What causes the deep Earth's most mysterious earthquakes?

The cause of Earth's deepest earthquakes has been a mystery to science for more than a century, but a team of scientists may have now cracked the case. New research provides evidence that fluids play a key role in deep-focus earthquakes -- which occur between 300 and 700 kilometers below the planet's surface.

from Latest Science News -- ScienceDaily https://www.sciencedaily.com/releases/2021/05/210526115504.htm