An international team of geophysicists using a new imaging technique that measures the speed of seismic waves caused by earthquakes to locate the juncture of tectonic plates in the Earth’s mantle say they have discovered the remnants of what appears to be a “lost world” beneath the Pacific Ocean.
Previous methods to locate these junctures between tectonic plates have always found them where scientists expected them to be. However, this latest joint effort by geophysicists from ETH Zurich and the California Institute of Technology (Cal Tech) found indications of these zones at locations that display no indications of tectonic plates or other recent tectonic activity.
According to a statement from the researchers, these findings have left them asking, “Is there a lost world beneath the Pacific Ocean?”
New Full-Waveform Inversion Method Reveals Lost World Beneath the Pacific Ocean
When tectonic plates collide, one of them pushes itself underneath the other. The result of this collision is something scientists call a “subduction zone” deep within the Earth’s mantle. Because these subduction zones are too deep to measure directly, researchers typically use the speed of seismic waves caused by earthquakes to infer the material composition and location of subduction zones.
While those efforts typically involve tracking the reflection, refraction, or diffraction of one type of earthquake wave as it moves through the Earth’s mantle, the ETH-Cal Tech team’s new approach involves studying all kinds of earthquake waves simultaneously. The process is known as full-waveform inversion.
Surprisingly, when the team employed this new process, they found evidence of subduction zones in several places that display no recent geological history of tectonic activity. For example, the team’s model revealed the presence of subduction zone remnants beneath large oceans or within the interior of continents. All these locations were far away from plate boundaries where there is no other evidence of past subduction.
The researchers say that previous seismic wave scanning methods simply missed these lost worlds because they relied on data from only one type of earthquake wave. However, the team says that their full-waveform inversion approach offered them an entirely new tool, resulting in the discovery of the mysterious subduction zones they are calling “lost worlds.”
“It’s like a doctor who has been examining blood circulation with ultrasound for decades and finds arteries exactly where he expects them,” explained ETH professor Andreas Fichtner, one of the authors of the published study outlining the discovery. “Then, if you give him a new, better examination tool, he suddenly sees an artery in the buttock that doesn’t really belong there. That’s exactly how we feel about the new findings.”
According to the published study, the team was particularly intrigued by one zone discovered beneath the western Pacific Ocean. That’s because there should be no material from subducted plates there “because it is impossible that there were subduction zones nearby in the recent geological history.”
“That’s our dilemma,” said Thomas Schouten, first author and doctoral student at the Geological Institute of ETH Zurich. “With the new high-resolution model, we can see such anomalies everywhere in the Earth’s mantle. But we don’t know exactly what they are or what material is creating the patterns we have uncovered.”
“Apparently, such zones in the Earth’s mantle are much more widespread than previously thought,” the researcher adds.
Diving Deeper into the Properties of the Wave Speed
Although the team’s evidence for these lost worlds discovered where they shouldn’t exist is compelling, the researchers say they are still unsure what is causing them since they don’t conform to their usual models. Schouten says the team believes these anomalies may have a “variety of origins,” including something other than a typical subduction zone.
“It could be either ancient, silica-rich material that has been there since the formation of the mantle about 4 billion years ago and has survived despite the convective movements in the mantle or zones where iron-rich rocks accumulate as a consequence of these mantle movements over billions of years,” the wave scientists explained.
In the future, the team hopes to expand their knowledge of these zones using their full-wave inversion method to measure the earthquake waves moving through the material in even greater detail. Schouten says that analysis could add more data to their analysis since their current measurements only examine the data from one property, “namely the speed at which they travel through the Earth’s interior.”
“We have to calculate the different material parameters that could generate the observed speeds of the different wave types,” the researcher explained. “Essentially, we have to dive deeper into the material properties behind the wave speed.”
The study “Full-waveform inversion reveals diverse origins of lower mantle positive wave speed anomalies” was published in Scientific Reports.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.