The dawn quake: How tremors in Delhi are different from earthquakes in the Himalayan region
Mostly, the tremors experienced in Delhi from time to time are from earthquakes that originate elsewhere. Monday’s magnitude 4 earthquake was located in Delhi itself. In fact, it was the strongest earthquake to have originated in Delhi in at least five year
Delhi is no stranger to earthquakes,
but the one that occurred before dawn on Monday was slightly different.
Mostly, the tremors experienced in Delhi from time to time are from earthquakes that originate elsewhere, sometimes as far away as Afghanistan. Monday’s magnitude 4 earthquake was located in Delhi itself, somewhere close to the Dhaula Kuan area.
In fact, it was the strongest earthquake to have originated in Delhi in at least five years, data from the National Centre of Seismology showed.
Another earthquake of similar magnitude occurred a couple of hours later near Siwan in Bihar. Magnitude 4 earthquakes are not very strong, and do not result in much damage. There were no reports of any damage due to Monday’s earthquakes
Seismic activity in Delhi
Delhi lies in a seismically active area, and thus an earthquake originating here is not a surprise. In the last five years, areas in and around Delhi have produced several small earthquakes, with magnitudes ranging between 2 and 3 or lower (chart). These are barely noticed, except by seismometers.
India’s official earthquake hazard map puts Delhi in Zone 4, the second highest classification of areas based on their susceptibility to shaking experienced during an earthquake.
Zone 4 in India includes areas that are expected to experience MSK-8-level intensity during an earthquake. The MSK, or Medvedev-Sponheuer-Karnik scale is a measure of intensity, rather than strength, or energy released, which is described by magnitude.
Simply put, the MSK scale measures the susceptibility of an area to an earthquake. Based on the local geology and other factors, two places can feel, and be impacted by, a quake very differently, even if they are equidistant from its epicentre.
Delhi falls in a region that has high susceptibility to quakes. MSK-8 means the area is vulnerable to major damage being caused to buildings and other infrastructure. Zone 5, the most vulnerable areas in India, correspond to MSK-9 levels of intensity or higher.
Aravalli-Delhi Fold Belt
Delhi lies in what is known as the Aravalli-Delhi Fold Belt, a seismically-active geological belt extending from southern and eastern Rajasthan to Haryana and Delhi.
This region is characterised by the presence of deformed layers of rock that have been folded or bent due to geological processes hundreds of millions of years ago. These deformities have created stress which is sometimes released in the form of earthquakes.
This is a very different mechanism from the one that triggers earthquakes in the Himalayan region, which is one of the most quake-prone regions in the world. The Himalayan region has been witnessing the subduction of the Indian tectonic plate under the Eurasian plate — one pushing against the other — which results in a lot of stress building up.
The Aravalli-Delhi Fold Belt was much more seismically active in the past than it is now. Over the years, the tectonic activity in the region has slowed down considerably, leading to greater geological stability. But some faults still remain, which give rise to occasional mild earthquakes.
“It is like old scars flaring up once in a while. In the past, this area has witnessed significant geological processes, but these have largely stopped now,” said Vineet Gehlot, Director of Dehradun-based Wadia Institute of Himalayan Geology.
“Most of the times people mistake other kinds of sounds as the sound of the earthquake. Big earthquakes, for example, can result in buildings and other things vibrating. This shaking can produce different kinds of sounds, and these are very often confused with the sound of the earthquake. I am not sure what people heard in Delhi, but not much should be read into this,” he said.
Large quakes unlikely
Gehlot said Delhi had a “good history” of quakes, but not large earthquakes of the kind that are witnessed in the Himalayan region.
“There is a mention in some historical records of a strong earthquake having occurred in Delhi around 1720. Another big earthquake is reported to have happened in 1803 in which the Qutub Minar is said to have suffered major damage. This one is supposed to have occurred around Mathura.
“Some of these past events are being reassessed now with newer data and technology. For example, it is now understood that the 1803 earthquake most likely happened in the Srinagar-Devprayag region of Uttarakhand and was felt in Delhi. The features of that earthquake are almost typical of a Himalayan quake,” he said.
The chances of a large earthquake originating in Delhi are quite slim, Gehlot said.
“There is always this discussion whether Delhi can be hit by a big earthquake. Indeed, a large earthquake could result in very heavy destruction in a densely populated and heavily built-up region like Delhi.
“But based on what we know of the area, I would say such an event is extremely unlikely. The tectonic setup, the structures underneath the surface, the energy stored, the stress are all not conducive to produce a big earthquake in or around Delhi,” he said.