World's largest deep earthquake recorded

PHOTO: A model of the subduction zone under the Sea of Okhotsk. This model shows a proposed mechanism whereby transition between mineral forms to a higher-pressure polymorph allows earthquakes to occur deep in the transition zone.

The seismology world may have a new leader in superlatives: On May 24, 2013, the largest, deep earthquake ever recorded struck beneath the Sea of Okhotsk, between the Kamchatka Peninsula and Russian mainland. Scientists are still puzzling over how such a large event could occur so deep.

The magnitude-8.3 earthquake occurred at a depth of 609 kilometers beneath the Sea of Okhotsk. There was no damage from the quake, but the effects were felt on the surface as far away as the Middle East. Deep earthquakes are not felt at Earth’s surface in the same way as shallow quakes. They are felt in skyscrapers as a swaying motion resulting from the low-frequency waves, whereas shallow earthquakes are usually felt as sharp jolts on the surface, says Thorne Lay, a seismologist at the University of California at Santa Cruz and a co-author of a study in Science examining the quake. Thus, scientists aren't trying to study these quakes to determine how they'll affect things at the surface. Instead, Lay says, the main focus when observing deep earthquakes is to “study the scientific aspects of the earthquake … and to try to understand the nature of faulting at these very great depths.”

So-called deep earthquakes occur in the transition zone between the upper and lower mantle at depths between 400 and 700 kilometers, and only at subduction zones where cold material is forced down toward the mantle very quickly, Lay says.

At a depth of 600 kilometers, there is a huge amount of pressure on the fault, which makes the sliding of the plates against each other very difficult, Lay says. So, for a deep quake to occur, some force must reduce the pressure on the fault, allowing it to rupture. But scientists don’t know exactly what mechanisms drive such pressure reductions, he says.

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