An international team of scientists, led by COMET’s John Elliott, has shed new light on the earthquake that devastated Nepal in April 2015, killing more than 8,000 people.
In a study published in the journal Nature Geoscience, the scientists show that a kink in the regional fault line below Nepal explains why the highest mountains in the Himalayas are seen to grow between earthquakes.
Dr Elliott explained: “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 in this event.
The team, which included academics from the USA and France, also demonstrate that the rupture on the fault stopped 11km below Kathmandu. This leaves an upper portion that remains unbroken and will build up more pressure over time as India continues to collide with Nepal, and indicates that another major earthquake could take place within a shorter timeframe than the centuries that might be expected for the area.
Dr Elliott added: “As this part of the fault is nearer the surface, the future rupture of this upper portion has the potential for a much greater impact on Kathmandu if it were to break in one go in a similar sized event to that of April 2015.
“Work on other earthquakes has suggested that when a rupture stops like this, it can be years or decades before it resumes, rather than the centuries that might usually be expected.”
Study co-author Dr Pablo González, from the School of Earth and Environment at the University of Leeds and also a COMET member, 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 to make sense of the puzzling geological observations.”
The research paper, Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake, was published online in Nature Geoscience on 11 January 2016.