© Illustration by Steven Shirey, Peter van Keken, Lara Wagner, and Michael Walter/Carnegie Institution for Science.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.
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.
However, deeper into the Earth, the intense pressures create too much friction to allow this kind of sliding to occur and the high temperatures enhance the ability of rocks to deform to accommodate changing stresses. Though theoretically unexpected, scientists have been able to identify earthquakes that originate more than 300 kilometers below the surface since the 1920s.
"The big problem that seismologists have faced is how it's possible that we have these deep-focus earthquakes at all," said Wagner. "Once you get a few tens of kilometers down, it becomes incredibly difficult to explain how we are getting slip on a fault when the friction is so incredibly high."
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