Tremble after the tremors

The NCR shook seven times in the last 20 days, fuelling fears of a high-magnitude earthquake. Seismologists have ruled out an immediate threat though they insist the region remains at risk of a ‘great’ Himalayan quake

Published - June 06, 2020 11:55 pm IST - New Delhi

Since May 15, the National Center for Seismology has recorded seven small earthquakes, ranging from 1.8 to 4.5 on the Richter scale, with epicentres at Faridabad, Rohtak and New Delhi. The spate of tremors — the most recent one occurring last Wednesday (June 3) — has fuelled speculation about the possibility of a bigger earthquake in this region.

The experts have discredited this theory but warned that the region — situated close to the ‘most active fault line on earth’ — would be at risk in the event of a widely anticipated ‘great’ Himalayan earthquake.

Misinterpreted threat

The Director of Wadia Institute of Himalayan Geology, Kalachand Sain, was recently quoted in a news article saying, “There is consistent seismic activity happening in the NCR and can trigger a major earthquake in Delhi.” But he told The Hindu that his words were misinterpreted. In fact, he clarified he meant quite the opposite and the recent earthquakes were a sign that “the region was unlikely to have a greater earthquake”.

Earthquakes in this region were due to “release of stress” accumulated from the movement of the Indian tectonic plate and its collision with the Eurasian tectonic plate, Dr. Sain explained. Consequently, the recent tremors would have diffused the accumulated stress, reducing the risk of a more serious earthquake, he added.

Vineet Gahalaut, former director of National Center for Seismology, too dismissed the fears of a devastating earthquake. “There is something called background seismicity level, which continues over a region over time and that is normal for it. Such tremors have been occurring in this region for the last 40-50 years. That would be a cause for concern only if they occurred in regions where tectonic plates met,” he said.

Only larger faults and larger systems trigger bigger earthquakes, explained Kusala Rajendran of Centre for Earth Sciences at Indian Institute of Science in Bangalore. “That is why you have great earthquakes only along the plate boundaries such as the Sumatra plate boundary [near Indonesia], Andaman plate boundary or Himalaya and California,” she said, while reiterating that Delhi had a history of only small tremors.

Not risk-free

Concerns about the risk of a major earthquake in Delhi, however, may not be unfounded. “An earthquake of 5.5 to 6 magnitude can never be ruled out anywhere,” said Dr. Rajendran, citing the instance of the 1993 earthquake in Latur, Maharashtra. At that time, the area fell under zone 1 of the country’s seismic zonal maps, which was the category with least risk. With a magnitude of 6.2 on the Richter scale, the earthquake left thousands dead. The same can’t be ruled out here, she said.

“Though there are pockets of localised seismicity in Delhi, that is not new… As a seismologist who has been working on earthquakes for a long time, my feeling is that the real fear for Delhi is from the Himalayas,” said Dr. Rajendran, adding that it is very close to the most active fault line in the world.

Several researchers have hypothesised the probability of a great earthquake, something of the magnitude of eight and above, striking the Himalayan region.

Based on historical, archaeological and geological data, Dr. Rajendran said that such an event has not taken place in the area for at least a 1,000 years. Others peg it at 500 years. This, along with GPS-based modelling of the speed of movement of the Indian plate, suggests that an earthquake is due, she said, and added: “That means it can happen any time.”

Impact on Delhi

“Even a strong earthquake in the Himalayan belt [as experienced in the recent past] may pose a threat to Delhi-NCR,” said Dr. Sain. He based this on the fact that this region is only 150-odd km from the active Himalayan seismic belt. Also, the “large sediment thickness (loose soil) in the Ganga Alluvial Plains” to the north of Delhi tends to amplify the impact of earthquakes. Given the presence of high-rises in the area, large number of buildings and a dense population, he said, it was imperative to strictly impose building codes as a precautionary measure.

Dr. Rajendran pointed to an earthquake in Uttarakhand’s Chamoli district in March 1999, measuring 6.5 on the Richter scale, which caused damage to some buildings in Patparganj in Delhi, 280 km from the epicentre. She also raised concerns over the vulnerability of buildings in Delhi-NCR and whether the authorities had taken steps to make them secure.

Drawing parallels with an earthquake that took place in 1803 near Garhwal in Uttarakhand and reportedly led to the toppling of a minaret of the Qutub Minar in Delhi, Dr. Gahalaut said that we need to be concerned about the consequences of a Himalayan earthquake on cities such as Delhi, Kanpur, Lucknow, Banaras and others in the Indo-Gangetic plains. He opined that the NCR would not be able to withstand such an earthquake, given the unplanned manner in which building constructions are carried out.

However, there is no definite answer to when a massive earthquake would occur. “At any time,” Dr. Rajendran said. The first paper saying that an earthquake can take place in the Himalayan region came out in 1981. “So already 30 years have passed,” she said. Thus, while some of the speculation are baseless, Delhi-NCR is not entirely invulnerable to earthquakes either.

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