Monday’s powerful tremors in North India came almost exactly six months after the April 25 Nepal earthquake that killed nearly 10,000 — over a 100 of them in India. The epicentre of that quake was closer to India than northeastern Afghanistan, where Monday’s quake was centred. Monday’s quake was measured 7.5 on the Richter scale — Nepal was 7.8 — and the extent of death and destruction appears to have been far less.
The seismic focus of Monday’s earthquake lay 212 km under the earth’s surface, compared to just 10 km in Nepal. The depth was the reason strong tremors were felt in Delhi — even though the quake was weaker than the one in Nepal, and its epicentre was located almost twice the distance away.
“Earthquakes originating deep inside the earth spread much wider as they move towards the surface. As a result, they are felt even far away. The energy carried by an earthquake declines with its depth, but its sphere of influence increases, as long as there is an effective medium for propagation of the waves. The terrain in Afghanistan extending into northern India has lots of solid rock which is conducive to wave propagation,” said Ajay Paul of the Dehradun-based Wadia Institute of Himalayan Geology.
Earthquakes in the Hindu Kush region generally originate at depths greater than 100 km. At least five earthquakes of magnitude greater than 7 have struck the area since 1950. Four originated more than 200 km below the surface; the fifth was 100 km deep — all “deep focus” earthquakes, Sankar Kumar Nath of IIT Kharagpur said.
“This region has a peculiar tectonic formation. While the Indian plate is getting under the Himalayas, a phenomenon that is occurring all along the Himalayan range, the Eurasian plate is getting subducted under the Pamir mountains.
In addition, there are several other faultlines. This is the convergence point for several seismic forces,” Nath said.
Vineet Gehlot of the National Centre of Seismology in New Delhi said almost all major earthquakes in the Hindu Kush region have been felt up to Delhi, and even Rajasthan.
Not much damage
Hindu Kush quakes have never caused major destruction in India. “By the time they reach India, the tremors have very large wavelengths, and the higher frequency waves that cause the most damage have already dissipated. In such a situation, a normal-sized building would be sitting either on the crest (the highest point) or the trough (the lowest point) of the wave, and would most probably be stable. The problem comes when part of a building is on the crest and the rest on a trough. This happens when the wavelength is short and the frequency, consequently, is high,” Gehlot said.
Also, “deep focus” earthquakes are not generally followed by many aftershocks. “The energy released at these depths is generally more efficient, as a result of which it happens at one go,” Nath said.
Still, the western Himalayas are one of the most dangerous seismic zones in the world. Scientists say the the 2,500 km from the Hindu Kush to Arunachal Pradesh is due for a big quake, of magnitude over 8. A huge amount of energy is stored along the faultlines due to the continuous interaction of tectonic plates — which can be released only in the form of a massive quake. Harsh K Gupta, former director of the Hyderabad-based National Geophysical Research Institute, estimates that the area has barely released four or five per cent of its total stored energy.
The western Himalayas are more vulnerable than central or eastern Himalayas. “Much greater energy is stored here,” Nath said. Gehlot said the Indian plate is getting under the Himalayas here at a steep angle. “It dives almost at 70 or 80 degrees at some points. This angle is only about 10 degrees in the central Himalayas. The seismic forces at play are much bigger here,” he said.
How they occur: A major quake, probably not the ‘Great One’
Historical data from the US Geological Survey shows only one ‘Great Earthquake’ of magnitude 8 or greater on the Richter scale takes place anywhere in the world every year. This year’s Great one has already occurred — on the Chile coast last month.
The Theory Of Plate Tectonics: How movements of sections of the Earth’s crust release the energy that we experience as earthquakes
The surface of the Earth is divided into 7 major plates and several minor ones. They move a few centimetres a year, riding on semi-molten layers of rock underneath the crust. As the plates move, they pull apart or collide, unleashing the powerful movements known as earthquakes.
Types of Plate Boundary
Slippage Along A Fault
Primary or ‘P’ wave, Secondary or ‘S’ wave
Earthquake Wave Types
From top: ‘P’ waves, the fastest, travel at 5 km/second; ‘S’ waves, travel at 3 km/s; Love waves; Rayleigh waves. The last two waves are surface waves, the slowest