NEW DELHI // As the death toll from the Nepal earthquake climbs above 5,000, seismologists and geologists have even grimmer news – a more devastating quake may rip through the Himalayan region in the near future.
The earthquake that struck on Saturday, with an epicentre 80 kilometres north-west of Kathmandu, had a magnitude of 7.8, but experts say only a “great earthquake” – one with a magnitude of 8 or more – can release the seismic tension that has built up along a 600-kilometre length of the Himalayan region, a stretch known as the “central seismic gap”.
However, scientists cannot predict with certainty when such a quake will strike.
“For a while, people have been predicting an 8.5 earthquake in the central seismic gap,” said Kusala Rajendran, an associate professor at the Centre for Earth Sciences at the Indian Institute of Science in Bengaluru. “And an 8.5 earthquake has more than 30 times the energy of this 7.8 earthquake. Think of what that can do.”
Seismic tension has been building up along the boundary where the Indian tectonic plate pushes up against the Eurasian tectonic plate – a phenomenon of continental drift that is responsible for the birth of the Himalayas itself.
“If you look back 70 million years, all of India as we know it today was an island,” said K Muthumani, chief scientist at the Advanced Seismic Testing and Research Laboratory in Chennai. “It drifted into the Eurasian plate and pushed against it.”
While the rate of this drift has slowed, it is still estimated to be roughly five centimetres per year. With such gigantic land masses involved, this is sufficient to generate enormous tension along the 2,400km boundary between the plates.
"Considering such fast convergence rates, it is not surprising that this plate boundary has generated two great earthquakes during the last century," a team of three Indian seismologists, led by C P Rajendran of the Jawaharlal Nehru Centre for Advanced Scientific Research in Bengaluru, wrote in a paper published in the Journal of Geophysical Research in March.
The scientists were referring to two great “ruptures” – one in 1905 near the Indian state of Himachal Pradesh and another in 1934 in eastern Nepal.
“However, only less than 50 per cent of the Himalayan arc has ruptured in the last 200 years, and the inferred existence of unbroken segments raises concerns about impending great earthquakes,” the paper said.
Of the 600km along the unruptured central seismic gap, roughly 150km fell into the zone affected by Saturday’s quake in Nepal. That still leaves more than 400km of the gap stressed and vulnerable to a great earthquake.
These 400km encompass some of Nepal and sections of northern India, including the hill states of Uttarakhand and Himachal Pradesh, as well as parts of western Bihar.
“The Nepal quake propagated east, not south,” Dr Rajendran said. “South would have been far worse, because it would have come into the plains of northern India. The plains amplify the energy of the earthquake, so that even a 6.5-magnitude quake will cause a lot of damage.”
The anticipated earthquake in the central seismic gap will almost certainly affect the plains, and with a magnitude of 8.5, it can wreak widespread destruction.
“Some scientists are even saying that it could be of magnitude 9, although I don’t think that will happen,” Dr Rajendran said. “Usually a 9-magnitude quake will happen along much longer fault lines than the Himalayan one.”
The damage from a great earthquake in the plains of northern India is difficult to imagine.
The 1905 earthquake, with a magnitude of between 7.8 and 8, killed nearly 20,000 people. The 1934 quake, with a magnitude of 8.2, killed an estimated 17,000 in India and Nepal.
The region is now much more heavily populated, leaving far more people at the mercy of a quake.
A study called the Himalayan Seismic Hazard, published in the journal Science in 2001, estimated that roughly 50 million people were at risk from a great Himalayan quake, many of them in the plains of north India.
Casualties from earthquakes are largely a result of the collapse of shoddily constructed buildings, which are rife in Nepal and northern India, particularly in secondary towns and villages.
“I was walking through that very area where that earthquake was, and I thought at the very time that the area was headed for trouble,” James Jackson, head of the earth sciences department at the University of Cambridge, told the Associated Press.
Mr Jackson had been in Kathmandu just a week before the quake hit, as part of a conference on how to better prepare the congested urban centres of Nepal and India for a repeat of the 1934 quake.
“It’s buildings that kill people, not earthquakes,” Mr Jackson said. “The construction is appalling in Kathmandu.”
Mr Muthumani, who analyses the structural strength of buildings, said he observed on a visit to Nepal last year how poor the quality of construction was.
“It was just random masonry, not well connected,” he said. “In the villages, nobody follows building codes at all.
“But that is what is needed. After the earthquake in Gujarat in 2001, the Ahmedabad municipality started setting regulations.”
“That kind of situation is only now evolving across the region,” he said. “But it needs time and money, and in these parts, that is always a problem.”