Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Indra Mohan ¹, M. V. D. Sitaram ¹, Harsh K. Gupta ¹

Affiliations
1 National Geophysical Research Institute, Hyderabad, 500007, IN

Abstract

The Peninsular Shield of India, generally considered to be aseismic, has recently experienced significant activity al Koyna (1967), Bhadrachalam (1969) and Broach (1970). Besides, over two hundred tremors, with magnitudes ranging from 2 to 5, have also been identified mainly in Andhra Pradesh, Karnataka and Maharashtra during 1967 to 1977, and are catalogued. Low magnitude activity (M < 4) is observed to be prevalent in the Peninsular Shield.

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V. Cermak ¹, J. Kubik ¹, M. M. Saikia ², M. V. D. Sitaram ²

Affiliations
1 Geophys. Inst., Czechosl. Acad. Sci, 141-31 Praha, Czechoslovakia, GB
2 Geoscience Div., Regional Research Laboratory, Jorhat, IN

Abstract

The empirical relationship between observed P_n (Sub-Moho) seismic velocities and the surface heat flow (including also the assessed crust-upper mantle temperatures) was used to improve the understanding of the regional geothermal activity of the North-Eastern Region of India. Vertical seismic velocity profile was further converted into radiogenic heat production and the knowledge of the crustal heat sources helped in both, to estimate deep temperature distribution within the crust and to evaluate the Moho heat flow. The most probable existing temperature at the crust-upper mantle boundary is between 800 and 900°C and the flow of heat from below the crust attains about 50% of observed surface heat flow, i.e, about 30mW. m^-2.

Keywords

North-East Region Heat Flow, Geothermal Model Seismology, Meghalaya, Assam, Nagaland, Arunachal Pradesh.

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M. V. D. Sitaram ¹, Dilip Kumar Yadav ¹, Kabita Goswami ²

Affiliations
1 Geoscience Division, Regional Research Laboratory, Jorhat - 785006, Assam, IN
2 Dept. of Applied Geology, Dibrugarh University, Dibrugarh - 786004, Assam, IN

Abstract

A study of travel times for Pg, P* and Pn waves is conducted for the three profiles along the Main Central Thrust (MCT) and Main Boundary Fault (MBF) in the Arunachal Himalaya and the Brahmaputra Lineament (BPL) in Assam, in order to obtain seismic velocities and the thicknesses of the upper and lower crust and the depth to Moho. These profiles roughly pass through the locations of seismic stations and the epicentres of the earthquakes used in this study. 143 earthquakes with magnitudes 2 to 5.5 and depths of 10 to 20 km during the period 1985-1997 were used to carry out the study. Velocities 6.01±0.15,6.57±0.13, 8.31±0.07 km/sec along the MCT, 6.0±0.1,6.54±0.13 and 8.24±0.14 km lsec along the MBF and 5.90±0.04,6.37±0.08 and 8.01±0.07 km/sec along the BPL for Pg, P* and Pn waves were obtained. The corresponding thicknesses of upper and lower crust are 28±1.6, 23.8±0.2 km for the MCT; 25.1±1.4, 23.8±0.2 km for the MBF and 21.5±1.5,23.8±0.2 km along the BPL (Fig. 2). A 6.5 krn thickening of upper crust from Assam towards Arunachal Himalaya is visualised.

Keywords

Travel Time, Crustal Structure, Main Boundary Fault, Main Central Thrust, Brahmaputra Lineament, Himalaya, Arunachal, Assam.

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