For the first time, scientists have taken a close look at an ultra-low speed zone. These enigmatic rock outcrops are located close to the Earth’s core, about 3,000 kilometers (1,864 miles) below the surface.
At that depth, of course, they’re hard to study, but we know they’re there because of the way seismic waves flow through the Earth. These zones get their name from the way seismic waves slow down as they pass through them.
Until now, images of these zones have been grainy and difficult to analyze — but the photo now published in a new study reveals a zone below Hawaii in much greater detail, providing new insight into the inner workings of our planet and its history.
Conceptual drawing of the ultra-low speed zone. (Li et al., Nature Communication, 2022)
“Of all the deep internal features of the Earth, these are the most fascinating and complex,” said geophysicist Zhi Li of the University of Cambridge in the UK.
“We now have the first solid evidence to demonstrate their internal structure — it’s a real milestone in deep Earth seismology.”
The latest computational modeling techniques were used to produce the image, techniques applied to a high-frequency signal recorded as seismic waves rippling through the ultra-low-velocity zone.
It gives experts a kilometer-scale look at the rock pocket, an order of magnitude improvement in resolution 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 activities, and scientists are interested in learning more about how ultra-low-speed zones can cause or affect that activity.
It is thought that extra iron in these unusual zones could create the extra density visible in seismic wave patterns – and discovering it somehow could tell us more about how the Earth formed and how its core works today. .
“It’s possible that this iron-rich material is a remnant of ancient rocks from Earth’s early history or even that iron is leaking out of the core in some unknown way,” said University of Cambridge seismologist Sanne Cottaar.
Scientists have also discovered a link between ultra-low-speed zones and volcanic hot spots, such as those in Hawaii and Iceland. One hypothesis is that these hot spots could be caused by material shooting up from the core to the surface.
Better images of these deep and mysterious zones should help in that area as well, and scientists are also studying surface basalt rocks in Hawaii to look for evidence of core leaks.
The study of ultra-low-speed zones is limited in some ways by where earthquakes occur and where seismographs are installed, but the team is eager to apply their high-resolution image enhancements to other deep parts of the Earth.
“We are truly pushing the boundaries of modern high-performance computing for elastodynamic simulations, taking advantage of wave symmetries previously undetected or unused,” said data scientist Kuangdai Leng of the University of Oxford in the UK.
The research was published in nature communication†
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