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Sunday, March 07, 2021

Explained: Why Delhi earthquakes aren’t a signal

Delhi earthquakes: 11 mild earthquakes in Delhi since May have raised speculation about a big one, but scientists say these are not unusual. A look at why predicting an earthquake is difficult, and how an area should stay prepared.

Written by Amitabh Sinha | Pune |
June 9, 2020 4:30:41 am
Explained: Why Delhi earthquakes aren’t a signal A mock drill for earthquake preparedness in Delhi, conducted by the National and Delhi Disaster Management Authorities. (Praveen Kanna/Express Archive)

An earthquake of magnitude 2.1 was detected near Delhi on Monday. It was the eleventh minor earthquake recorded in and around Delhi since May, the most powerful of which happened to be of magnitude 3.4. These recent earthquakes have triggered discussions on the possibility of increased seismicity around Delhi, and fears of an impending big earthquake sometime soon. None of these apprehensions have any scientific basis.

Is it unusual for Delhi to experience so many earthquakes in such a short time?

Scientists are unequivocal in asserting that no unusual seismic activity is taking place around Delhi in the last few months. “There is absolutely nothing happening in Delhi that can be called unusual or abnormal,” said Vineet Gehlot, former head of the National Centre for Seismology in Delhi, who is now at the Hyderabad-based National Geophysical Research Institute (NGRI).

“If you look at the earthquake catalogue, Delhi and its surrounding areas, and this would extend till Jaipur, Ajmer, Mount Abut and the Aravallis, usually experience between two and three earthquakes of magnitude 2.5 and above every month. But there are monthly and annual variations as well. Geological and seismological processes are not very smooth. So sometimes you would expect to see higher number of earthquakes as well. I am pretty sure nothing special has happened in Delhi in the last couple of months,” he said.

Detection of earthquakes, especially those of smaller magnitude, being recorded in an area also depends on the number of seismic recorders installed in that area. The area around Delhi has the most dense concentration of seismometers anywhere in the country, even more than the Himalayan region which is seismically much more active. Out of the 115 detectors installed in the country, 16 are in or around Delhi. As a result, even the earthquakes of smaller magnitude, those that are not even felt by most people, are recorded, and this information is publicly accessible.

Do these small earthquakes foretell a bigger one?

Earthquakes of magnitude four or below hardly cause any damage anywhere and are mostly inconsequential for practical purposes. Thousands of such earthquakes are recorded around the world every year, and most of them are uneventful. And, they certainly do not signal any big upcoming event.

“The concept of foreshocks is something that is largely applied in hindsight. When a big event happens, all the smaller earthquakes that have occurred in that region in the near past are classified as foreshocks. Foreshocks are post-event definitions. The description does not exist before any big earthquake has happened. So all this talk of these being foreshocks of a big earthquake in Delhi have no basis at all,” said Harsh Gupta, one of India’s foremost experts on earthquakes and a former director of NGRI.

“A big earthquake might still occur. No one can rule it out. But they cannot be predicted. So to say that these small earthquakes are precursors to the big one is totally unscientific,” he said.

What, then, is a signal to an upcoming earthquake?

Scientists have been working for years to identify “precursors” to an earthquake, but have so far met with no success. Some special earthquakes, the ones that are triggered by volcanic activity, can be predicted to some extent — Gehlot calls them “much more well-behaved” than others — but nothing else.

Predicting earthquakes in a region like Delhi is all the more difficult because the place does not lie on any faultlines. “I would say we still understand a little bit about the tectonics in the Himalayan region, where two tectonic plates are meeting each other. But Delhi does not lie on a plate boundary. It is located on a single plate, and the seismic activity is generated by internal deformities. Here, we understand even less. Therefore, predicting earthquakes in advance is out of the question,” Gehlot said.

The Himalayan region, extending from the Hindu Kush to the Northeast and going south to Southeast Asia, is seismically one of the most active regions in the world. The region has experienced several big earthquakes in the past, most recently in 2015 in Nepal.

Is a big one is coming to the region?

Scientists say that the Himalayan region is due for a big earthquake, of magnitude 8 or even higher. That is because they have been able to measure the energy that is getting trapped under the surface as a result of one tectonic plate trying to move beneath the other one. But even here, scientists have no idea when this big earthquake will occur. The prediction about the big one is based only on the estimate of the energy that is ready to be released.

A magnitude 6 earthquake is typically associated with the kind of energy that was released by the atom bomb in Hiroshima. Since the magnitude of earthquakes are measured on a logarithmic scale, a magnitude 7 earthquake is about 32 times more powerful than a magnitude 6 earthquake. Accordingly, a magnitude 8 earthquake would be almost 1,000 times more powerful than a magnitude 6 event.

Gupta said even the argument that the smaller quakes are helping release the energy bit by bit so that a big one would not be necessary also does not hold. “Delhi area has experienced earthquakes of magnitude between five and six, but fortunately not in the last 50-60 years… You would need 1,000 earthquakes of magnitude 4 to release the energy equivalent of a magnitude 6 earthquake. These arguments have no basis,” he said.

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So, is a big one coming to Delhi?

No one knows, but a more relevant question, Gupta said, is what even if we knew. “Supposing we know that a magnitude 6 earthquake is going to occur in Delhi at 11 am tomorrow. What can we do after that? Can we get the entire city evacuated? Is that possible? Prediction is not going to make us safe against earthquakes. What is important is that we need to make our structures earthquake resistant, we need to follow prescribed drills when an event happens, everyone must know what is the best place to run to when we are in office, or at home, or in open spaces. It is these kinds of discussions that are meaningful. Instead, what we see is speculation, rumours and half-baked information in public discussions,” he said.

So a big earthquake is very much possible in Delhi. No one is ruling out that possibility. But they would occur when they have to. Earthquakes still like to come unannounced. They do not like to knock on our doors with foreshocks.

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