Guwahati: People in Assam spent a sleepless night as a series of aftershocks continued to shake Assam, following the strong 6.4 magnitude earthquake that caused extensive damage across the state on Wednesday.

Swapnamita Choudhury, who has been involved in tectonic geomorphology and earthquake precursor studies for decades, was intrigued by how the seismic event in Assam and neighbouring states unfolded before her on Wednesday morning.

A senior scientist at the Wadia Institute of Himalayan Geology in Dehradun under the Ministry of Earth Science, Choudhury remained glued to her office computer since the tremor struck.

During an intense seismic event, soil saturated with moisture can lead to structural collapse of houses in Northeast India despite solid foundation

“Based on the data, it is still difficult to assign this seismic event to the Kopili faultline. We need more data over the coming days,” Choudhury, who originally hails from Assam, told EastMojo

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Although no casualties were reported, one large tremor followed by 18 aftershocks jolted the state between Wednesday and Thursday, and all originating in and around Sonitpur. After the first quake struck at 7.51 am on Wednesday, a total of 18 aftershocks hit the district and its nearby areas on both sides of the Brahmaputra in central Assam.

Choudhury said aftershocks are inevitable as all geological faults accumulate strain and during an earthquake, the stress is released. Strain on the fault causes ruptures, and a rupture may propagate further on the fault. This goes on until the strain is released.

“As the ruptures progress, the next one will be of a lesser magnitude,” she explained.

According to a preliminary analysis by the National Centre for Seismology (NCS), the quake epicenter at Dhekiajuli in Sonitpur district was located near the Kopili faultline, closer to Himalayan Frontal Thrust (HFT). The earthquake has also brought the focus back on smaller faultlines such as Kopili along the Himalayan areas, as scientists stress on the need for more studies and data on the region’s lesser known seismicity.   

Faults are essentially cracks on the Earth’s crust. When these cracks or faults are active, they move. The movement results in earthquakes. Named after one of most important tributaries of Brahmaputra, the Kopili fault, according to Dr Nilutpal Bora, a seismographer at the Civil Engineering Department of Indian Institute of Technology, Guwahati (IIT-G), was initially identified as lineament in 1980.

Also read: Massive earthquake shakes Assam, other NE states

D.R. Nandy, a renowned seismologist and Sujit Dasgupta, the former Director General of Geological Survey of India (GSI) found several new faults along the Brahmaputra valley and the Himalayan region with the help of Landsat satellites belonging to United States Geological Survey (USGS). The duo used remote sensing to detect hypocenter, the source of earthquakes, beneath the Earth’s crust, below the epicenter of the tremors. Kopili was among one of those faults discovered in North Eastern India.

Cycle of thrust  

Sandwiched between the Shillong plateau and the Himalayan Mountain complex, interest in Kopili peaked much later in the 20th century due to smaller quakes in the region. It is the Kopili fault that separates the Shillong plateau from the Mikir Hills, where the autonomous council districts of Karbi Anglong and Dima Hasao lie, and it moves further south towards Manipur.

Seismicity (M ≥ 4.0) in the Assam and surrounding region during 1960 -March 2020 [earthquake data source: seismo.gov.in and Geological faults and lineaments source: Bhukosh, GSI] and the Mainshock of 28th April 2021 is also shown. Source: NCS

Due to several small and devastating quakes such as the Imphal earthquake of 2016 that killed about 20 people, Kopili fault is notorious among Assam’s seismologists. The fault is spread over 300 kilometres in length and 50 kilometres in width, from the western part of Manipur to the tri-junction of Bhutan, Arunachal Pradesh and Assam.

“We can say that the recent seismicity discovered along the Kopili fault has led to speculations that this is one of the most seismically active faults in the region and a major earthquake could be expected in the future, which was also somehow validated by today’s event,” IIT’s Bora told EastMojo

Pictures from the 1950 Assam earthquake

At its northwestern end, Kopili fault could easily be affected by active faults in the Himalayan complex situated in the north, researchers say.

Around 50 million years ago, when the Himalayas collided with the Eurasian Plate, the India Plate moving towards North buckled under the Eurasian plate through a process called subduction. Starting from the Karakoram Ranges, to the Eastern Himalayan Syntaxis, where the Himalayas take sharp Southern turn towards Myanmar to Indonesia, this Himalayan belt can generate big earthquakes.

“Here, the Indian Plate produced three north-dipping fault systems stacked on each other. These faults, namely, Main Central Thrust (MCT), Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), run along the length of the Himalayas for about 2400 km,” Choudhury told EastMojo.

She says the HFT is where seismic activities are presently active.

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The past century saw several great earthquakes. The 1905 Kangra earthquake, 1934 Bihar-Nepal earthquake and the 1950 Assam earthquake.

“In case of the Assam earthquake of 1950, we found that the reason for widespread devastation in the eastern Himalayan belt was due to dual ruptures on the Himalayan Frontal Thrust near Pasighat and on the Mishmi Thrust, in a study we published recently in the Earth and Planetary Science Letters,” she said.

Preliminary data on Wednesday’s quake in Assam indicated that the quake occurred along the Kopili fault itself.

“However, the preliminary locations of the main shock and the aftershocks for this quake are linearly aligned in north-south direction near Dhekiajuli, suggesting a possible source on the Kopili fault. The Kopili fault is studied to be a right-lateral strike-slip fault, running north-south in the Kopili basin. In the past, it has generated many major to great earthquakes,” she said. 

“The Kopili fault should have generated more of a strike-slip component. With more data, this solution can be clearer as to which fault can be assigned to a strong earthquake and its aftershocks. More study on yesterday’s earthquake and the aftershocks might bring a better picture on rupture propagations on the Kopili fault, and whether it does produce movements other than pure strike-slips,” she added. 

Assam earthquake and the liquefying wave

On the other hand, Dr Nilotpal Bora, who is also a Raman–Charpak Fellow, said the Kopili fault zone has witnessed major seismic activities, including the 7.8 magnitude 1869 earthquake, the 1943 earthquake of magnitude 7.3 and the January 2016 earthquake of the 6.7 magnitude in Imphal, which occurred at certain time intervals.

“Considerable portion of the Kopili fault zone and its neighbouring areas is characterized by alluvial soils that have higher potential of trapping seismic waves,” Bora told EastMojo.

When seismic waves get trapped, in Northeastern region, it leads to soil liquefaction – a phenomenon seen during the Nepal earthquake of 2015.

According to Sarat Phukan, Head of the Department of Geology at Gauhati University, soil liquefaction occurs commonly during big earthquakes in the region.

“Soil here in Assam or rest of the Northeast has moisture present in them, which lose cohesion during a seismic event. So the soil often liquefies under the stress caused by the tremor,” Phukan told EastMojo

People witnessed water oozing out of the ground near the epicenter at Dhekiajuli in Sonitpur district.

Choudhury maintains that liquefaction is a common seismic phenomenon in flood plains like the Brahmaputra. “Because Assam has open fields, the soil will be restored as water is leaving the soil under sudden stress. If blocked with structures and cement, the soil would behave differently and structures would sink to the ground,” she added.

“Houses in Northeast India have solid foundations as compared to the houses in North or Northwest India. During an intense seismic event, however, soil saturated with moisture can lead to structural collapse,” she said while stressing on the need to enforce strict building codes in the entire Northeast region.

Geologists from the region feel there is a lot of scope to improve data collection and dissemination for seismic events. “Despite being in a top education institution, there is always a dearth of data because there is hardly any collection or even devices to monitor smaller seismic events. If such data is collected, we do not get access to them all the times,” Phukan said.  

For senior scientists such as Choudhury, the entire Kopili river basin needs a more extensive landform study, combined with seismic study to understand the propagation of the fault.

“We are more focused on the major faults on the Himalayan Front and concentrate less on fault systems like the Kopili fault until one day we wake up to jolts like this,” she said.

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