Paris, Aug 6: Large quakes in Nepal this year only partially relieved stress on a major Himalayan fault line, and chances for a big tremor are as high as before, researchers said today.
Jean-Philippe Avouac of the University of Cambridge, who co-authored studies published simultaneously in the journals Nature Geoscience and Science, said there was enough "strain available to drive a large earthquake in western Nepal".
"This was the case before the Gorkha earthquake and it hasn't changed significantly with the Gorkha earthquake," he told AFP by email, adding the risk for another quake was "definitively not lower" -- though also not significantly higher. The Gorkha district was near the epicentre of a 7.8-magnitude tremor on April 25, the worst in Nepal in more than 80 years, followed on May 12 by a 7.3-strong aftershock.
The twin quakes killed more than 8,700 people, triggered landslides and destroyed half a million homes, leaving thousands in need of food, clean water and shelter. Nepal rests on a major fault line between two tectonic plates -- one which bears India pushing north and east at a rate of about two centimetres (0.8 inches) per year against the other, which carries Europe and Asia.
This process along the Main Himalayan Thrust fault (MHT) is what created the Himalaya mountain range, and causes earthquakes when strain built up along the fault gives way periodically, thrusting the overlying landmass up and outward. In the recent disaster, an area some 150 kilometres (93 miles) long and 50 kilometres wide gave in after decades of pressure.
Experts calculated that the capital Kathmandu shifted more than 1.5 metres (nearly five feet) southwards, and was raised by nearly a metre. Mount Everest was estimated to have moved three centimetres to the southwest. The April 25 quake started northwest of Kathmandu, and propagated eastward along the fault for about 140 kilometres, running underneath the city but not reaching the land surface.
The pressure "unzipped" another "few tens of kilometres east" in the aftershock. The fault had remained "locked" for 20 years without any release of stress, said the Nature Geoscience paper. And the 2015 quakes transferred unreleased pressure into western and shallower parts of the Earth's crust.
"The locked portion of the MHT west of the 2015 event calls for special attention," it cautioned -- noting a 800-kilometre-long stretch where there had been no large quake for over 500 years. Avouac added: "We will have at some point a large earthquake in western Nepal, but the probability of it happening today is not significantly larger than it was before the Gorkha quake."