Local seismic activity in the region of the Assam Gap, northeast 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 M_D ≧ 3_¼. 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 M_D≧3_¼. 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.