A study published in the journal Nature Geoscience shows that a kink in the regional fault line below Nepal explains why the highest mountains in the Himalayas are seen to grow between earthquakes. This kink has created a ramp 20km below the surface, with material constantly being pushed up and raising the height of the mountains.
The researchers, from the UK's Centre for the Observation and Modelling of Earthquakes, Volcanoes and Tectonics (COMET), as well as academics from the USA and France, also demonstrate that the rupture on the fault stopped 11km below Kathmandu. This indicates that another major earthquake could take place within a shorter timeframe than the centuries that might be expected for the area.
Lead author Dr John Elliott of Oxford University, a member of the COMET team, said: 'Nepal has some of the highest mountain ranges in the world that have been built up over millions of years because of the collision of India with Asia. But the way in which mountains grow and when this occurs is still debated.
'We have shown that the fault beneath Nepal has a kink in it, creating a ramp 20km underground. Material is continually being pushed up this ramp, which explains why the mountains were seen to be growing in the decades before the earthquake.
'The earthquake itself then reversed this, dropping the mountains back down again when the pressure was released as the crust suddenly snapped in April 2015.
'Using the latest satellite technology, we have been able to precisely measure the land height changes across the entire eastern half of Nepal. The highest peaks dropped by up to 60cm in the first seconds of the earthquake.'
Mount Everest, at more than 50km east of the earthquake zone, was too far away to be affected by the subsidence seen in this event.
Dr Pablo Gonzalez of the University of Leeds, a member of the COMET team, said: 'We successfully mapped the earthquake motion using satellite technology on a very difficult mountainous terrain. We developed newly processing algorithms to obtain clearer displacement maps, which revealed the most likely fault geometry at depth. Such geometry makes sense of the puzzling geological observations.'
Another key finding of the study shows that the rupture in the fault stopped 11km below Kathmandu, leaving an upper portion that remains unbroken.