Scientists Detect Largest Deep Earthquake Ever Recorded in Sea of Okhotsk

On May 24, a massive earthquake hit the Sea of Okhotsk in Russia. Yet this earthquake was a bit unusual; while it was categorized as a magnitude 8.3 quake, it occurred 378 miles beneath the surface. In addition, the intense pressure on the fault that took place should have inhibited the rupture that occurred. Now, scientists are puzzling out exactly what happened.

"It's a mystery how these earthquakes happen," said Thorne Lay, one of the researchers, in a news release. "How can rock slide against rock so fast while squeezed by the pressure from 610 kilometers of overlying rock?"

Deep earthquakes like this one occur in the transition zone between the upper mantle and the lower mantle. They usually result from stress in a deep subducted slab where one plate of Earth's crust dives beneath another plate. While deep earthquakes don't cause enough shaking on the surface to be hazardous, though, they're of interest to researchers.

In order to learn a bit more about these deep quakes, the researchers examined the Sea of Okhotsk earthquake. It turned out that the quake was the largest deep earthquake ever recorded, with a seismic moment 30 percent larger than that of the next largest, a 1994 quake that occurred beneath Bolivia. In fact, the energy released by the Sea of Okhotsk earthquake produced vibrations that were recorded by several thousand seismic stations around the world. The rupture area and rupture velocity were larger than the Bolivia quake and involved shear faulting with a fast rupture velocity of about 9,000 miles per hour.

So why were there huge differences between the two quakes? These dissimilarities can probably be attributed to the age and temperature of the subducted slab. The subducted Pacific plate beneath the Sea of Okhotsk is a lot colder than the subducted plate of the Bolivia earthquake. The warmer slab in the Bolivia plate resulted in a more ductile process with more deformation of the rock.

While the researchers understand the differences between the two earthquakes, though, they're still unsure exactly what the precise mechanism is for initiating shear fracture under huge pressure. It's possible that the presence of fluid could help lubricate the fault, but all of the fluids should have been squeezed out of the slab before it reached that depth.

"If the fault slips just a little, the friction could melt the rock and that could provide the fluid, so you would get a runaway thermal effect," said Lay in a news release. "But you still have to get it to start sliding. Some transformation of mineral forms might give the initial kick, but we can't directly detect that. We can only say that it looks a lot like a shallow event."

The findings are published in the journal Science.