A massive earthquake struck near Sonitpur in Assam on April 28 2021 at 7.51 am. National Centre for Seismology reports a magnitude of 6.4 on April 28 at 7.51 PM corresponding to 26.69, 92.36 (Lat. and Long.) at a depth of 17km (43km W of Tezpur, Assam, India – See Figure 1).

While the recent 2nd wave of Covid 19 pandemic is hitting Assam this additional natural calamity coming in the form of a major earthquake near Sonitpur has created panic due to this multi-hazard occurring at the same time making life disastrous for the people of Assam.

Followed by a massive earthquake, a collapse of a free-standing rocky hill in Bhirabkund along with liquefaction of ground occurred. These are some of the multi-hazard events that have happened in this area on April 28 and followed by events on April 29.

Such multi-hazards that hit Dhekiajuli, Assam and the surrounding area indicate the possibility of hazardous events occurring simultaneously, cascading or cumulatively over time, and taking into account the potential interrelated effects of these hazards.

The high-intensity earthquake early Wednesday has caused damage to houses and buildings with people running out of their homes and other places in panic, obliterating social distancing and other Covid guidelines amid a raging pandemic.

From the initial reports, the massive quake has caused only light damage to buildings and there have been no fatalities reported so far and only a few injuries have been reported. Massive earthquake impact was likely reduced by the fact that no major cities are located close to the epicentre. Even though it is a shallower earthquake, followed by strong or even strong tremors (intensity up to VII on the Mercalli scale), which occurred within a radius of approx 50-70 km around the epicentre.

Close to it, many ground cracks opened in fields and open spaces. Many photographs and videos on social media show fissures exposing liquified soil. Part of the large freestanding rocky hill known as Bhairabkunda in the Udalguri district also has collapsed. Water seeping out from a paddy field in the Narayanpur area of Dhekiajuli, close to the epicentre of the massive earthquake has also been witnessed, indicating liquefaction of soil underneath.

This massive earthquake of 6.4 event along or near the Kopili fault closer to Himalayan frontal thrust was followed by 15 quakes so far and the event of 4.7 magnitude earthquake at 8.03 p m, 4.0 magnitude at 8.13 pm and many smaller events afterwards.

The area was shaken by 1 major quake of magnitude 6.4, 1 quake of magnitude 4.7, 4 quakes between 4.0 and 5.0, 7 quakes between 3.0 and 4.0, and 10 quakes between 2.0 and 3.0.

Among these events, the biggest quake 6.4 quake occurred 7.7 km northwest of Dhekiajuli in Sonitpur, Assam.

The biggest quake on April 29 , measured as 4.9, occurred 25 km west of Tezpur near Sonitpur at 1:20 am, and later a quake of magnitude 2.8, 33 km northeast of Tezpur occurred near Sonitpur on April 29 at 01:43 GMT.

Location of Sonitpur earthquake (source NCS, Govt of India)

Sonitpur district has a population of 19.5 lakhs. Physio graphically the area can broadly be divided into three parts, i.e., the hilly tract, the foothill region and the extensive flood plain created by the river Brahmaputra and its tributaries. The hilly tracts comprise Siwalik sediments of lesser Himalayas. The epicentre of the massive earthquake near Sonitpur spreads over plain areas interspersed with hills. The hills and the isolated hill locks are made of Precambrian granitic rocks.

Quaternary alluvium occupies the valleys, deposited over the undulated and faulted basement of granitic rocks, forming the plain area inter-woven with several paleo channels and static water bodies, over which the present-day habitation stands.

Tezpur town (with a population of approx. 60,000) and other smaller towns near the epicentre, numerous buildings suffered light to moderate damage: parts of wall plaster, bricks, roof tiles fell, windows broke, few small and weakly built walls collapsed, pipes were damaged, objects fell off from their position and similar effects, but overall, the damage is comparably limited considering the size of the quake.

According to media reports, a multi-storied building in Nagaon tilted on the adjacent structure, triggering panic. Healthcare facilities such as Dispur Hospital, Apollo Clinic, Down Town Hospital and Excel Care Hospital also saw damages.

Few similar minor damages were reported in Guwahati city as well. The luxury hotel Taj Vivanta in Guwahati also witnessed massive damages with several glass panes, ceilings and walls falling apart.

Eyewitness reports on social media highlight that the duration near the epicentre was relatively long, up to approximately 1 minute. Even in IIT Guwahati, strong tremors were felt by a large number of people.

In fact, the earthquake was not only felt in almost every corner of Assam, but also in parts of neighboring Bhutan, which is only 50 km from the epicentre and Tibet, which is  150 km, and Bangladesh, which is 250 km south from the epicentre.

As mentioned earlier, the earthquake which is close to Kopili fault, is an NW-SE trending strike-slip fault which separates Shillong plateau from the Mikir hills and extends the north up to the main central thrust (MCT).

The seismic tectonic setting of Northeast India is shown in Figure 2, which depicts the Kopili fault. The Kopili fault zone is under compressional stress from the Indo-Burma arc to the east and from the Himalayan arc to the north, characterized by transverse tectonics.

In the past, seismic activity has been reported around this fault by several seismologists. As per the USGS report, the event is reported as 6.0 magnitude earthquake and the Moment tensor solution shows that the earthquake is of reverse type with strike-slip components.

In the past, two large earthquakes occurred in this area or near the Kopili fault. One in 1869 (M 7.7) towards the southeastern end of the fault and the other in 1943 (M 7.2) that occurred further north of the 1869 event.

Seismotectonic Setting of Northeast India (After Raghu Kant and Dash 2010).

Liquefaction is one of the major effects of the earthquake, where saturated sand and silt behaves like a liquid, resulting in loss of soil strength. A major part of Assam lies in the Brahmaputra river valley and other parts are bounded by the Northeast Himalayan region, where the seismic activity is very high.

Our studies on Assam sand have indicated that the shear moduli and damping properties of soils are strongly affected by the magnitude of shear strain amplitude and it was evident from past studies that areas in Assam in Brahmaputra valley possess lower cyclic strength of in-situ sands for a given number of cycles for initial liquefaction.

Further, geotechnical studies concluded that relative densities of these soils do not significantly affect the damping ratios. Assam state has many oil refineries, industries, and advanced infrastructural facilities. Hence, in the areas which come under zone 5, the seismic risk in the present-day situation is very high, and the degree of damage due to both primary and secondary effects with multiple hazards taking place simultaneously would be very high.

Seismic microzonation of urban areas should be carried out and Earthquake resistant design should be strictly enforced in these areas. The old time-tested techniques like Assam type wooden houses should be popularized and encouraged more and more to reduce the risk in this area.  The community needs to be prepared with preparedness and disaster management at all levels. Considering such perpetual disasters this region has faced, and based on the advice of Honourable PM during the 22nd convocation of IIT Guwahati (22 September 2020), the IIT Guwahati has started a Centre for Disaster Management and Research (CDMR) which will begin Master’s and Ph D programme from July 2021 session and will provide trained human resources for future preparedness.

This Centre would work in a purely multidisciplinary manner and faculty from various disciplines working on disaster management related fields will contribute through research, innovation, consultancy, capacity building and advocacy, by assimilating the recent developments in both climate change and disaster management fields (see for details at: https://www.iitg.ac.in/cdmr/dir-msg.html).

Prof T G Sitharam, Director IIT Guwahati and President of Indian Society for Earthquake Technology. His group has done extensive work in the North East related to seismic hazard assessment and liquefaction of soils.

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