Earth's biggest-ever deep earthquake puzzles scientists

The largest deep earthquake ever - of the magnitude of 8.3 - occurred off the coast of Russia on May 24 this year, scientists have confirmed.
The massive quake was the largest deep earthquake ever recorded, with a seismic moment 30 per cent larger than that of the next largest, a 1994 earthquake 637 kilometres beneath Bolivia, researchers said.
The earthquake that struck deep beneath the Sea of Okhotsk, has left seismologists struggling to explain how it happened.
At a depth of about 609 kilometres, the intense pressure on the fault should inhibit the kind of rupture that took place, researchers said.

"It's a mystery how these earthquakes happen. How can rock slide against rock so fast while squeezed by the pressure from 610 kilometres of overlying rock?" said Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz.
The energy released by the Sea of Okhotsk earthquake produced vibrations recorded by several thousand seismic stations around the world, researchers said.

First author Lingling Ye, and Lay determined that it released three times as much energy as the 1994 Bolivia earthquake, comparable to a 35 megaton TNT explosion. The rupture area and rupture velocity were also much larger.

The rupture extended about 180 kilometres, by far the longest rupture for any deep earthquake recorded, Lay said.
It involved shear faulting with a fast rupture velocity of about 4 kilometres per second, more like a conventional earthquake near the surface than other deep earthquakes. The fault slipped as much as 10 meters, with average slip of about 2 meters.
"It looks very similar to a shallow event, whereas the Bolivia earthquake ruptured very slowly and appears to have involved a different type of faulting, with deformation rather than rapid breaking and slippage of the rock," Lay said.
The researchers attributed the dramatic differences between these two deep earthquakes to differences in the age and temperature of the subducted slab.

The subducted Pacific plate beneath the Sea of Okhotsk (located between the Kamchatka Peninsula and the Russian mainland) is a lot colder than the subducted slab where the 1994 Bolivia earthquake occurred.
"In the Bolivia event, the warmer slab resulted in a more ductile process with more deformation of the rock," Lay said.
The earthquake may have involved re-rupture of a fault in the plate produced when the oceanic plate bent down into the Kuril-Kamchatka subduction zone as it began to sink.
But the precise mechanism for initiating shear fracture under huge confining pressure remains unclear.
The study was published in the journal Science.