The last major earthquake to strike Sikkim was the September 18, 2011 temblor, which measured 6.9 on the Richter Scale and caused widespread devastation. In the 15 years that have passed since, Sikkim had largely forgotten the trauma of that event.
That was until last month, when the State was shaken by a swarm of more than forty quakes. The intensity of the quakes was low, but their frequency has triggered widespread panic.
Prof. Dr. Malay Mukul of the Department of Earth Sciences, Indian Institute of Technology Bombay, explains that the earthquakes occurring in Sikkim right now are shallow thrust earthquakes related to the continuous slip along the Main Himalayan Thrust (MHT), which extends from the Siliguri region in North Bengal, in the foothills of Darjeeling, to Tibet.
“Global Positioning System (GPS)-based measurements from 1999–2012 reveal that the continuous slip along the MHT in the Darjeeling-Sikkim region is arrested between Gangtok and Lachen (Mukul et al., 2018, Journal of Structural Geology), triggering moderate thrust earthquakes in the region between Jorethang and Lachen, like the ones we are currently witnessing,” he states.
He adds that the Lesser Himalayan Duplex has formed in this region, and the shallow earthquake swarms of less than 5 magnitude that we are currently witnessing are related to faults within this active structure.
“It is generally expected that at some point in the future, the MHT will also slip south of Jorethang because of activity in the Lesser Himalayan Duplex, resulting in a great earthquake. This is the reason for the seismic Zone VI classification of the region,” he mentions, further adding that one cannot say when the big earthquake will happen, but there are past records of such earthquakes in the region dating back around 1,000 years.
He further warns that it is because of this risk that infrastructure must be built to survive great earthquakes and other natural disasters, such as landslides and glacial lake outburst floods (GLOFs), that will likely accompany such an event.
Dr. Mukul goes on to add that the earthquake Sikkim witnessed in 2011 was of another type, related to strike-slip faults in the region. These kinds of earthquakes are likely to be moderate events not exceeding magnitude 7, he mentions.
According to the State Relief Commissioner and Secretary of the Disaster Management Department, Rinzing Chewang, the state recorded a total of 41 earthquakes between February 9 and 26. Including the wider region of Bhutan and Nepal, there were 57 earthquakes during that period.
The strongest among these was the 4.6 magnitude tremor that struck 4 km northeast of Yuksom in West Sikkim on February 26 at 11:34 AM, according to the National Centre for Seismology. The rest were all below magnitude 4 on the Richter Scale. February 26 also recorded 16 earthquakes, the highest number of tremors in a single day, most being aftershocks of the 4.6 magnitude quake.
At a press conference held on February 27, Mr. Chewang urged the people of Sikkim not to panic but to remain alert. He also asked people to steer clear of rumours and misinformation.
Regarding damage, he informed that a few reports have come in and district authorities are verifying them. A school in West Sikkim reported a false ceiling collapse, but no one was harmed.
On the possible impact of such earthquakes on high-altitude lakes, Mr. Chewang said that the Science and Technology Department is regularly monitoring these lakes.
Meanwhile, tremors continue to rattle Sikkim, as one measuring 3.2 magnitude was recorded on March 1 in Gangtok, while another measuring 2.0 magnitude was recorded in Mangan earlier that day.
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