Just a week ago on Saturday at 11.08 pm, an earthquake measuring 5.3 on the Richter scale jolted Northeast India including Assam, Manipur, Tripura, and Mizoram. This is the third earthquake that rocked Assam and the second one in Manipur only in the last two months alone.
It is a well-known fact that the region of Northeast India is highly vulnerable to earthquakes as geomorphologically this region falls under Zone V in the seismic map of India. The Northeastern Region has experienced 18 large earthquakes (M>7) during the last hundred years.
This including the great earthquakes of Shillong (1897) and the Assam-Tibet border (1950) both with M=8.7. This is leaving several small and microearthquakes which has also been recorded frequently in the region. The high seismicity in this region can also be attributed to the collision tectonics between the Indian Plate and the Eurasian Plate in the north and subduction tectonics along with the Indo-Myanmar range (IMR) in the east.
Historically speaking, earthquakes during the non-instrumental period occurred during the years of 1548, 1596, 1601, 1642, 1663, 1696, 1756, 1772, 1838, and 1841. Earthquakes during the instrumental period can be listed during the years of 1869, 1897, 1923, 1930, 1943, 1947, 1950, 1985, 1984, and 1988. This can also be a sign of the fact that frequent and major earthquakes, in the long term basis prediction, is likely to occur in this region.
The high vulnerability and seismic risk in India can be attributed to the fact that 10.9% of its geographical area falls in the seismic Zone V. This zone is vulnerable to very high seismic risk. Add on to it that 17.3% of India’s geographical area falls in seismic Zone IV which is also vulnerable to high seismic risk. Clubbed together, these two zones alone covers 28.2 % of India’s geographical area.
229 districts of India fall within seismic zones IV and V according to the Vulnerability Atlas of India prepared by the Building Materials Technology Promotion Council (BMTPC).
Now, these values are a wholesome view of the entire earthquake scene of India, particularly Northeast India. So to know a bit about the reason behind the recent successive tremors team EastMojo was able to interact with a seismologist who is a government official and under conditions of anonymity stated, “What northeast India has been experiencing these past few months is nothing new. We are anyway in a dynamically active zone and so the change in the frequency of earthquakes should not be surprising.”
He went on to state that we cannot stop earthquakes as it is a natural phenomenon. All one can do according to him is to sensitize ourselves to the dangers of living in an earthquake-prone area which includes the need to construct earthquake-resistant structures.
“Many believe that these small tremors are a sign of a major earthquake to come but it is not a scientifically backed fact and so there is no need to panic,” he said adding, “it’s the panic that causes death and not the earthquake.”
He concluded by saying that in scientific terms what we are facing is a normal phenomenon for Northeast faces tremors of a minimum 5 magnitude every 100 days.
It is a common saying that it’s not the earthquake that kills but it’s the buildings that do. Being in a seismically active zone, the residents of the Northeastern states of India must enforce building codes in the construction of houses or high rise flats. If not, even a moderate intensity earthquake may result in widespread destruction of property and life.
Although, the 1887 earthquake in Shillong which caused widespread devastation and crippled the hilly town. However, upon further observation, it was found that some traditional Khasi houses survived the tremor. This launched a study in the earthquake-resistant feature of such traditional houses. Basically, the traditional Khasi houses are built on raised platforms and the roofs are of thatch while the walls may be from materials such as stone masonry with lime mortars, lime rendered mud walls, or thatch depending on prevailing weather conditions.
Some of the characteristics of the traditional Khasi houses in Shillong that conform with modern building code requirements include the fact that they are not usually built on hilltops. Additionally, they have symmetrical oval shapes which are devoid of sharp corners, thereby avoiding stress concentrations. These are a major source of failure at wall corners during earthquakes. Meanwhile, nails are not used in the constructions, and the grooves and tongues that are utilized allow for dissipation of seismic loads. The roofs are made of light materials so that fatalities from failed roofs are limited.
Additionally, on the same lines of earthquake-resistant houses are the Assam Type ones. Back in 2009 Hemant Kaushik and KS Ravindra Babu (housing specialists) wrote in the World Housing Encyclopaedia that these houses are made with walls of bamboo or reed mesh and plaster set in a wooden framework are “highly earthquake resistant.” Additionally, with the light tin roof and wooden floors, the chances of getting injured reduce drastically. The usage of such materials and construction techniques makes the buildings light, flexible, and able to move with the swaying caused by the tremors, thereby reducing extensive damage or casualties.
National Institute of Disaster Management of the Ministry of Home Affairs has put out advisories for the construction of Earthquake resistant houses in different seismic zones of India. MHA is keen that all new buildings should be made earthquake resistant in the first instant. The advisory focuses on the usage of good cement mortar with the ration of one part being cement and four parts being sand.
Additionally usage of horizontal seismic bands- consisting of the reinforced concrete flat runner through all external and internal masonry walls at
The plinth level of the building
The levels of lintels of doors and windows
The ceiling level of roofs, etc. are encouraged for safety. Here is the list of the additional details for the construction of houses in the Seismic Zone V.
Northeast India is seismically one of the six most active regions of the world, the other five being Mexico, Taiwan, California, Japan and Turkey. . Earthquake hazards in this northeastern region cannot be changed, however the disaster can be mitigated. One of the most important steps for mitigation of hazard is the building up of capacity in Civil Engineering and Architectural Professionals. This is to ensure that earthquake resistant constructions are easily available and applicable for everyone. As for the the existing buildings and structures retrofitting is one the optimum options for the mitigation of disaster as most of the destructions/ damages are caused due to the collapse of structure.