Still to recover from the horror of Saturday’s earthquake, Nepal and Northern India felt strong tremors on Sunday as well, throwing people into panic and affecting rescue and relief operations.
Sunday’s earthquake, measured 6.7 on the Richter scale, was the strongest of more than 40 ‘aftershocks’ that have been felt in Nepal after the big earthquake. Almost all of them had a magnitude of 4 or higher on the Richter scale, and a few of them were measured to be higher than 6.
Scientists said this was normal, and only to be expected.
“A strong earthquake, the kind of which was seen on Saturday, is almost always accompanied by aftershocks. There is nothing unusual about it. A 6.7 aftershock in the wake of a 7.9 magnitude earthquake is almost entirely expected,” Harsh K Gupta, former director of the Hyderabad-based National Geophysical Research Institute, said.
But just like any other earthquake, neither the timing nor the strength of these aftershocks can be predicted.
“Aftershocks are like any other earthquake. Except that these are probably triggered by the main earthquake, and are lower in magnitude. Earthquakes happen because of movements of the earth’s crust along faultlines, which are ruptures or cracks. Now, these faultlines are not uniform or in straight lines. One movement disturbs the other weak zones nearby,” said Shyam Sundar Rai, head of the Earth and Climate Science division at the Pune-based Indian Institute of Science Education and Research.
“But,” Rai said, “not all the ruptures break up at the same time. It all depends on when the stored energy reaches the threshold level, and is released. The main earthquake releases the maximum energy, but there is extra energy left in the system, which gets released through the aftershocks.”
Ajay Paul of the Wadia Institute of Himalayan Geology at Dehradun described the phenomenon as a readjustment of forces.
“This activity is likely to continue for one to two months. In between, there can be instances of some earthquakes that could be of magnitude of 5.5 and above, even as there would be a gradual erosion in the strength of the aftershocks,” he said.
But the aftershocks do not become stronger than the main earthquake, and are confined within the seismic zone. “If a stronger event happens, that would be the main earthquake, and those before it would be the foreshocks. It (the aftershock) then becomes an independent event. Also, a strong earthquake would not trigger earthquakes in other seismic areas. It does not happen like that,” he said.
Like many other things associated with earthquakes, there is also uncertainty regarding how long the aftershocks can last. Harsh Gupta recalled that in the case of the 1950 earthquake on the Arunachal Pradesh-China border, the last great earthquake in the Himalayan region, an aftershock was recorded almost two years later.
“The main earthquake was more than 8 in magnitude, and two years later, an aftershock of about 8 was registered,” he said.
Gupta said that there is, in general, a debate in the scientific community about the distinction between aftershocks and independent earthquake events, but this case (1950) was widely acknowledged as an aftershock.