- A 6.4 magnitude earthquake, measured on the Richter scale, rocked Assam in the early hours of April 28 this year. Two people died of shock and there was a lot of damage to property. There were at least 20 aftershocks.
- Soil liquefaction, when water seeps from the ground, was seen in places near the epicentre, near Dhekiajuli in the Sonitpur district.
- Scientists point that anthropogenic factors could contribute to earthquake triggers, although of smaller magnitude.
- More stringent monitoring of construction activities to ensure that the seismic code of safety is followed and awareness among people, are ways to mitigate the rising vulnerability of Assam as it lies in the highest seismic hazard zone.
When a strong, 6.4 magnitude earthquake shook Assam on April 28 this year, 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 northeast India, is categorised under seismic zone 5, which means it’s 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 (ASDMA). Six aftershocks, of magnitude 3.2 to 4.7, occurred within hours of the main tremor. The National Centre for Seismology (NCS) has in fact continued to record seismic activity of magnitude 2.6-2.7 in the region on the eighth day of the main tremor.
The main tremor, said the NCS, occurred near the Kopili Fault, close to the Himalayan Frontal Thrust (HFT) 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 (USGS) “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 and Assam’s vulnerability
Earthquakes are not uncommon in Assam, with the NCS 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 (IIT) 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 leads to the 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 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.
When water guzzled out of the earth
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. And its magnitude was 5.7 on the Richter scale,” Gahalaut said.
Anthropogenic factors at play?
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. 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,” Gahalaut told Mongabay-India.
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 ASDMA 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,” he said.
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 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.
Banner image: Seismic activity has a role to play in adding sediment load to the Brahmaputra river. Photo by VINOYBLOG/Wikimedia Commons.