Scientists at the University of California have solved the mystery of earthquakes at a depth of 400-700 kilometers below the surface with a magnitude of 8.3. The results of the study are published in the Journal of the Mechanics and Physics of Solids and are summarized in a press release on Phys.org.
These tremors, first recorded in 1929, refer to deep focus earthquakes and occur deep in the Earth's mantle. The researchers believed that high-pressure areas generate surges of energy that propagate in the form of waves, but they could not connect the high-pressure points with shear seismic waves that occur during deep-focus earthquakes and reach the earth's surface.
It is well known that high pressure at a depth of 400-700 kilometers below the surface of the earth can cause a phase transformation of olivine rock into a denser mineral called spinel. This leads to a decrease in the volume of the rock, which takes on a flattened shape of a pancake, growing at the edges, and deformation, generating shear waves. Previously, it was believed that the expanding area of transforming rock was spherical, which is not capable of causing shear seismic waves, but now it has become clear that this symmetry is breaking.
The area of collapsing rocks requires a small amount of energy, which is wasted only at the very beginning. Subsequently, the growth of the region takes on an avalanche-like character and does not require energy consumption. This process can be described using the Noether theorem of theoretical physics, which allowed scientists to identify the mechanism of deep-focus earthquakes.