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Assam quake: Experts from IIT Guwahati proposes Seismic micro zonation of urban areas

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The Economic Times
30th April, 2021 21:26 IST

Following earthquake in Assam on Wednesday, experts from IIT Guwahati proposed Seismic micro zonation of urban areas and earthquake resistant design should be strictly enforced in these areas.

Experts stated that old time-tested techniques like Assam type wooden houses should be popularized and encouraged more and more to reduce the risk in this area.

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

The IIT stated that a massive earthquake struck near Sonitpur, Assam on 28th April 2021 at 7.51 AM. National Centre for Seismology, MoES Government of India reports a magnitude of 6.4 on 28th April 2021 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 massive earthquake. a collapse of 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 28th April 2021 and followed with events on 29th April 2021. Such multi-hazards that hit Dhekiajuli, Assam and 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 IIT added.

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, massive quake has caused only light damage to buildings and there have been no fatalities reported so far and only 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 Narayanpur area of Dhekiajuli, close to the epicentre of the massive earthquake has also been witnessed, indicating liquefaction of soil underneath.

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 eyewitness reports on social media highlight that the duration near the epicentre was relatively long, up to approx. 1 minute. Even in IIT Guwahati, strong tremors were felt by large number of people and name boards of departments tremors for about 30 to 40 secs.

In fact, the earthquake was felt in almost all of Assam state, but also in parts of neighboring Bhutan, only 50 km away to the northwest, Tibet 150 km to the north, and Bangladesh 250 km to the south of the epicentre. Social media reports a weak tremors as far as 400 km distance from the epicentreepicentre.

Liquefaction is one of the major effects of 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 North East 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," IIT added.

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