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What led to Assam Earthquake; why the area is seismically unstable

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Assam, earthquakeAssam, earthquakeThe area falling in the highest Seismic Hazard zone V is seismically very active. (image: PTI)

On Wednesday at around 8:00 AM, several buildings and houses were damaged after a 6.4 magnitude earthquake hit Assam. Following the main tremor, six aftershocks occurred of magnitude ranging between 3.2 and 4.7 in two-and-a-half hours. The earthquake’s epicentre was in Dhekiajuli town in the district of Sonitpur, according to an IE report. The National Centre for Seismology (NCS) said that the primary earthquake had its epicentre at latitude 26.690 N and longitude 92.360 E, around 80 kilometres northeast of the state’s capital- Guwahati, and a focal depth of 17 kilometres. It further mentioned that as per the preliminary analysis, the events are situated near to Kopili Fault closer to Himalayan Frontal Thrust. The area falling in the highest Seismic Hazard zone V is seismically very active.

What is the fault?

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Himalayan Frontal Thrust (HFT), which is also known as the Main Frontal Thrust (MFT), is a geological fault along the Indian and Eurasian tectonic plates’ boundary. The Kopili Fault is known to be a 300-kilometre northwest-southeast trending fault from the Bhutan Himalaya to the Burmese arc. A scientific agency of the US federal government, United States Geological Survey (USGS) defines a fault as a fracture along which either side’s crust blocks have moved relative to one another parallel to the fracture. USGS says that when an earthquake takes place on one of these faults, the rock on one of the fault’s side slips with respect to the other. The surface of the fault can be horizontal, vertical or at some angle to the surface of the earth.

Why is the area seismically unstable?

According to Prof Chandan Mahanta of the Department of Civil Engineering at IIT-Guwahati, the northeast region is situated in the highest seismological zone, so there should be constant earthquake preparedness at all levels. The professor said tectonic stress keeps building up continuously, especially along the faultlines. This earthquake was because of accumulated stress release. At this epicentre, stress was probably constrained for a fairly long time, hence there was relatively higher intensity release.

The earthquake’s timing and its duration ensured the damage was restricted, Mahanta stated. The earthquake took place early in the day when most of the people were home. If it occurred during working hours, say when there were labourers at under-construction sites of high-rise buildings, the earthquake might have taken lives, he said. According to him, the duration is also important. Had the main tremor continued with the same intensity for over 30 seconds, the resultant acceleration, as well as resonance, could have caused greater damage to buildings, he added.

History of earthquakes

According to the NCS report, historical and instrumentally recorded data of earthquakes show the region has witnessed several moderate to large earthquakes in the past. The worst of these earthquakes that was recorded was the great Assam-Tibet Earthquake that took place on Independence Day in the year 1950. The earthquake had a 8.6 magnitude and was in the same league as the Great Chile Earthquake, with its 9.5 magnitude, ten years later or the Tohoku-oki earthquake off the coast of Honshu in the year 2011, for which the magnitude was determined to be 9.0, it said.

Earlier on June 12, 1897, another massive earthquake of magnitude 8.1 had shaken the state of Assam. As per a scientific paper on the earthquake in the journal Nature, the earthquake of 12 June 1897 in Assam reduced to rubble all masonry buildings within the country’s northeastern region roughly England’s size and was felt over an area exceeding that of the great Lisbon earthquake of 1755.

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