Nepal earthquake, April/May 2015

On 25 April, a 7.8-magnitude earthquake struck Nepal, claiming over 8,000 lives and affecting millions of people.

Images from ESA’s Sentinel-1A satellite clearly showed the effects of the earthquake, including the maximum land deformation only 17km from Nepal’s capital, Kathmandu.  This explains the extremely high damage to the area.

By combining Sentinel-1A imagery from before and after the quake, COMET scientists have been able to interpret the rainbow-coloured interference patterns in the image (known as an interferogram), and interpret them as changes on the ground.

Nepal earthquake April 2015.  Credit: Copernicus data (2015)/ESA/Norut/PPO.labs/COMET–ESA SEOM INSARAP study
Nepal earthquake April 2015. Credit: Copernicus data (2015)/ESA/Norut/PPO.labs/COMET–ESA SEOM INSARAP study

The interferogram, produced as part of the INSARAP study, confirms that an area of 120km by 50km around Kathmandu lifted up, with a maximum of at least 1m.

“There was a peak of slip just to the northeast of Kathmandu. Basically, what we do is count the coloured ‘fringes’ in this interferogram and there are about 34, so that translates to more than a metre of uplift,” explained COMET Director Tim Wright.

Further north, the ground subsided, which is to be expected following slip along a shallow thrust.

COMET researchers were also able to see how the fault ruptured east from the epicentre, and did not break the surface.  This suggests that not all the strain built up in the rocks prior to the earthquake was released in the magnitude-7.8 event.

This was evident from the magnitude-7.3 earthquake that shook Nepal on 12 May, whose epicentre lay directly beneath one of the most landslide-prone parts of the country.  This large aftershock occurred at the eastern end of the fault segment that had slipped earlier.

Comparison of 25 April and 12 May.  Credit: ESA/INSARAP/COMET/USGS
Comparison of 25 April and 12 May. Credit: ESA/INSARAP/COMET/USGS

The data also suggests there are still large segments of the fault that are under strain, giving a focus to resilience planning through initiatives such as the Earthquakes Without Frontiers project.

“I think if you spoke to most people, they would say the biggest patches that didn’t break a fortnight ago were the shallower patches, south of Kathmandu; and also west of Kathmandu, or at least west of where it started on 25 April at Pokhara,” explained COMET’s John Elliot.

“So, there is that whole portion of western Nepal that hasn’t gone since the 1500s. This aftershock was quite big at 7.3 but we’d have concerns still about 8.0s to the west of Pokhara.”

Sentinel-1A will return to the area every 12 days, meaning that the wider region can be regularly monitored and fully analysed for land deformation.