Why Northeast is one of the most seismically vulnerable regions in India?
Seismicity is a term which expresses the frequency of occurrence of earthquake in a region in the past as well as probable occurrence in the future. Some region experience higher magnitude of earthquake, whereas some experience lower magnitude. There are some regions which may not experience earthquake at all. Depending on magnitude and frequency, we generally define a place as a region of high seismicity and low seismicity. Seismicity of a particular region depends on the tectonic condition
pertaining to number and type of movement of faults in that region.
In India, one of the most seismic hazard zones is North east part of India (Table 1). This region has witnessed several major earthquakes causing large scale devastation. The main cause of this high
seismicity in North East India is its unique placement of tectonic plates and their interactions. This region consists of eastern and north-eastern Himalayas to the north, Indo-Burma ranges to the east, Bangladesh to the south and the Andaman-Sumatra region to the southeast (Fig. 6.1). This region
comprising of Shillong Plateau, Mikir hills, Assam valley, Tripura fold belt and the Bengal basin
(Bangladesh) is jawed in 3-ways between the three tectonic arcs, the Himalayan arc to the north,
Eastern Himalayas Syntaxis (Assam Syntaxis) to the east , the Indo-Burma arc to the south east. The
interaction of these three uniquely placed tectonic arcs makes North East India an interlocked region
of high seismicity. Moreover, the Eastern Himalayan Syntaxis is a complex triple junction that joins
the Indian and Eurasian plates with the northern end of the Burma plate where the Himalayan arc
takes a sharp turn of about 900 and meets the Indo-Burma ranges (Fig. 1).
Fig. 1 Tectonic map of NE India (Kayal, 1998)
Table1: Details of Major Earthquake Experienced by North-east India
Earthquake is a natural phenomenon which is responsible for continuous geological formations and modifications of Earth. Therefore assessing earthquake is necessary in terms of location, frequency, magnitude to evaluate the extent of devastation and threat that it posses. Deaths and destruction during an earthquake mostly occurs due to the collapse of the buildings. North-east India has seen a significant increase in population density and also witnessed a growth of unplanned urbanization. This
has caused human population and different structures to be vulnerable. Seismicity of Northeast India is one of the six most active regions of the world, the other five being Mexico, Taiwan, California, Japan and Turkey. Thus, it is very essential to assess the status of seismicity in the north-eastern region and also to evaluate seismic performance of existing buildings.
References:
Kayal, J.R. (1987). Microseismicity and source mechanism study: Shillong Plateau, Northeast India, Bull. Seismological Society of America, 77:(1): 184-194
Kayal, J.R. (1996). Earthquake source process in Northeast India: A review. Him. Geol., 17: 53-69
Kayal, J.R. (1998). Seismicity of Northeast India and surroundings – development over the past 100
years, Jour. Of Geophysics, 19(1): 9-34.
Paulami Das Choudhury
Assistant Professor
Department of Civil Engineering
UEM, Jaipur