SEISMOTECTONICS OF NORTHEASTERN INDIA

Intraplate seismicity accounts for about 20% of the earthquake activit y of northeastern India. The Shillong plateau, northern Bengal basin, Tripura fold belt and Kopili lineament zone together contribute 87% of the intraplate seismicity of the region while Upper Assam is free fro m earthquake activity. The high seismic activity of the Shillong plate au appears to be related to the gently northward dipping seismogenic t hrust fault, rupturing of which caused the great Assam earthquake of 1 897. Excessive stresses induced in the Shillong plateau, northern Beng al basin and the Kopili lineament zone due to under-thrusting along th e Shillong thrust fault and beneath the eastern Himalaya appear to be the root cause of the intraplate seismicity in the region, The entire intraplate area excepting Upper Assam and the Kopili lineament zone is identified as a separate stress region characterized by a mean S-Hmax orientation of N3 degrees E, The mean S-Hmax orientation in Upper Ass am is N26 degrees E. The Kopili lineament zone forms a broad stress bo undary between them. Fractures/faults associated with only the Sylhet lineament, Eocene limestone hinge, Kopili, Tista and Padma lineaments are favourably oriented for their reactivation in strike-slip mode, Th e fact that only the first three amongst them are seismically active m ay be attributed to the high stresses induced in the northen Bengal ba sin and Kopili lineament zone, It appears that the crust beneath Upper Assam is a rigid block with unfavourably oriented fractures/faults, a nd is being deformed at a much slower strain rate, and as a result it is seismically stable.