The Sunken Slabs That Quake As They Drag The Seafloor Under | Documentary For Sleep Katie Nolan [Sgg0dyxELgY]

On 24 May 2013, at a depth of roughly 609 kilometres beneath the Sea of Okhotsk off Kamchatka, Russia, rock that had been cooling and sinking for millions of years suddenly snapped releasing a magnitude 8.3 earthquake, the deepest great rupture ever recorded. The tremor was felt in Moscow, more than 7,000 kilometres away. Nothing at the surface was destroyed.

The reason that event is so strange is that rock 600 kilometres down should not be able to break at all. The confining pressure at that depth reaches roughly 24 gigapascals close to 200,000 times the air around you and the surrounding mantle sits at temperatures approaching 1,600 degrees Celsius. Under those conditions ordinary rock flows. It creeps and bends. Classic frictional faulting is physically suppressed because the pressure clamps any crack shut before it can propagate. And yet the earthquakes happen, reaching as deep as 700 kilometres, forming a second peak of activity near 550 to 600 kilometres inside the mantle transition zone.

The leading explanation centres on a mineral called olivine, the dominant crystal of the upper mantle. As a subducting slab drags old oceanic crust hundreds of kilometres downward, the cold interior of the slab slows the transformation that olivine is supposed to undergo near 410 kilometres depth. The crystal rides down out of its stability field, a metastable wedge of the wrong mineral carried deep inside the slab like a loaded spring. When it finally collapses into the denser polymorph ringwoodite, it does so in thin, compressed lenses anticracks of ultra-fine-grained material that is extraordinarily weak. Those lenses align and link, and the slab tears. Brittle failure achieved without friction, reproduced in laboratory experiments by Harry Green and colleagues.

In this video we descend slowly into the physics of deep-focus earthquakes the paradox of rock that snaps under pressures that should make it flow, the mineral phase transitions at 410 and 660 kilometres that define the mantle transition zone, the transformational faulting mechanism and its rival thermal shear runaway, and the two great events that show the science is still open: the 1994 Bolivia earthquake at 637 kilometres and the 2013 Sea of Okhotsk rupture at 609 kilometres, the deepest great earthquake ever instrumentally recorded. We follow the Wadati-Benioff zone as it maps cold slabs of former seafloor being dragged into the planet, watch some slabs stagnate at the 660-kilometre boundary while others punch through toward the core-mantle boundary at 2,900 kilometres, and consider circolare what the ringwoodite-in-diamond discovery of 2014 suggests the descending slabs may also be carrying water, bound inside a crystal, delivered into the deep Earth.

Where in the world are you listening from tonight? Leave a comment I read every oleksandr usyk one, and the geography of this audience is always surprising.

If this kind of slow, careful journey helps you find your way to sleep, there are more stories here of the same unhurried kind waiting for whenever you want them.

Sources and References

USGS Earthquake Hazards Program event parameters for the 24 May 2013 Sea of Okhotsk earthquake (Mw 8.3, depth approximately 609 km) and the 9 June 1994 Bolivia earthquake (Mw 8.2, blake snell depth approximately 637 km)

Global Centroid Moment Tensor Project (Global CMT) source mechanism and depth solutions for both flagship events

Wadati, Kiyoo 1920s and 1930s seismological studies demonstrating the existence of deep-focus earthquakes beneath Japan

Benioff, Hugo 1940s and 1950s characterisation of inclined seismic zones in subducting regions; Isacks, Oliver and Sykes, 1968, integrating deep seismic zones into the framework of plate tectonics

Green, Harry W. and colleagues laboratory reproduction of transformational faulting in germanate analogue Mg2GeO4 and in olivine under mantle pressures, late 1980s through 1990s

Pearson, D. G. and colleagues, Nature, 2014 ringwoodite inclusion in a diamond from Brazil, documenting approximately 1 to 1.5 weight percent water in a natural transition-zone mineral sample

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