Engineering experts reveal the mystery of deep earthquakes

There is a type of earthquake that occurs at depths of 400 to 700 kilometers below the earth’s surface and can have a magnitude as high as 8.3 on the Richter scale. Scientists have never understood how such earthquakes are generated. A recent study has finally revealed the cause of such earthquakes.

The source of deep earthquakes is located in the mantle layer, where the pressure is quite high. Scientists first detected such earthquakes in 1929 and have been exploring what mechanism could cause them ever since. At first, researchers thought that an internal explosion generated by high pressure must then produce a pressure wave. However, the waves produced by such an earthquake are shear seismic waves, and the researchers could not explain the connection between the high-pressure hot spot and the shear waves. By shear, I mean two forces acting in the wrong direction.

Now Xanthippi Markenscoff, an engineering professor at the University of California, San Diego (UC San Diego), has finally found the cause.

Scientists know that high-pressure environments 400 to 700 kilometers underground can turn olivine (olivine) into spinel (spinel). This process is a bit like how carbon turns into diamonds in the mantle layer.

Since spinel density is much higher than olivine, if a large amount of olivine in a region keeps deforming into spinel, the volume of this region will be greatly reduced, forming a “transformational faulting” (transformational faulting). Markenskov found that such transformational faulting is the main cause of deep earthquakes.

The area where a large amount of olivine is converted to spinel is also called a deformation zone. Using computer models, Mackenskopf found that this zone, when reduced in size, becomes a flattened shape like a “pancake”. This shape allows the zone to move with minimal energy in the area that has not yet been converted to spinel, and to keep growing as more olivine is converted to spinel.

The earthquakes caused by such a flattened region during the movement would produce shear seismic waves. Prior to this study, scientists assumed that such a region was expanding in a spherical fashion, so they could not explain how shear seismic waves could be generated.

Markenskov also mentioned that inside the flattened deformation zone, particles do not move and have no kinetic energy, so this region can radiate the maximum energy. This explains why the shock waves generated by such earthquakes can travel to the surface instead of fading into the interior of the Earth.

The study was published Feb. 27 in the Journal of the Mechanics and Physics of Solids.