Natural hazards in the north-east region of India

Abstract

North-East India is afflicted by three main natural hazards: floods, earthquakes and landslides. Of these, the earthquakes are most unpredictable. Earthquakes are generally grouped under four categories depending on their severity, measured on the Richter scale. In common parlance, these are known as ‘slight’, ‘moderate’, ‘great’ and ‘very great’ earthquakes. These categories, on the Richter scale, measure as <4.9, 5–6.9, 7–7.9 and >8, respectively. The frequency of these earthquakes varies with the intensity of the earthquakes, slight earthquakes occurring more frequently than the great or very great ones. The low-intensity tremors occur quite frequently in the contact zone of plate boundaries or along the lines of structural weakness, like the Main Boundary Fault (MBF) or the Main Central Thrust in the Himalayan region or along a chain of thrusts extending from Lohit district (with Mishmi fault) in the north to Manipur and further south. From the mid-nineteenth century till date, there have been seven earthquakes with an intensity of >7.0 on the Richter scale, of which the two occurring in 1897 and in 1950 were very severe, the first one measuring 8.7 and the second one 8.5 on the Richter scale. The earthquake of 1950 not only caused tremendous loss of property and life, but even changed the course of many rivers including the morphology, especially depth profile of Brahmaputra. Floods are a recurring annual feature of Assam when Brahmaputra and its tributaries, with very large catchments, are flooded exceeding the limit of bankful discharge and submerge a substantial part of Brahmaputra plain. In very severe floods, three to four million hectares of land are affected. These floods occur between May and September, the period of summer monsoon. The floods affect the crops, cause erosion, breach embankments, wash away cattle, destroy houses, uproot trees and even affect the wildlife sanctuaries.