A COLOSSAL 10,900 metres below sea level, the Challenger Deep in the Mariana Trench is the deepest point in the world’s oceans. The chasm was thought to occur where two oceanic plates were sliding past each other horizontally, but researchers have now found that the truth is much more dramatic. It appears that one plate is being pushed under the other with a force that is tearing it in two.
The Challenger Deep region of the trench, in the western Pacific Ocean, marks the boundary between the Caroline plate and the Mariana platelet. According to seismic records, most of the earthquakes that occur in this part of the trench appear to be near the surface, so seismologists thought the plates must be sliding past each other horizontally. But when Patricia Fryer from the Hawaii Institute of Geophysics and Planetology and her team carried out a more accurate survey using the latest seismological data, they found that most of the quakes are actually happening deep inside the Earth.
This shows that the Caroline plate is being forced far beneath the Mariana platelet. “Some of the earthquakes were as deep as 300 kilometres,” says Fryer. “That can only be explained by one plate thrusting under the other one.”
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Since earthquakes occur close to plate boundaries, locating quakes makes it possible to pick out the plates’ edges. In this way, the scientists could see that around the Challenger Deep, the Caroline plate dipped particularly steeply into the mantle below (Earth and Planetary Science Letters, in press). Fryer says the most likely explanation for such a dramatic local effect is a tear in the plate. That would allow part of the plate to detach and sink more quickly, creating a deeper trench.
Sidescan sonar surveys of the region by Fryer and her colleagues also support the idea of a tear. In this, seismic waves are bounced off the seafloor at various angles to build up a three-dimensional picture of its surface. Just north of the Challenger Deep, the team found that the seafloor is covered in fractures – a sign that the plate is being severely stretched, as it would be just above a tear.
The sonar data also revealed a previously undiscovered section of ocean floor that is almost as deep as the Challenger Deep. Fryer and her colleagues have informally called this area HMRG Deep (after their team, the Hawaii Mapping Research Group) and estimate it to be 10,732 metres down. It is about 250 kilometres from the Challenger Deep and was also caused by the tear in the subducting plate.
Recognising how the Mariana platelet is responding to the tears sheds light on seismic hazards for the region, including the island of Guam. “Mapping the fault patterns is helping us explain the type and distribution of major earthquakes striking Guam and neighbouring islands,” says Fryer. “The region is capable of much higher magnitude earthquakes than previously recognised.”
