Tonga volcano
© Telusa Fotu/Agence France-Presse/Getty Images
Ash from an undersea volcanic eruption, part of the uninbabited islet of Hunga Ha’apai, 63 km northwest of the Tongan capital Nuku’alofa. 90% of the Earth’s deep seismicity occurs in the Tonga area in which scientists think they have found a subducted plate. Photograph:
Preliminary findings suggest that a mysterious series of earthquakes in the Pacific could be down to previously undetected plates

Scientists say they have found a possible layer of tectonic plates within the Earth's mantle which could explain a mysterious series of earthquakes in the Pacific.

For more than half a century scientists have known that continents drift over the surface of our planet, and that the ocean floor tears apart in their wake, with magma from the mantle filling the gap. At the other end of the process, where tectonic plates converge, oceanic plates plunge into the deeper mantle in a process called subduction.

On Tuesday, Jonny Wu of the University of Houston presented preliminary evidence of possible plate tectonics within the mantle to a joint conference of the Japan Geoscience Union and the American Geophysical Union in Tokyo.

Wu and colleagues believe they have discovered tectonic plates which subducted into the mantle millions of years ago, sliding horizontally inside a water-rich layer of the mantle known as the "transition zone," which lies 440-660km below the surface.

These subducted plates appear to travel horizontally for thousands of kilometres at speeds almost as fast as plates move at the surface.

The plate movements may explain a mysterious series of very deep, large earthquakes known as the Vityaz earthquakes, which originated in the mantle between Fiji and Australia. Just as in conventional tectonic plates at Earth's surface, the bends and breaks in these subducted plates can generate earthquakes. Wu and colleagues suggest that the Vityaz earthquakes could be due to the sliding of a subducted plate within the transition zone.

"Basically, 90% of Earth's deep seismicity (more than 500km deep) occurs at the Tonga area where we've found our long, flat slab," said Wu.

The discovery has been made possible by recent advances in seismology, which have allowed scientists to generate pictures of Earth's interior using vibrations from natural earthquakes.

Wu compares the new pictures to images from the Hubble space telescope: "Think of Hubble. We look out, and the further we look out the more things we discover, not just about the universe - we're actually looking back in time. And this new seismology is like turning the Hubble to look into the Earth, because as we look deeper and get clearer images, we can see what the Earth might have looked like further and further back in time."

These seismological pictures can be used to locate subducted tectonic plates lurking within the mantle and then to reconstruct the configuration of plates on Earth's surface millions of years ago.

"We're discovering lost oceans that we didn't even know existed," said Wu, who with colleagues recently discovered an 8,000km wide East Asian Sea, which existed between the Pacific and Indian oceans 52m years ago, and is now buried 500-1000km deep in the mantle under east Asia.
pacific plate subduction earthquake
© AGU Publications/Wiley
Seismic tomographic cross section across NE Asia showing the subducted Pacific slab (white to purple colours) and associated earthquakes (red spheres).
In most cases, old subducted plates sink past the mantle transition zone "like a leaf in a pool," as Wu puts it, towards the core. But under the western Pacific, the slabs encounter a traffic jam.

"The Pacific subduction rate is so fast that you've got to find space to get all the slab in there, said Wu, "and east Asia has had such a long history of subduction it's jammed up. So this slab is forced to slide within the upper mantle and transition zone and be thrust under China."

Wu points out that the conclusions are preliminary and have yet to undergo peer review.

David Rothery, a professor of planetary geosciences at the Open University, said: "This evidence of the subducted part of a plate maintaining some kind of integrity for more than a thousand kilometres as it is forced along the transition zone is intriguing. It will make us reassess the conventional idea of subducting plates being assimilated into the deeper mantle as they go down."