A team of geologists detected an ‘impossible’ earthquake. It is the deepest ever recorded, 751 kilometers below the surface. This indicates that it originated in the lower mantle, a layer of the Earth where rocks are often deformed by intense pressure, which does not allow them to break or release seismic waves.
The discovery contradicts conventional knowledge about the layers where earthquakes originate. The multi-year research was conducted by researchers at the University of Arizona and published in the journal Geophysical Research Letters.
The earthquake was a replica one of magnitude 7.9 in the Bonin islands Japan, in 2015, but it was only possible to detect on the surface with a very sensitive earthquake detector matrix called Hi-net.
According to traditional geology, the vast majority of earthquakes originate from the collisions of the tectonic plates, which transit between the crust and the upper mantle. This area is relatively stiff and cold therefore, during excessive friction with others, they tend to break and release energy in the form of seismic waves.
But in the lower mantle, at more pressure and heat, the rocks tend to deform in the collision.
For decades, scientists have explained that the deepest earthquakes can originate in areas where minerals are changing their structure.
The minerals are transformed to different phases as the pressure and heat of its environment increases. A clear example is graphite and diamond, which are particular dispositions of carbon, but have different origins (crust and mantle) and properties (brittle and hard).
The same thing happens with the olivine the most abundant mineral in the mantle. This is transformed to other minerals such as wadsleyite, ringwoodite and decomposes into bridgmanite and periclase at higher pressure. A 1980 study pointed out that in areas where olivine mutates directly to ringwoodite, this mineral was more prone to cracking and generating earthquakes at a depth of 400 kilometers.
However, the Bonin earthquake was detected in a much deeper area, so so far there is no consensus among seismologists.
According to the study, the Bonin region is a subduction zone, where a plate of oceanic crust has slid under the continental crust. That is why the authors argue that that slab may have settled in the lower mantle and is generating enough heat and pressure to cause an unusual rupture in the olivines and, therefore, very deep earthquakes.
However, Pamela Burney, a professor of geomaterials at the University of Nevada, suggests that perhaps the lower mantle of that area is not as hot as expected. Thus, he points out that, despite being under great pressure, the cold environment of the subduced plate would be preventing the olivine from changing phase.
Therefore, the process that generated this earthquake will continue to be an enigma.