is that convergence across the Himalaya east of Sikkim decreases in velocity eastward from 18 to ≈12 mm/yr and convergence between the Shillong Plateau and Bangladesh across the Dauki fault increases from 3 mm/yr in the west to >8 mm/yr in the east. This fast convergence rate is inconsistent with inferred geological uplift rates on the plateau (if a 45°N dip is assumed for the Dauki fault) unless clockwise rotation of the Shillong block has increased substantially in the past 4–8 Myr. Such acceleration is consistent with the reported recent slowing in the convergence rate across the Bhutan Himalaya. The current slip potential near Bhutan, based on present-day convergence rates and assuming no great earthquake since 1713 A.D., is now ~5.4 m, similar to the slip reported from alluvial terraces that offsets across the Main Himalayan Thrust and sufficient to sustain a Mw ≥ 8.0 earthquake in this area.

Seismic Strain Energy Release Pattern in Northeast India and its Adjoining Region

Northeast India and its adjoining region constitutes an important geotectonic element of Southeast Asia and is connected to India via a narrow corridor squeezed between Nepal and Bangladesh. Geomorphologically, the entire NE India is located in an earthquake prone zone ( Zone - V ) of the Indian subcontinent. The strain energy release has been studied by dividing the region into in the six geo - tectonic block. It has been found that the Arakan - Yoma to be seismically active followed by Naga Hills region. It also showed that the probability of occurring an earthquake is more in the Shillong Plateau than the other five tectonic blocks. For the region as a whole there is the probability that an earthquake of intensity around 6.64 mb may occur as determined from strain energy bearing capacity of the region. Moreover, it has also been found that if strain energy released by a tectonic block is large it might effect the stress building process in the rocks of adjacent tectonic blocks. The iso-strain release map depicted areas of high and low seismic activity.

Physico-Chemical characteristics of drain-water of open cast coal mining area in the Ledo-Margherita range of Assam

Coal mine drainage is a major problem in coal mining areas. It may adversely affect the water-bodies in and around mining areas. Oxidation of pyrites, from coal seams and overburdens, in presence of air and water leads to very low pH and results in sulphate and heavy and rare earth metals dissolved into the water. In the present study, Physico-Chemical Characteristics of water of drains carrying mine-waters in Ledo and Tirap open cast coal mining areas have been investigated. Water samples were analysed for pH, Electrical Conductivity (EC), hardness SO42-, NO3-, dissolved oxygen (DO), Fe2+, Cr, Pb, Ni and Zn. The results indicate highly acidic condition of the drain-water with low DO, high EC, high hardness and higher concentrations of SO42-, Fe2+ and Pb.

Applicability of surface wave magnitude scale, Ms, and body wave magnitude scale, mb, in the study of seismicity of Northeast India and its adjoining region

Richter gave an empirical relation between surface wave (Rayleigh Waves) magnitude , Ms, and body wave magnitude, mb, ( generally short P type body waves of period 1 sec). At the outset, we have tried to modify the Richters relation by taking into account of the earthquake data of the North-East India and its adjoining region. Then we analyse the applicability of surface wave magnitude scale and body wave magnitude scale for the seismicity study of the region. Lastly the completeness of the earthquake datafile is checked. Various statistical methods are adopted in this present study. It has been seen that there is not much difference between the values of surface wave magnitude obtained from Richters relation and modified Richters relation. Since error is less,it would be better if modified Richters relation is applied. From the study of applicability it is found that error is less if body wave magnitude is taken in the seismicity study. From the completeness analysis of the datafile it has been found that the datafile is complete from different period onwards for different magnitudes.