Why the recent 6.4 magnitude earthquake should wake up Assam

When a 6.4 magnitude earthquake struck Assam on April 28, there was panic and chaos. The ground cracked near the epicenter in the Suntpur area, and so did the walls and roofs of people’s homes tens of miles in radius; The buildings swayed “like betel trees in the wind,” a hill collapsed, and water gushed out from the rice fields.

Trapped in the second wave of the COVID-19 pandemic, the earthquake – and multiple aftershocks of the day – unleashed fear. Two people died from shock, and there were several reports of severe damage to homes and buildings. It also revealed, once again, the vulnerability of Assam to seismic activity and how human activities can contribute more to it.

Assam, and the entire northeast of India, is classified within seismic zone 5, which means that it is extremely vulnerable to high-intensity earthquakes. On April 28, a 6.4-magnitude earthquake was followed by 20 aftershocks of various degrees during the day, according to Gianendra Dev Tripathi, chief executive of the Assam Disaster Management Authority. Six aftershocks with a magnitude of 3.2 to 4.7 occurred within hours of the main quake. In fact, the National Seismology Center continued to record 2.6 to 2.7 magnitude seismic activity in the region on the eighth day of the main earthquake.

The National Seismology Center said the main earthquake occurred near the Kobe Fault, near the intrusion forward of the Himalayas, a very seismically active region “associated with collision tectonics where there are sub-channels of the Indian plane under the Eurasian plate.”

A fault, according to the US Geological Survey, “is a fracture or region of fissures between two blocks of rock. The imperfections allow the blocks to move relative to each other.” The Kobili Fault is a fault heading about 300 km between northwest and southeast from Bhutan in the Himalayas to the Burmese Arc.

Assam weakness

Earthquakes are not uncommon in Assam, as the National Seismological Center said the region has seen several earthquakes of medium to high intensity. One of the worst of these was the Great Assam-Tibet earthquake of 1950, which measured 8.6 on the Richter scale.

Approximately 4,800 people were killed as a result, and several landslides occurred that closed the Brahmaputra tributaries and changed the terrain of the area. The Kashar earthquake of 1869, which had a magnitude of 7.4, was another major seismic activity that struck the region.

Experts said recent earthquakes detected along the Koppeli fault have led to speculation that it is one of the most seismically active faults in the region. Scientist Nilotpal Pura said that a large part of the Kobili fault zone, and the adjacent areas are characterized by alluvial soils that have a greater potential to trap seismic waves, thus making it one of the most earthquake-prone areas in Northeast India.

“The state of Assam, which is at the highest seismic zone, is constantly challenged by the potential for earthquakes as an expression of the release of accumulated tectonic stress,” said Chandan Mahanta, professor in the Department of Civil Engineering, Indian Institute of Technology in Guwahati. Mongbay India. He said that constant tectonic stress is building up along fault lines, and a magnitude 6.4 earthquake was a release of such accumulated stress.

An earthquake measuring 6.4 on the Richter scale struck 43 km west of Tizpur, Assam, India. Image credit: National Seismology Center one thing for another

Aside from very intense tremors, the region is subject to seismic activity of various sizes and intensity. Maanta said this could be a contributing factor to the erosion, because “seismic activity can disturb the properties of the Earth’s material, such as strength and cohesion, and add slope instability to that.”

Pictures of a section of a hill breaking and falling into a river in the Udalguri region after the 6.4 magnitude earthquake over the rest of the world showed the severity of the earthquake.

The 1950 earthquake triggered many landslides, and Mahanta said that with the exception of the major earthquakes mentioned, “many landslides are due to earthquakes.”

This means that seismic activity has a role to play in adding the sediment load to the Brahmaputra River. “Landslides in the upper part of Brahmaputra are known to add a high sediment load to the river,” Maanta said.

This is important because the high sediment load on the Brahmaputra is known to cause frequent floods since the river bed rises and the river is broadened. The Assam government has always said that dredging the river is a solution to this problem, and has allocated huge sums of money for this.

In 2017, Confederation Minister of Transport, Nitin Gadkari, announced Rs 250 crore for dredging Brahmaputra. However, experts argue that dredging the river completely is neither a feasible nor permanent solution because the silt returns after it is removed.

Strange event

The earthquake in April of last year also led to what appeared to be a “strange” event: water flowing from rice fields near the epicenter, near Dikyaguli, in the state of Assam. The current state chief minister Himanta Pessoa Sarma has posted a video of this on social media.

Water seeps from a rice field in Narayanpur District in Dikyaguli, epicenter of the massive 6.7 in Assam earthquake pic.twitter.com/BOD6bfCp6s

Himantabiswa April 28, 2021

Scientists call this phenomenon liquefaction of soil. By definition, it means that when saturated or partially saturated soil loses strength and stiffness dramatically in response to applied stress, such as earthquake vibration.

In this case, Vineet Gahalaut, chief scientist, CSIR-National Geophysical Research Institute, said that nothing is out of the ordinary, especially because it is “common in places with shallow water levels.”

“In 2017, the Manu earthquake in Tripura caused soil liquefaction as far as Bangladesh,” said Gahlot. “Its strength was 5.7 on the Richter scale.”

Human impact

What must be taken into account, and most importantly, is the possibility of anthropogenic influence on seismic activity. “It is difficult to determine whether anthropogenic factors could trigger an earthquake of this magnitude (6.4),” Ghalut told India’s Mongabhai. “But there have been a few cases in the United States, where the seismic monitoring is good, as it has been observed that anthropogenic factors induced small earthquake and seismic activity.”

For example, he said, excessive mining, geothermal heat pumps, building dams and injecting water under high pressure into oil reserves to fracture the area in order to release liquids and oil may “trigger” seismic activity. Research paper, “Impact of anthropogenic groundwater discharge in the Indo-Gangetic plains of April 25, 2015 Mw 7.8 Nepal earthquake” – co-authored by Gahalaut – speaks along similar lines.

“Tectonic process and human factors are two completely different things, but like the last straw on a camel’s back, when the pressure is already high and in a critical situation, human activity can cause an earthquake,” Gahlot said.

Mitigate weaknesses

For north-eastern India and Assam, which is the most likely region in terms of earthquake risk, vulnerability is high and increasing. “Building on hills and the proliferation of tall buildings increases the vulnerability factor,” said Tripathi of the Assam State Disaster Management Authority, Mongabai, India. “The way to mitigate this vulnerability is to make sure that all construction works follow the Seismic Safety Act and make people aware of safety measures.”

Whether every building in Assam and in particular Guwahati – “especially new buildings,” as one expert put it – follows the code, however, is questionable.

“There is no robust oversight mechanism or regulatory policy to ensure that this law is followed in all construction work,” Tripathi said.

Mahanta suggests that “high-precision building design and layout based on zoning of small areas following the correct seismic code” is the key to building safety which is critical in a place like Assam. An example is the 2007 micro-earthquake atlas of the Guwahati region.

Interestingly, the earthquake also shed light on the good old one-story “Assam” houses that were built with local materials and were more earthquake-resistant.

The rapid tremor vanished from urban areas, sparking nostalgia on social media and sparking conversations about restoring these grandparents’ homes.

This article first appeared on Mongabay.

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