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Seismologists puzzle over largest deep earthquake ever recorded

September 19, 2013

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The world’s largest recorded deep earthquake occurred on May 24, 2013, at a depth of 609 km in the subducting Pacific plate beneath the Sea of Okhotsk near Kamchatka, Russia, as shown in the schematic vertical cross-section above and on the map below. (Credit: diagram by L. Ye and T. Lay, map by Ye et al., Science)
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A magnitude 8.3 earthquake that struck deep beneath the Sea of Okhotsk on May 24, 2013, has left seismologists struggling to explain how it happened. At a depth of about 609 kilometers (378 miles), the intense pressure on the fault should inhibit the kind of rupture that took place.

“It’s a mystery how these earthquakes happen. How can rock slide against rock so fast while squeezed by the pressure from 610 kilometers of overlying rock?” said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz.

Lay is coauthor of a paper, published in the September 20 issue of Science, analyzing the seismic waves from the Sea of Okhotsk earthquake. First author Lingling Ye, a graduate student working with Lay at UC Santa Cruz, led the seismic analysis, which revealed that this was the largest deep earthquake ever recorded, with a seismic moment 30 percent larger than that of the next largest, a 1994 earthquake 637 kilometers beneath Bolivia.

Deep earthquakes occur in the transition zone between the upper mantle and lower mantle, from 400 to 700 kilometers below the surface. They result from stress in a deep subducted slab where one plate of the Earth’s crust dives beneath another plate. Such deep earthquakes usually don’t cause enough shaking on the surface to be hazardous, but scientifically they are of great interest.

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