Abstract:
The North-Eastern Region (NER) of India is the most varied but picturesque and beautiful area of rolling forested hills and lush green fields. It has remained a touchy region both politically and geologically. Encompassing a sensitive border zone where India meets Bhutan, Tibet/China, Myanmar and Bangla Desh, this picturesque region is having mountainous terrain all around with Assam Plains in the center. The NER is a land of social, cultural and ecological diversity. Endowed with rich natural resources, it has peculiar characteristic of environmental conditions and fragile ecosystem. Furthermore, the NER is under threat from seismic point of view, and thus, is restless. To understand the occurrence of earthquakes in NER, it is essential to know the tectonic set up of the region. The NER comprises nearly seven geo-tectonic terranes, viz., Eastern Himalaya, Mishmi Block, Naga Hills, Assam Valley, Shillong Plateau & Mikir Hills, Arakan- Yoma Folded Belt, and Bengal Basin. The Eastern Himalaya shows typical Himalayan tectonic belts, such as Siwalik, Gondwana, Miri-Buxa, Bomdila and Sela from south to north, each separated by a thrust. Mishmi Block, which in fact is part of the Shan-Thailand- Malayian Block, is seen in juxtaposition with the Himalayan Block along a Suture Zone. In Arunachal this block shows the Mishmi Massif comprises of granodiorite, diorite and granitic gneiss. Naga Hills shows suppean structure of Tertiary sediments and the ophiolitic mealange. Assam Valley lies between the Himalaya & Naga Hills in the eastern part and between the Himalaya & Shillong Plateau in the western part. The valley has been formed as a result of deposition of sediments brought by the rivers from rising Himalaya, Mishmi & Naga Hills, and Shillong Plateau. Shillong Plateau & Mikir Hills form a part of the main land. Mikir Hills are detached from the Shillong Plateau by NNW-SSE trending Kopili Lineament. Shillong Plateau is delimited by E-W trending Dauki Fault in the south, Yamuna Lineament in the west. There has been a history of continuous uplift of the Plateau since Cretaceous time and submergence of the area lying to the north & south. Arakan-Yoma Folded Belt consists of a series of thrusts and thrust slices that lies to the west of Central Burma molasse basin. Bengal Basin lies south of Shillong Plateau and is bounded by Tripura Folded Belt in the east and Indian Peninsula in the west, and was formed during Eocene to Pleistocene time. Arunachal Pradesh, representing the Himalaya, is trending SW-NE and then bends acutely near the Siang River in NW-SE direction. Along its southeastern corner, it turns again, trending NE-SW. This bend was earlier known as the 'Syntaxial Bend' geologically. However, it is not the 'Syntaxial Bend' in true sense. In fact, it is because of the geological set up of three mountain systems of different origin that occur in juxtaposition. These are : i) Himalayan ranges, ii) Mishmi hill ranges (NW part of Shan-Malayasia Plate), and iii) Naga- Patkai-Arakan ranges. These tectonic zones are geodynamically active. In fact, the Indian sub-continent is moving in northeastern direction @ approx 5.4 cm per year and pressing the Himalaya very hard. In the NER, the Meghalaya-Mikir block is pushing, and consequently, the land of Assam is sliding northwards under the Arunachal Himalaya and eastwards beneath the Indo-Myanmarese Ranges. It is evident from uplift of the Meghalaya Block, sudden narrowing of the Brahmaputra River near Guwahati, severe deformation and recurrent seismicity in the region. Thus, strain is building up, which is being released periodically, as such seismicity is very common in this region, which has been experiencing frequent earthquakes ranging from major to minor intensity. The seismic records show that this region has experienced two great earthquakes of Magnitude >8.7; 22 earthquakes of Magnitude >7; 32 earthquakes of Magnitude between 6 to 7; and innumerable number of smaller earthquakes till 2007. As a result, the entire NER falls under the high seismic zone, i.e. Zone V. The NER, thus, has remained restless and consequently the geo-environment is under threat. Owing to the active geodynamic condition of the terrane, even small tampering with the ecological- geological balance can initiate environmental changes. Thus, there is a need to take extraordinary care, especially in the developmental activities, to minimize the adverse impact on the geo-environment of the region.