NEW DELHI: In the wake of the recent tremors in Delhi-NCR, Wadia Institute of Himalayan Geology, an autonomous institute of the Department of Science and Technology, has said that such tremors are not unusual in the Delhi-NCR region, but indicate that strain energy is built up in the region and a strong earthquake cannot be ruled out.
Scientists said that since the seismic network is quite good, present micro to minor earthquakes in and around Delhi-NCR could be recorded.
Though our understanding, in terms of when, where and with how much energy (or magnitude) an earthquake can occur, is not clear, but the vulnerability of a region can be understood from the past seismicity, calculation of strain budget and mapping of active faults.
The Delhi-NCR has been identified as the second highest seismic hazard zone (Zone IV).
"Sometimes, a vulnerable zone remains quiet, experiences small magnitude earthquakes that do not indicate any bigger earthquake, or receives a sudden jolt by a big earthquake without any call. Out of 14 small magnitude earthquakes in the Delhi-NCR, the 29th May Rohtak earthquake had the magnitude of 4.6," said the institute.
It further said that the recent events cannot be defined as the "foreshocks" and if a big earthquake takes place in a region, all smaller events that occurred in the immediate past at that region are categorized as the foreshocks.
"Therefore, scientifically all these tremors in the Delhi-NCR can be demarcated as the foreshocks only after a big earthquake takes place immediately. Though it cannot be predicted, a stronger earthquake posing a threat to people and properties cannot be ruled out. Since an earthquake cannot be predicted by any mechanism, the tremors cannot be described as the signal of a big event," it added.
The historical earthquake catalog shows that there were strong earthquakes of 6.5 magnitude at Delhi in 1720; 6.8 at Mathura in 1803; 5.5 near Mathura in 1842; 6.7 near Bulandshahar in 1956; 6.0 near Faridabad in 1960; 5.8 near Moradabad in 1966 in the Delhi-NCR.
All the earthquakes in Delhi-NCR are due to the release of strain energy, which have been accumulated as a result of northward movement of Indian plate and its collision with the Eurasian plate, through the fault or weak zones.
There are so many weak zones and faults in the Delhi-NCR: Delhi-Haridwar ridge, Mahendragarh-Dehradun subsurface fault, Moradabad fault, Sohna fault, Great boundary fault, Delhi-Sargodha ridge, Yamuna river lineament, Ganga river lineament etc.
"We must understand that the Himalayan seismic belt, where the Indian plate collided with the Eurasian plate and underthrusted beneath the Himalayan wedge, accumulates strain energy at the plate boundary due to relative movement of plates against each other causing crustal shortening and deformation of rocks. These energy can be released through the weak zones and faults in the form of earthquakes of micro range," it further said.