Residents of Delhi woke up to tremors in the early hours of Wednesday. While the shaking was distinctly felt in Delhi, and the entire National Capital Region, the epicentre of the earthquake was far away, near Dipayal Silgadhi in the Doti district of western Nepal.
India's National Centre for Seismology said that the epicentre was over 300 kilometres away from Delhi. The earthquake left six people dead in Nepal and hundreds panicked in the NCR region. Residents described the experience as scary. Some said that it felt like the tremors lasted for a long time.
However, the earthquake was only a moderate one. CP Rajendran, adjunct professor at the National Institute of Advanced Studies Bangalore, told BOOM, "It is a known fact that whatever happens in the Himalayas, which is the main source of big earthquakes or moderate earthquakes. What we just had was a moderate earthquake, but you have an impact on the alluvial plains."
Delhi isn't new to tremors and low-intensity earthquakes. In June 2021, a low-intensity earthquake of a magnitude of 2.1 was recorded in the Punjabi Bagh area, later in July, another minor earthquake of magnitude 3.7 in Jajjhar, Haryana was felt in Delhi. In 2020, Delhi felt several mild earthquakes in June and later in December.
In light of the latest tremors, here's a look at why minor and moderate-intensity earthquakes are a regular feature in the Indian National Capital.
Where do Delhi's earthquakes originate?
Delhi is in a position where it is not only affected by earthquakes in the Himalayan region but also has fault lines closer to it. Rajendran said, "This particular earthquake is sourced from faultlines in the Himalayas, but that doesn't mean that there are no faults near Delhi. Two to three years back there were small tremors in Delhi. Those tremors are sourced from nearby faultlines. So there are faultlines in Delhi, but those are separate from what you find in the Himalayas."
The Delhi-NCR region has several faults or weak zones that include the Delhi-Haridwar Ridge, Mahendragarh-Dehradun Subsurface Fault, Moradabad Fault, Sohna Fault, Great Boundary Fault, Delhi-Sargodha Ridge, Yamuna River Lineament, Ganga River Lineament and the Jahazpur Thrust.
Vineet Gahalaut, chief scientist at the CSIR-National Geophysical Research Institute in Hyderabad, told BOOM that the hilly topography between Delhi and Udaipur through Gurgaon, Jaipur and Ajmer is proof that there has been geological activity in the region in the past that gave rise to these mountains and faults. "Apart from the mountains, geological activities could have led to faults being developed. Faults were created at that point in time. Many of them have healed now and become inactive."
Faults are essentially cracks in the earth's crust. The Himalayas were formed when the Indian tectonic plate collided with the Eurasian plate millions of years ago. The Delhi-Arravalli range were formed through similar geological activity, and are older than the Himalayas. In the case of the Delhi-Aravalli range — also known as the Aravalli-Delhi Fold Belt — such tectonic movement in recent times may not give rise to new hills or mountains but may cause minor earthquakes.
Add to that the fact that the entire Indian tectonic plate itself is still moving northeast at the rate of about 5 cm per year towards the Eurasian plate. Gahalaut said, "The movement of this plate itself also causes some buildup in the faults that are not in the Himalayan region, but outside of it, for example, the Delhi-Aravalli region. For that matter the Kutch Region, the Narmada-Son region or the Godavari region — old fault lines — that get reactivated, that's why you have earthquakes in the region where you expect them not to occur."
Both experts said that earthquakes originating from the Delhi region may be frequent, they're not the ones to be worried about.
Rajendran said, "I would rather worry more about the sources in the Himalayas, rather than the ones in Delhi. The sources in the Himalayas can generate big earthquakes, but you will have a big impact on towns and cities like Delhi."
A look at the map below shows how Delhi is located right between the Himalayas and the Aravalli-Delhi Fold Belt, which could be causing frequent tremors.
Delhi's tryst with earthquakes
Rajendran told BOOM, "It was good that it was a moderate earthquake, if it had been 7 or 7.5 magnitudes, the damage could have been much greater."
Rajendran's words may sound alarming, but has historical significance. While Wednesday's tremors were intense enough to scare Delhi residents, historically Delhi has seen damages from earthquakes in the last 300 years that took originated in the Delhi region and the Himalayan region.
Gahalaut said, "The earthquakes originating from the Himalayan region can be tricky. If you go to Qutub Minar, it's written there that Qutub Minar lost its upper cupola because of an earthquake that occurred in 1803."
While the 1803 earthquake was located in British Uttarkashi, it is said to have been of magnitude 7.7 and was felt across the Indo-Gangetic plain. The paper 'Revisiting the 1991 Uttarkashi and the 1999 Chamoli, India, earthquakes: Implications of rupture mechanisms in the central Himalaya' by Kusala Rajendran, Revathy M Parameswaran and CP Rajendran, highlights that the effects were felt as far away as in Kanpur which was 750 kilometres away from the epicentre.
The impact of the 1999 Chamoli earthquake of 6.6 magnitude, that killed over a hundred people, was also felt in Delhi. In 2001, an article in The Hindu had said that some buildings in the capital had sustained non-structural damages because of the earthquake two years earlier.
Rajendran said, "I would rather worry more about the sources in the Himalayas, rather than the ones in Delhi. The sources in the Himalayas can generate big earthquakes, and you will have a big impact on towns and cities like Delhi."
Why do Himalayan earthquakes impact Delhi?
The particular faultline which generated the earthquake on Wednesday extends from western Nepal to the Indian Himalayas. Rajendran said, "It is mostly loose soil around the Ganga plain where Delhi is located. So you'll always have amplification of energy there. So if you have an earthquake in the Himalayas, it will have an impact on the alluvial plains because of the loose sediments."
So it is not just Delhi, but other cities on the Indo-Gangetic plain may be affected by such earthquakes. Rivers flowing through the region from the Himalayan mountains deposit alluvium, which is loose fertile sediments, on the beds of flowing streams.
Gahalaut explained, "In the case of earthquakes, we call elastic waves, and the elastic waves travel in a solid medium. They get reflected or refracted whenever they see a boundary in which there is a contrast or a change in physical properties."
So when seismic waves travel from a rocky mountaineous region to the alluvial plain, they get amplified. The topographical location of Delhi is unique as even within the city there is a difference in the amount of sediments found. While the sedimentation is thin in south Delhi, and becomes more rocky towards Gurgaon, the sedimentation thickens towards Noida and Ghaziabad as it is close to the Yamuna river.
"Compare this to when you go north of Delhi where, if you go to Meerut or Muzaffarnagar area, you have thick sediments in the region that are almost four to five kilometres thick. Below these four to five kilometres you have hard rocks," Gahalaut said.
So certain areas closer to the Yamuna plains may be more affected by the seismic waves and the damage may be exacerbated.
Can earthquakes be predicted?
While scientists have carried out research to identify regions that may be more prone to earthquakes, there is still no technology to predict their timings.
Rajendran said, "A long-term forecast is made about the Himalayas. From whatever has been studied, scientists have a consensus that there are going to be big earthquakes there... It could happen tomorrow or after 10 years, we don't know. But it is certain that this area has the potential to generate a very large earthquake that can have an impact on various cities in northern India.