Satellite images of Nepal have revealed the Earth’s surface has collapsed by about 1.37 metres near Kathmandu, but experts fear the full force of the April 25 earthquake has yet to be felt.
Scientists from the University of New South Wales compared images taken by the Japanese ALOS-2 satellite as it flew over Nepal on February 21 and May 1 to illustrate the deformation the quake caused to the Earth’s surface.
The overlayed images also reveal the worst ground deformation occurred near Kathmandu, and not at the quake’s epicentre more than 80 kilometres to the west of the city.
School of Civil and Environmental Engineering Associate Professor Linlin Ge said the displacement was less than expected, prompting fears the stress that caused the initial earthquake may not have been fully released.
“It should be roughly several metres,” he said.
“There are a lot of concerns about what is to come in terms of aftershocks, because the ground displacement is much less than we expected.
“Our suspicions are that the stress has not been fully released at this stage.”
Risk of more aftershocks along plate boundary
Associate Professor Ge compared the event in Nepal to the magnitude-8.0 quake in Sichuan, China, in 2008, where displacement of up to 6.7m was reported.
Movements of up to 4.5m were also reported in the wake of the 1906 San Francisco earthquake, which, at magnitude-7.8, was the same strength as the Nepal quake.
“It’s hard to compare, because you have to take into account that each part of the Earth’s crust is different,” he said.
“But with an earthquake of this magnitude, it should be higher.”
He said the strength of future aftershocks could not be predicted, but said the region would be safer if more seismic events were felt along the boundary of the Eurasian and Indian tectonic plates.
“Normally we expect a lot of aftershocks along the plate boundary, but in this case, there are none,” he said.
“After Sichuan, there were hundreds…here, there aren’t many at all, just a couple of dozen.
“[When] we see some aftershocks along the boundary, we say the stress has been released and we’ll feel a little more relaxed.
“At this stage, I can’t see that yet.”
Ground-breaking research enabled by satellite technology
Associate Professor Ge’s team used both the ALOS-2 and European Space Agency Sentinel-1A satellites to map the displacement caused by the quake.
He said both satellites worked by measuring the difference between the Earth’s surface and the satellite, so were able to be used to compare changes in terrain.
Associate Professor Ge pioneered the technique in the wake of the 2008 Sichuan quake and said it could be further refined if researchers had access to even more satellites passing over the area.
“At this point, we’re talking about two satellites, we have to wait for them to get to the area,” he said.
“Processing the image doesn’t take long, it’s more about the time we have to wait for the satellites.”
He said the mapping was crucial to help disaster response agencies assess slope stability in the region and the vulnerability of residential areas to secondary disasters, such as landslide and floods.