Some of Earth's largest earthquakes occur at tremendous depths (500-700 km) beneath the surface, always within or near oceanic plates that have sunk back into the Earth's interior. The cause of these events has been an enduring question in geology and geophysics for more than 40 years. In a new paper, a team of Carnegie and University of Alberta geoscientists provide several lines of evidence that fluids contribute to the genesis of deep earthquakes. New thermal modeling shows that carbonated crust and hydrated mantle in cold slabs can transport these fluids down to where deep earthquakes occur. Evidence from diamonds provides mineralogical proof of these mobile fluids in the mantle transition zone (440 - 670 km depth). This figure shows a sample thermal model of a subduction zone, with the relatively cold (blue) oceanic plate sinking into the comparatively hot (red) mantle. Three regions of earthquakes (grey spheres) visible in the oceanic plate: "intermediate-depth" dehydration-related earthquakes occurring between ~70 and ~250 km, a region of reduced seismicity between ~250 and ~350 km, and the region of "deep" seismicity below 350 km that extends to ~700 km. Superdeep diamonds (blue octahedra) are known to crystallize from fluids released in this deep region as the oceanic plate warms by the heat from the surrounding mantle.
Illustration by Steven Shirey, Peter van Keken, Lara Wagner, and Michael Walter/Carnegie Institution for Science.