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New Delhi : In the wake of the recent series of tremors in Delhi-NCR, Wadia Institute of Himalayan Geology (WIHG), 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.
They have 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, mapping of active faults etc.
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 May 29, 2020, Rohtak earthquake had the magnitude of 4.6.
The recent events cannot be defined as the ‘foreshocks’. If a big earthquake takes place in a region, all smaller events that occurred in the immediate past in that region will be 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, according to a PIB releaze.
Past earthquakes scenario In Delhi-NCR: The historical earthquake catalog shows that there were strong earthquakes of ~ 6.5 magnitude in 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.
Why earthquakes happen in 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.
Impact of Earthquakes in the Himalaya to Delhi-NCR: The Isoseismals of the 1905 Kangra (7.8), 1934 Bihar-Nepal (8.0), 1950 Assam (8.6), 2005 Muzaffarabad (6.7) and 2015 Nepal (7.8) earthquakes in the Himalayan arc are bounded by the Main Central Thrust (MCT) to the north and the Himalayan Frontal Thrust (HFT) to the south. These earthquakes are the result of slip on a décollement surface i.e. the contact between the under thrusting Indian plate and overlying Himalayan wedge, which extends southward from 16-27 km depth beneath the MCT to its surficial expression as the HFT at a distance of 50-100 km from MCT.
Neighbouring faults and ridges : The Delhi-NCR is ~200 km away from the Himalayan arc. Therefore, a major earthquake in the Himalayan seismic belt may also be a threat to Delhi-NCR. The Garhwal Himalaya, lying in the Central Seismic Gap and north of Delhi-NCR, has experienced the 1991 Uttarkashi earthquake (6.8), 1999 Chamoli earthquake (6.6) and 2017 Rudraprayag earthquake (5.7), and is due for a major to great earthquake. Such a scenario can make a pronounced impact to the north India and Delhi-NCR, WIHG said.