Kopili Fault zone 'very active', may trigger earthquake over 7 magnitude: Researchers

Guwahati, Mar 1: The 300-km-long Kopili Fault zone has been "very active" and is vulnerable to generating an earthquake of magnitude over 7, according to researchers.

Preliminary analysis of February 27's magnitude 5.0 quake showed that it was located near the Kopili Fault and closer to the epicentre of the magnitude 6.4 Sonitpur earthquake that had occurred on April 28, 2021.

"The Kopili Fault is identified as the most seismically active fault that caused two (Cachar, 1869, and Mikir hills, 1943) large earthquakes (M 7.0) in the past and several severely felt earthquakes (5.0 and above) in recent years. The implication of the 2021 Sonitpur earthquake sequence possibly reminds us of the vulnerability of the Kopili fault zone to generate a larger earthquake of magnitude 7.0 in the near future," according to the vulnerability of the fault assessed by a team of researchers. The study was published in Pure and Applied Geophysics.

Over the past few decades (1984-2019), the Kopili Fault zone produced some eleven severe and damaging earthquakes (M 5.0-6.7) by "strike-slip faulting.”

On April 28, 2021, a severely felt and damaging earthquake (M 6.1) occurred at the northern segment of the Kopili Fault in Sonitpur district. It caused two casualties and six injuries, with considerable damage to houses and ground deformations, including ground fractures, fountain formation, landslide, and soil liquefaction. The main shock was followed by the largest aftershock (M 4.7) within six hours and by six felt aftershocks, including an aftershock M 3.8 on October 4, 2021, after some five months of the main shock.

The intense seismicity along this 100-km-wide zone indicates that the Kopili Fault extends from the western part of Manipur up to the tri-junction of Bhutan, Arunachal Pradesh, and Assam. The Kopili Fault zone is a mantle-reaching, deep-rooted transverse structure, pre-existing in the Indian plate before it collided head-on with the Tibetan plate. It reactivates due to the tectonic stresses caused by the collision tectonics in the Himalayas and subduction tectonics in the Indo-Burma ranges and generates earthquakes in the Assam valley at depths down to 50 km.

"The central segment of the Kopili Fault zone produced two large earthquakes (1869 and 1943) in the past at intervals of some 75 years. The southern segment ruptured in the 2016 Manipur earthquake of 6.7 magnitude. The northern segment also ruptured as a result of the 2009 Bhutan earthquake and the 2021 Sonitpur earthquake. It appears that the central segment may have ruptured from a large earthquake like that of the 1869 or 1943 event. As we have no way of predicting earthquakes with time, space, and magnitude, we can only assess the vulnerability of this zone for an impending large earthquake," the study concluded.

Admitting that the entire Kopili Fault system is very active, seismologist Saurav Baruah, however, said that it was a good sign that accumulated energy was being released through smaller quakes.

"The vulnerability of the region calls for manifold increases in investments in research on seismic hazard assessment and mitigation. Guwahati particularly is vulnerable as it is surrounded by a number of seismic faults. More or less, new infrastructure follows seismic design, but we need to assess the seismic vulnerability of old buildings. Building bylaws also need to be strictly followed," Baruah said.

By

Rituraj Borthakur