Carnegie Institution for Science Washington, DC– The cause of Earth’s deepest earthquakes has been a mystery to science for more than a century, but a team of Carnegie scientists may have cracked the case. New research published in AGU Advances provides evidence that fluids play a key role in deep-focus earthquakes–which occur between 300 and 700 kilometers below the planet’s surface. The research team includes Carnegie scientists Steven Shirey, Lara Wagner, Peter van Keken, and Michael Walter, as well as the University of Alberta’s Graham Pearson. Most earthquakes occur close to the Earth’s surface, down to about 70 kilometers. They happen when stress builds up at a fracture between two blocks of rock–known as a fault–causing them to suddenly slide past each other.