N-E within Asia’s most active seismic zone

“Until recently, only two to three earthquakes used to be reported every month in the region. But with the installation of a network telemetric seismic stations, RRL has been able to record 100 to 150 tremors a day,” studies carried out by RRL scientists have said.

The North-Eastern region, it may be recalled, has experienced some of the most devastating earthquakes in the country during the past hundred years. According to Dr M.M. Saikia, who has recently retired as deputy director of RRL, Jorhat, has quoted statistics to show that between 1897 and 1952, there were 44 earthquakes which measured 6.5 or more on the Richter scale. Similarly, between 1953 and 1992, the region had 21 earthquakes of the same intensity or more.

RRL, Jorhat, in collaboration with the National Geophysical Research Institute (NGRI), Hyderabad, has recently established a network of 16 seismic stations in the entire region, which has immensely helped in recording more details of earthquakes occurring here.

The scientists at RRL, Jorhat, have confirmed that the seismic activities in the region are associated with five well-defined major tectonic lineaments constituting the crustal weakness.

These lineaments are:
(a) The Himalayan thrust belts bordering the Assam Valley;
(b) The North-East-South-West directed Naga-Dissang group of thrust faults bordering the Upper Assam Valley;
(c) The Mishmi thrust system;
(d) The East-West Dawki lineament of the southern border of the Shillong Plateau;
(e) The tectonic lineaments associated with interior Nagaland and Manipur having genetic relationship with the Arakan-Yoma-Chin belt.
The scientists have also come to the conclusion that the seismic activities in the region have shown a well-defined arcuate belt. This, in turn, is analogous to an island-arc system with well-defined volcanic lines, a belt of intermediate depth earthquakes and a negative gravity anomaly.

They have also pointed out that the mountain arc is considered to be the outcome of collision between a continent and island-arc system. The folds of Arakan-Yoma and Western Sumatra and also the tertiary folds of the Upper Assam, Andaman-Nicobar and Meantewai chains represent the outgrowth of South-East Asia.