NDTV Explainer: Why Delhi Felt Strong Tremors Despite a Low-Magnitude Quake

Residents of Delhi and surrounding areas awoke to intense vibrations triggered by a magnitude 4.0 earthquake this morning.

This morning, residents in Delhi and surrounding areas experienced powerful tremors triggered by a magnitude-4.0 earthquake. According to the National Centre of Seismology, the quake struck at a depth of approximately 5 kilometers. Videos showed the ground and buildings shaking, forcing many to flee their houses.

There have been no reports of injuries or property damage, but many residents said the vibrations seemed like a bridge collapsing. However, the quake was not of significant strength. In general, earthquakes with a magnitude less than 2.5 do not create tremors; earthquakes with a magnitude between 2.5 and 5.4 cause minor vibrations but minimal damage. Any earthquake larger than that is expected to inflict severe vibrations and destruction.

So why did the 4.0-magnitude earthquake in Delhi feel so intense?

There are numerous explanations for this, but the epicenter of the quake was Delhi itself. According to the United States Geological Survey (USGS), the most violent shaking during an earthquake occurs near the epicenter. However, vibrations can be felt hundreds of kilometers away. The Dhuala Kuan area, the epicenter of today's quake, has been suffering more minor, low-magnitude earthquakes every two to three years. It recorded a 3.3 magnitude earthquake in 2015.

Another cause for the severe shocks was that the earthquake occurred at shallow depths. Shallow earthquakes, which originate five or ten kilometers below the surface, typically do more damage than deep earthquakes. This also caused a loud grumbling sound during the quake. According to the USGS, high-frequency vibrations from shallow earthquakes produce a booming sound as the ground vibrates, resulting in a short-period seismic wave motion that travels into the air and becomes sound waves. The shallower the epicenter, the greater the potential for energy and sound production.

However, earthquakes can produce booming noises even when there are no tremors present.

Delhi's dense population contributed to significant tremors since seismic waves travel a shorter distance and reach structures faster in such places.

The national capital is also vulnerable to earthquakes because it is located in Seismic Zone IV. According to the Delhi Disaster Management Authority, this zone has a relatively high seismicity, with earthquakes ranging in magnitude from 5 to 6 and occasionally up to 7-8. However, zoning is a continual and ever-changing process.

Seismicity in North India, including the Himalayas, is caused by the collision of the Indian and Eurasian plates. These colliding plates bend and store energy like a spring, and when the plate's margin eventually slips to release energy, an earthquake occurs.

'No Cause for Concern': Expert on Delhi Earthquake

Dr. OP Mishra, Director of the National Centre for Seismology, has assured residents of the national capital that minor aftershocks are "natural."

"Delhi has been experiencing minor earthquakes." The earthquake happened at Dhaula Kuan. In 2007, there was a 4.7 magnitude earthquake there. There's nothing to worry about. There is a seismic zone there. The earthquake happened because of in-situ material heterogeneity. After a 4.0 magnitude earthquake, the aftershock will be 1.2 magnitudes smaller, which is expected since it will repair the damage," he noted.