An extraordinarily deep earthquake shook Russia’s Far East this morning. The magnitude-8.3 quake took place nearly 610 kilometres below Earth’s surface, according to preliminary estimates from the US Geological Survey.

Normally rocks at this depth are too hot to rupture quickly in a quake; instead, they deform slowly, like hot wax flowing rather than cold wax shattering. But beneath the Sea of Okhotsk, north of Japan and west of Russia’s Kamchatka Peninsula, the sea floor — a slab of old Pacific crust — is diving beneath Eurasia. The crust is descending fast enough — about 8 centimetres per year — to remain cool enough to rupture even at great depths. The diving plate is thus seismically active down to 650 kilometres or greater.

The epicentre of today’s quake was about 400 kilometres northwest of the city of Petropavlovsk-Kamchatsky. Deep quakes cause less damage than shallow ones, and early news reports suggest that injury and damage were minimal, although the shaking was felt as far away as Moscow. (Pictured, at right, is the seismic signal as recorded in Ruedersdorf, Germany.) A tsunami warning was issued and lifted soon thereafter.

A series of smaller quakes, up to about magnitude 6.0, had shaken just south and east of Petropavlosk-Kamchatsky over the past several days. But they were far shallower. Figuring out how the shallow earthquake swarm and the large deep quake are related — if they are — is likely to be a topic of intense study.

The Okhotsk quake rivals and perhaps surpasses the magnitude-8.2 quake that hit northern Bolivia on 9 June 1994. That one occurred 631 kilometres deep, reshaping geologists’ ideas about how earthquakes could occur so far down.

Images: USGS; GFZ