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Local seismic activity in the region of the Assam Gap, northeast India

EARTH SCIENCES DEPARTMENT ROORKEE UNIVERSITY, ROORKEE, UP 247672, India
CIRES UNIVERSITY OF COLORADO, BOULDER, COLORADO 80309
GEOPHYSICS DIVISION NORTH EASTERN REGION GEOLOGICAL SURVEY OF INDIA, SHILLONG, MEGHALAYA 793003, India

Abstract

A tripartite array of vertical seismographs was operated for 5.5 months in late 1979 in the Shillong Massif area of northeast India. During this time, 719 local earthquakes were recorded. Of these, 359 were located (2.4 M_D 5.1), using for some events additional arrival times from the WWSSN/IMD station Shillong and some portable stations operated by the NGRI/RRL. The area of the Shillong Massif and the Mikir Hills (60 km radius within our array) showed a high seismicity rate of about four earthquakes per week with . This activity rate contrasted strongly with the Assam Gap where there were practically no earthquake sources detected by our array. A seismic asperity seems to exist in the Kopili Valley (between Shillong and Mikir Hills) which may have been the termination of the 1897 rupture, and which could perhaps become the nucleation point for the next great Assam earthquake. Based on these seismicity results and macroseismic data (Seeber and Armbruster, 1981), we estimate the length of the Assam Gap to be about 240 km. Frequency versus magnitude plots revealed b values of about 1.5, and suggested that within the array the locations were complete for . A comparison of local and teleseismic locations and magnitudes could be made only for the largest event which was located 300 km outside the array. This comparison suggests that M_D m_b + 0.5, and that the epicenter location errors outside the array are of the order of 40 km.

This article has been cited by other articles:

				J. R. KAYAL and R. DE Microseismicity and tectonics in northeast India Bulletin of the Seismological Society of America, February 1, 1991; 81(1): 131 - 138. [Abstract] [PDF]

				A. BAPAT Comments on "seismic gap in Assam, Northeast India" by K. Khattri, M. Wyss, V. K. Gaur, S. N. Saha, and V. K. Bansal Bulletin of the Seismological Society of America, October 1, 1985; 75(5): 1469 - 1470. [PDF]