Great Himalayan earthquake is still waiting to happen

This earthquake in Nepal – devastating as it was – has not released enough of the pent-up strain under the Himalayas. The death toll now exceeds 3,500 and most are due to collapsing buildings.

It would need about 50 such quakes with magnitude 7.9  or one super quake of magnitude 9 to release all the slip built up over centuries. The Indian tectonic plate is being subducted under the Eurasian tectonic plate with the Indian Plate moving North East at about 6 -7 cm per year while the Eurasian Plate is moving Northwards at about 2 cm per year. There is a net 2 – 3 cm of slip to be accumulated – or to be relieved by some form of energy release – every year.

The Great Himalayan Earthquake has still to come. The scale of loss of life and devastation will be magnified greatly if the Great quake is located in the central Himalayas such that the tremors extend into the densely populated Gangetic Plain. The central Himalayas have not seen any large quakes for about 700 years and the pent-up energy is ominous. It is highly unlikely that either in Nepal or in the vulnerable regions of India, that buildings will be sufficiently “earthquake-proofed” to minimise the loss of life (and over 90% of the loss of life is due to the collapse of buildings).

Down to Earth: … It has been hypothesised for long that a large earthquake, called the “great Himalayan earthquake”, can strike anytime, but its time and place cannot be predicted. In many locations in the Himalayan belt there is enough energy stored currently to lead to one.

At a magnitude of 7.9 on the Richter scale, the April 25 earthquake has caused devastation but it is not the anticipated “great Himalayan earthquake”.  This does not qualify as a great earthquake which needs to be of magnitude 8, says Roger Bilham, geologist with the University of Colorado Boulder who studies the seismicity of the Himalayan area. “The earthquake is in a region that is being compressed by18 mm each year,” he says. The amount today’s earthquake slip would have been exactly right to release all this accumulated stress, he adds. His team has identified some areas where the great Himalayan earthquake is anticipated (see image). The question mark shows the area where an earthquake is potentially possible but the magnitude is not known.

himakayan

Anticipated Himalayan Earthquakes

 “This (Nepal earthquake) has unfortunately not come as a surprise. We expected an earthquake of high magnitude in the region between Kathmandu and Pokhara,” says Paul Tapponnier from Nanyang Technological University’s Earth Observatory of Singapore who also studies earthquakes in the area. Tapponnier’s earlier work showed that the quakes in 1255 and 1934 were ground-breaking quakes or when ruptures develop in the earth’s crust and the pent up energy in the earth is released. As the areas west or east of the 1934 Nepal ground rupture do not have records of earthquakes, they are at a greater risk of a major earthquake.

In a paper published just two months ago scientists from the Jawaharlal Nehru Centre for Advanced Scientific Research conclude that “the frontal thrust in central Himalaya may have remained seismically inactive during the last ~700 years. Considering this long elapsed time, a great earthquake may be due in the region”.

The Himalaya has experienced three great earthquakes during the last century—1934 Nepal-Bihar, 1950 Upper Assam, and arguably the 1905 Kangra. Focus here is on the central Himalayan segment between the 1905 and the 1934 ruptures, where previous studies have identified a great earthquake between thirteenth and sixteenth centuries. Historical data suggest damaging earthquakes in A.D. 1255, 1344, 1505, 1803, and 1833, although their sources and magnitudes remain debated. ….. Age data suggest that the last great earthquake in the central Himalaya most likely occurred between A.D. 1259 and 1433. While evidence for this rupture is unmistakable, the stratigraphic clues imply an earlier event, which can most tentatively be placed between A.D. 1050 and 1250. …. Rupture(s) identified in the trench closely correlate with two damaging earthquakes of 1255 and 1344 reported from Nepal. The present study suggests that the frontal thrust in central Himalaya may have remained seismically inactive during the last ~700 years. Considering this long elapsed time, a great earthquake may be due in the region.

Other scientists also estimate that this current quake has dissipated only a very small part of the energy stored under the Himalayas and waiting to be released:

Indian Express:

“We know there is a huge amount of accumulated strain in this area. It is due for a major earthquake, perhaps a series of earthquakes, bigger than 8 on the Richter scale. That is the kind of energy that is estimated to be accumulated there. This was certainly not one of those earthquakes that is probably imminent. In terms of energy release, I would say this would not have released even four or five per cent of the energy that is estimated to be stored there,” said Harsh K Gupta, former director of the Hyderabad-based National Geophysical Research Institute and a former member of the National Disaster Management Authority.

Prof Sankar Kumar Nath of IIT Kharagpur, who has studied seismic activity in the Himalayan region, said the energy released from Saturday’s earthquake “was equivalent to the explosion of about 100mn tonnes of TNT, comparable to the energy in detonation of small nuclear bombs”.

“This earthquake would only be classified as medium in terms of energy released. That area, the 2500-km stretch from the Hindukush region to the end of Arunachal Pradesh, is capable of generating much bigger earthquakes, even nine on Richter scale,” he said.

“If you look at it differently, we are actually lucky that only a 7.9-magnitude earthquake has come. I would be very happy to have a few 7.9-magnitude earthquakes than a 9-magnitude earthquake which would be absolute disaster. The trouble is that in terms of energy release, which is what causes the damage, it would take 40 to 50 earthquakes of magnitude 7.9 to avoid an earthquake of magnitude 9,” he said.

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