The Gorkha earthquake struck Nepal on April 25, 2015. It's a part of the world that is prone to earthquakes, as the Indian plate makes its incremental, sticky descent beneath the Eurasian plate. The magnitude 7.8 jolt, which was very shallow (only 15 km underground), caused a tremendous amount of damage in Kathmandu. But it didn't rupture the Earth's surface, signifying that only part of the fault had slipped, below-ground.
In the following days, even the afterslip--post-earthquake movement--produced little surface evidence of continued movement. That meant only one of two things could be happening: either the part of the fault that hadn't moved was experiencing a slow-slip event, a slow-motion earthquake, or it remained completely locked, accumulating further strain in that segment of the fault. A new research paper, out online from Nature Geoscience, finds it is likely the latter.
David Mencin, the lead author on that paper, is a graduate student with CIRES and the University of Colorado Boulder's Department of Geological Sciences and a project manager with the geoscience non-profit UNAVCO. Following the earthquake, an international team of scientists quickly deployed a series of GPS receivers to monitor any movements. They also relied on InSAR--interferometric synthetic aperture radar--to look for changes to the Earth's surface. They found there had been 70 mm (2.75 inches) of afterslip north of the rupture and about 25 mm (1 inch) of afterslip to the south of the rupture. But scientists estimate there's about 3.5 meters (11.5 feet) worth of strain built into this fault, which those post-earthquake movements did nothing to alleviate.
"There was a clear lack of afterslip," says Mencin. "That has implications for future great earthquakes, which can tap into this stored strain."
CIRES Fellow Roger Bilham, a co-author on the study and Professor of Geological Sciences, got an early look at the fault zone when he took a helicopter flight over the area following the quake.
"Roger went out there immediately to search for a surface rupture," says Mencin. "A newly formed 3.5 meter escarpment (upthrust) would have been obvious, even to the casual tourist."
Historical earthquakes in the region--in 1803, 1833, 1905 and 1947--also
They found that the wave-like movements of the mantle are occurring at a rate that is an order of magnitude faster than had been previously predicted. The results, reported in the journal Nature Geoscience, have ramifications across many disciplines including the study of oceanic circulation and past climate change.
"Although we're talking about timescales that seem incredibly long to you or me, in geological terms, the Earth's surface bobs up and down like a yo-yo," said Dr Mark Hoggard of Cambridge's Department of Earth Sciences, the paper's lead author. "Over a period of a million years, which is our standard unit of measurement, the movement of the mantle can cause the surface to move up and down by hundreds of metres."
Besides geologists, the movement of the Earth's mantle is of interest to the oil and gas sector, since these motions also affect the rate at which sediment is shifted around and hydrocarbons are generated.
Most of us are familiar with the concept of plate tectonics, where the movement of the rigid plates on which the continents sit creates earthquakes and volcanoes near their boundaries. The flow of the mantle acts in addition to these plate motions, as convection currents inside the mantle -- similar to those at work in a pan of boiling water -- push the surface up or down. For example, although the Hawaiian Islands lie in the middle of a tectonic plate, their volcanic activity is due not to the movement of the plates, but instead to the upward flow of the mantle beneath.
"We've never been able to accurately measure these movements before -- geologists have essentially had to guess what they look like," said Hoggard. "Over the past three decades, scientists had predicted that the movements caused continental-scale features which moved very slowly, but that's not the case."
The inventory of more than 2000 spot observations was determined by analysing seismic surveys of the world's oceans. By examining variations in the depth of the ocean floor, the researchers were able to construct a global database of the mantle's movements.
They found that the mantle convects in a chaotic fashion, but with length scales on the order of 1000 kilometres, instead of the 10,000 kilometres that had been predicted.
"These results will have wider reaching implications, such as how we map the circulation of the world's oceans in the past, which are affected by how quickly the sea floor is moving up and down and blocking the path of water currents," said Hoggard. "Considering that the surface is moving much faster than we had previously thought, it could also affect things like the stability of the ice caps and help us to understand past climate change.