When a strong, 6.4 magnitude earthquake shook Assam on April 28, there was panic and mayhem. The earth cracked near the epicentre in the Sonitpur district, and so did walls and ceilings of people’s houses scores of miles in the radius; buildings swayed “like betel nut trees in the wind”, a hill broke down, and water seeped out of paddy fields.
Already under the siege of the second wave of the Covid-19 pandemic, the earthquake – and the multiple aftershocks through the day – unleashed fear. Two people died of shock and there were several reports of considerable damage to houses and buildings. It also exposed, once again, the vulnerability of Assam to seismic activity and how anthropogenic activities could be further contributing to it.
Assam, and the entire North East India, is categorised under seismic zone 5, which means it is extremely prone to high-intensity earthquakes. On April 28, the 6.4 magnitude earthquake measured on the Richter scale, was followed by 20 aftershocks of different magnitudes through the day, according to Gyanendra Dev Tripathi, CEO of Assam State Disaster Management Authority. Six aftershocks, of magnitude 3.2 to 4.7, occurred within hours of the main tremor. The National Centre for Seismology has in fact continued to record the seismic activity of magnitude 2.6 to 2.7 in the region on the eighth day of the main tremor.
The main tremor, said the National Centre for Seismology, occurred near the Kopili Fault, close to the Himalayan Frontal Thrust which is a seismically very active area “associated with collisional tectonics where Indian plane sub-ducts beneath the Eurasian Plate”.
A fault, according to the United States Geological Survey “is a fracture or zone of fractures between two blocks of rocks. Faults allow the blocks to move relative to each other.” The Kopili fault is a 300 km northwest-southeast trending fault from the Bhutan Himalaya to the Burmese arc.
Earthquakes are not uncommon in Assam, with the National Centre for Seismology saying that the region has seen several moderate to high-intensity earthquakes. One of the worst among them was the great Assam-Tibet earthquake in 1950 which measured 8.6 magnitude on the Richter scale.
Approximately 4,800 people were killed as a result, and there were several landslides that blocked the tributaries of the Brahmaputra and changed the topography of the region. The 1869 Cachar earthquake, measuring 7.4 magnitude, was another major seismic activity to hit the region.
Experts have said that recent seismicity discovered along the Kopili fault had led to speculations that it is one of the most seismically active faults of the region. A large portion of the Kopili fault region, said scientist Nilutpal Bora and its neighbouring areas are characterised by alluvial soil that has a higher potential of trapping seismic waves and therefore making it one of the most earthquake-prone zones in northeast India.
“Assam, being located in the highest seismic zone, is perpetually challenged by the possibility of occurrence of earthquakes as an expression of release of accumulated tectonic stress,” Chandan Mahanta, professor in the department of civil engineering, Indian Institute of Technology Guwahati, told Mongabay-India. Continuous tectonic stress keeps building up along the fault lines, he said, and the 6.4 magnitude tremor was a release of such accumulated stress.
One thing to other
High-intensity tremors aside, the region is vulnerable to seismic activity of different magnitude and intensity. This, said Mahanta, could be a contributing factor to erosion, since “seismic activity can disturb earth material properties, like strength and cohesion, and slope instability adds to this”.
Images of a portion of a hill breaking and falling into a river in the Udalguri district following the 6.4 magnitude tremor showcased to the rest of the world the intensity of the quake.
The 1950 quake had led to many landslides and Mahanta said that apart from the major quakes mentioned, “many landslides are seismologically induced”.
This means that seismic activity has a role to play in adding sediment load to the Brahmaputra river. “Landslides in the upper Brahmaputra are known to add high sediment load to the river,” Mahanta said.
This is significant because a high sediment load on the Brahmaputra is known to cause recurrent floods since the river bed rises and the width of the river increases. Dredging of the river, the Assam government has long said, is a solution to this problem, allocating huge amounts of funds to this end.
In 2017, Union minister of transport, Nitin Gadkari, had announced Rs 250 crores for dredging the Brahmaputra. Experts, however, opine that dredging the entire river is neither a feasible nor a permanent solution since the silt makes its way back after being removed.
This April’s tremor also led to a seemingly “strange” event: of water springing out of paddy fields, close to the epicentre, near Dhekiajuli, in Assam. A video of this was shared on social media by the present state chief minister Himanta Biswa Sarma.
Scientists call this phenomenon, soil liquefaction. By definition, it means when saturated or partially saturated soil substantially loses strength and stiffness in response to applied stress, like the shake of an earthquake.
And in this case, Vineet Gahalaut, chief scientist, CSIR-National Geophysical Research Institute, said it was nothing out of the ordinary particularly because “it is common in places with shallow water table”.
“In 2017, the Manu earthquake in Tripura caused soil liquefaction till Bangladesh,” Gahalaut said. “And its magnitude was 5.7 on the Richter scale.”
What should be considered, more importantly, is the possibility of anthropogenic factors impacting seismic activity. “Whether anthropogenic factors can cause an earthquake of this magnitude (6.4) is difficult to establish,” Gahalaut told Mongabay-India. “But there have been few cases in the US, where seismic monitoring is good, where it has been seen that anthropogenic factors may have caused small magnitude earthquake and seismic activity.”
As an example, he said, excessive mining, geothermal pumps, construction of dams and injecting water under high pressure in oil reserves to crack the area in order to release fluid and oil may “trigger” seismic activity. A research paper, “Influence of anthropogenic groundwater unloading in Indo-Gangetic plains on the April 25, 2015 Mw 7.8 Nepal Earthquake”– of which Gahalaut is a co-author– talks along similar lines.
“Tectonic process and anthropogenic factors are two very different things, but like the last straw on the camel’s back, when the pressure is already high and in a critical position, human activity can trigger an earthquake,” Gahalaut said.
For northeast India and for Assam which is in the highest probability zone in terms of earthquake hazard, the vulnerability quotient is high and increasing. “Construction on hills, mushrooming high-rises increase the vulnerability factor,” Tripathi of Assam State Disaster Management Authority told Mongabay-India. “The way to mitigate this vulnerability is to ensure all construction follow the seismic code of safety and making people aware of safety measures.”
Whether every building in Assam and in particular Guwahati – “especially the new ones”, said an expert – is following the code, however, is questionable.
“There is no strong supervisory mechanism or regulatory policy to ensure that this code is followed in all construction,” Tripathi said.
Mahanta suggests that “high-resolution, micro zonation-based planning and building design following the correct seismic code” is the key to building safety which is crucial in a place like Assam. A 2007 seismic microzonation atlas of the Guwahati region is a case in point.
Interestingly, the tremor also brought into limelight the good-old, single-storeyed “Assam-type” houses which were built with indigenous material and were more capable of resisting earthquakes.
Fast disappearing from urban areas, the jolt has revved up nostalgia on social media and has spurred conversations around the restoration of such ancestral houses.
This article first appeared on Mongabay.