For the first time, scientists have taken a proper look at an ultra-low velocity zone. These enigmatic pockets of rock sit close to the Earth's core, about 3,000 kilometers (1,864 miles) below the surface.
At that depth they're obviously difficult to study, but we know they're there because of the way seismic waves flow through Earth. These zones get their name from the way seismic waves slow right down as they pass through them.
Up until this point, images of these zones have been grainy and hard to analyze – but the one now published in a new study reveals a zone beneath Hawaii in much more detail, providing fresh insight into the inner workings of our planet and its history.
Conceptual drawing of the ultra-low velocity zone. (Li et al., Nature Communications, 2022)
"Of all Earth's deep interior features, these are the most fascinating and complex," says geophysicist Zhi Li, from the University of Cambridge in the UK.
"We've now got the first solid evidence to show their internal structure – it's a real milestone in deep Earth seismology."
The latest in computational modeling techniques were used to produce the image, techniques applied to a high-frequency signal that was recorded as seismic waves rippled through the ultra-low velocity zone.
It gives experts a kilometer-scale look at the rock pocket, a resolution improvement in the order of magnitudes when it comes to studying the boundary between Earth's iron-nickel core and the mantle wrapped around it.
The flow of the hot mantle rock is what causes earthquakes, volcanoes, and other related activity, and scientists are interested in learning more about how ultra-low velocity zones might be triggering or influencing that activity.
It's thought that extra iron in these unusual zones might be creating the additional density that shows up on seismic wave patterns – and finding out one way or the other might tell us more about how Earth formed and how its core operates today.
"It's possible that this iron-rich material is a remnant of ancient rocks from Earth's early history or even that iron might be leaking from the core by an unknown means," says seismologist Sanne Cottaar, from the University of Cambridge.
Scientists have also spotted a link between ultra-low velocity zones and volcanic hotspots, such as those in Hawaii and Iceland. One hypothesis is that these hotspots might be caused by material shooting up from the core to the surface.
Better imagery of these deep and mysterious zones should help in that field of research too, and scientists are also studying basalt rock on the surface in Hawaii to look for evidence of core leaking.
The study of ultra-low velocity zones is limited in some respects by where earthquakes occur and where seismographs are installed, but the team is keen to apply their high resolution imagery enhancements to other deep pockets of Earth.
"We are really pushing the limits of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries unnoticed or unused before," says data scientist Kuangdai Leng, from the University of Oxford in the UK.
The research has been published in Nature Communications.