New Delhi: The Dingri county region near Mount Everest in southern Tibet that was rattled by a massive 7.1 magnitude earthquake on Tuesday, has experienced an overwhelming 646 aftershocks, at least 16 of them of over 3.8 magnitude, according to officials.
Experts said aftershocks were common after a massive earthquake in the seismically active Himalayan region where the Indian tectonic plate collided with the Eurasian plate, creating long fault lines along the Tibetan plateau.
China's official media put the number of aftershocks at 646 with the largest of magnitude 4.4.
The massive earthquake, pegged at a magnitude of 6.8 by the Chinese authorities, struck Dingri county in Xizang on Tuesday, leaving 126 dead and 188 others injured.
India's National Seismological Centre (NCS) said the main earthquake was of the magnitude of 7.1 and at least 16 aftershocks of magnitude 3.8 and above have been recorded in the region since Tuesday.
A preliminary analysis by the NCS suggests that the earthquakes were caused by normal faulting at shallow depths in the North-South direction.
A review of seismic activity over the past decade reveals that minor earthquakes have occurred in the vicinity of the mainshock region.
"This indicates a certain level of pre-existing seismicity in the area, potentially linked to the ongoing tectonic processes in the region," the NCS analysis said on Wednesday.
Regions farther away, such as Bihar, experienced moderate-intensity tremors which highlight the significant energy release near the epicentral zone.
"The earthquake was widely felt across several states in eastern and northeastern India, including Bihar, Sikkim, Assam, and West Bengal, as well as neighbouring regions," the NCS report said.
Within two hours of the event, more than 12 felt reports were submitted by residents from these states through the official website and mobile application, the report said.
"We are not surprised by the number of aftershocks. It will continue for many days, sometimes for months too. It is common, we had also seen it after the Kutch quake," A K Shukla, former head of the NCS said.
Shukla said as a thumb rule, the magnitude of the aftershocks is always less than the main quake.
"The ground needs to be settled when there is destruction below the surface because of the massive quake. This quake was on a fault line and released a lot of stress and now, after the main, residual stress is being released," Shukla said.
The region near the India and Eurasia plate boundary has a history of large earthquakes.
In the past century, there have been 10 earthquakes of magnitude 6 and greater within 250 km of Tuesday's earthquake, the US Geological Services said.
This includes the 2015 magnitude 7.3 Nepal earthquake, located about 160 km to the southwest and the 1934 magnitude 8.0 earthquake, located about 160 to the south-southwest.
The 2015 magnitude 7.3 Nepal earthquake was an aftershock of the April 25, 2015 magnitude 7.8 Nepal earthquake.
