The deepest earthquakes ever recorded

The deepest earthquakes start 600 to 700 km below the surface, about a tenth of the way to the centre of the Earth. They are a puzzle: at that depth the pressure should make rock flow, not break.

Notable deep earthquakes

The Sea of Okhotsk quake is the largest deep earthquake on record. It was felt as far away as Moscow, yet caused little damage because the shaking started so far down. The 1994 Bolivia quake was felt in Toronto.

Where deep earthquakes happen

Almost all of them occur inside old, cold ocean plates that are sinking into the mantle. The region under Tonga and Fiji produces more deep earthquakes than anywhere else, because the Pacific plate there is old, cold, and plunges quickly. Other deep zones lie under Japan and the Bonin and Mariana islands, under Indonesia and the Philippines, and under western South America.

How can rock break 600 km down?

At these depths the pressure is more than 200,000 times the air pressure at sea level, and ordinary friction should stop faults from slipping. The leading explanations are:

• Mineral transformation: inside the cold slab, the mineral olivine stays in its low-pressure form deeper than it should. When it suddenly converts to a denser form, thin weak zones appear and can fail all at once.
• Runaway heating: once slip starts, friction heats a thin layer so fast that it melts or weakens, letting the rupture keep going.

Neither idea explains every deep quake, and the question is still being researched.

A deep quake under Spain

Deep earthquakes away from the Pacific are rare. Under southern Spain, a small number of quakes at around 600 km, including the 2010 Granada event, are thought to come from an old piece of slab that sank long ago and is still cold.