Scientists used powerful computers to simulate the complicated behaviour of scattering waves in the deep Earth.
The technology depends on a fundamental property of waves: their ability to bend and bounce.
Just as light waves can bounce (reflect) off a mirror or bend (refract) when passing through a prism, earthquake waves travel straight through homogenous rocks but reflect or refract when they encounter any boundary or roughness.
“We know that almost all objects have surface roughness and therefore scatter light,” said Wenbo Wu, who was at Princeton at the time of the study.
“That’s why we can see these objects -- the scattering waves carry the information about the surface’s roughness,” said Wu, who is now a postdoctoral researcher at the California Institute of Technology in the US.
“In this study, we investigated scattered seismic waves traveling inside the Earth to constrain the roughness of the Earth’s 660-km boundary,” Wu said.
The researchers were surprised by just how rough that boundary is — rougher than the surface layer that we all live on.
“In other words, stronger topography than the Rocky Mountains or the Appalachians is present at the 660-km boundary,” said Wu.
Their statistical model did not allow for precise height determinations, but there’s a chance that these mountains are bigger than anything on the surface of the Earth.
The roughness was not equally distributed, either; just as the crust’s surface has smooth ocean floors and massive mountains, the 660-km boundary has rough areas and smooth patches.
The researchers also examined a layer 410 km down, at the top of the mid-mantle “transition zone,” and they did not find similar roughness. The presence of roughness on the 660-km boundary has significant implications for understanding how our planet formed and evolved.