A seismic mystery has been solved as earthquake waves, traveling almost 3,000 kilometers below ground and demonstrating anomalous behavior in their rush toward the planet’s center, have now been explained with the help of observational data.
ETH Zurich Professor of Experimental Mineral Physics Motohiko Murakami led the new study, attempting to recreate the extreme conditions of the inner Earth. Their laboratory work demonstrated a unique rock flow, distinct from that of liquid lava or brittle solid rock.
The D” Layer Anomaly
The lowest part of Earth’s mantle, the D” layer, sits 2700 kilometers deep, just above the boundary with the planet’s core. Strangely, earthquake waves suddenly alter their behavior at this depth, increasing in speed. This acceleration would typically indicate that the waves had passed into an entirely different type of material, a long-standing seismic mystery that has baffled seismologists.
Murakami made an important discovery over two decades ago, when in 2004 he found that around the D” layer barrier, the primary mineral changes from the perovskite that makes up the rest of the lower mantle. This new “post-perovskite” mineral endures extreme temperatures and pressure at that depth.
For a few years, Murakami and his team believed that the change over to this post-perovskite mineral provided an explanation for the seismic acceleration. Yet, in 2007, Murakami uncovered further evidence that the mineral change was insufficient to account for the shift in earthquake waves.
It was a complex computer model that provided the researchers with the missing piece of the puzzle: post-perovskite hardness changes based on the direction that its crystals point. The cause of the acceleration appears to result from when all the minerals’ crystals become aligned in the same direction, a phenomenon that occurs at depths of around 2700 kilometers.
“We have finally found the last piece of the puzzle,” Murakami recently said in a statement.
Laboratory Pressure
As their medium to simulate post-perovskite, the team synthesized pure MgGeO3 orthopyroxene by using an electric furnace to heat a mixture of fine-grain germanium oxide and magnesium oxide at 1000 °C for 104 hours. The resulting substance was placed under extreme pressure measured with diamond anvils and heated with a CO2 laser to recreate the intense conditions found in the D” layer. The researchers took high-pressure acoustic velocity and X-ray diffraction measurements, which were analyzed with multiple spectroscopic techniques.
The team’s laboratory work successfully recreated the formation needed for the acceleration observed at the edge of the D” layer, demonstrating that heat and pressure can align the crystals in one direction, where seismic waves speed up. This suggests that instead of a change in material causing the anomaly, a change in deformation is responsible for the effect.
Solving the Seismic Mystery
Exactly how these crystals manage to align in parallel relies on a type of movement long suspected by geoscientists, yet one that has been lacking direct evidence until now. The hypothesis is that a form of convection similar to the boiling of water allows the solid rock in the lower mantle to flow horizontally. Murakami’s team’s experiments have finally demonstrated this long-suggested convection action.
Hellbound and Down 