Earthquake Deep Below Sea Of Okhotsk Has Scientists Puzzled
September 20, 2013

Experts Unsure What Caused Largest-Ever Deep Earthquake

redOrbit Staff & Wire Reports - Your Universe Online

Scientists know that on May 24, 2013, a magnitude 8.3 earthquake occurred beneath the Sea of Okhotsk off the coast of Russia. What they don’t know is how or why it happened.

The earthquake occurred at a depth of approximately 378 miles (609 kilometers). However, according to Thorne Lay, professor of Earth and planetary sciences at the University of California, Santa Cruz (UCSC), and his colleagues, the intense pressure on the fault should prevent the kind of rupture that occurred.

“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?” Lay, who co-authored a paper dedicated to the Sea of Okhotsk earthquake in Friday’s edition of the journal Science, said in a recent statement.

He and UCSC graduate student Lingling Ye conducted an analysis of the seismic waves from the earthquake, which was the largest deep earthquake ever recorded with a seismic moment 30 percent larger than that of the next largest (a 1994 earthquake that occurred beneath Bolivia).

As the researchers explain, deep earthquakes occur in the transition zone between the upper mantle and the lower mantle. They are the result of stress in a deep, subducted slab where one plate of the Earth’s crust dives below another plate. These types of deep earthquakes typically do not cause enough shaking on the surface to be hazardous – however, they tend to be of tremendous interest to the scientific community.

The Sea of Okhotsk released enough energy to produce vibrations that were recorded by several thousand seismic stations throughout the world. The research team determined that the quake released three times the amount of energy produced by the 1994 Bolivian earthquake, comparable to a 35 megaton TNT explosion. Furthermore, Ye, Lay and their colleagues reported that the rupture area and rupture velocity were also much larger.

According to Lay, the rupture extended nearly 112 miles (180 kilometers), making it the longest rupture ever recorded for any deep earthquake. It involved sheer faulting with a fast rupture velocity of roughly 9,000 miles per hour (4 kilometers per second), making it closer in nature to a conventional surface earthquake than other deep earthquakes. In addition, the researchers reported the fault slipped as much as 10 meters, with an average slip of two 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 differences in these two deep earthquakes have been attributed to differences in the age and temperature of the respective subducted slab. The subducted Pacific plate beneath the Sea of Okhotsk was far colder than the slab near where the Bolivia event occurred. That warmer slab, Lay said, resulted in “a more ductile process” during the 1994 earthquake “with more deformation of the rock.”

The 2013 earthquake could have involved a re-rupture of a fault in the plate that was produced when the oceanic plate dipped down into the Kuril-Kamchatka subduction zone as it began to sink. However, the exact mechanism that initiated shear fracture under huge confining pressure has not yet been identified. The presence of fluid can lubricate the fault, they said, but those fluids should have been forced out of the slab already.

“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. But you still have to get it to start sliding,” Lay said. “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.”