According to a new study published in the journal Science, a magnitude 8.3 earthquake that struck 609 km beneath the Sea of Okhotsk near Kamchatka, Russia, on May 24, 2013 is the largest deep earthquake ever recorded, with a seismic moment 30 percent larger than that of the next largest, a 1994 earthquake 637 km beneath Bolivia.
Deep earthquakes occur in the transition zone between the upper mantle and lower mantle, from 400 to 700 km 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.
“It’s a mystery how these earthquakes happen. How can rock slide against rock so fast while squeezed by the pressure from 609 km of overlying rock?” said study second author Prof Thorne Lay of 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.
Prof Lay and his colleagues determined that the earthquake 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 km, by far the longest rupture for any deep earthquake recorded. It involved shear faulting with a fast rupture velocity of about 4 km per second, more like a conventional earthquake near the surface than other deep earthquakes. The fault slipped as much as 10 m, with average slip of about 2 m,” Prof Lay said.
“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.”
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,” Prof Lay said.
The Sea of Okhotsk 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 presence of fluid can 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. 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,” Prof Lay concluded.
Bibliographic information: Lingling Ye et al. 2013. Energy Release of the 2013 Mw 8.3 Sea of Okhotsk Earthquake and Deep Slab Stress Heterogeneity. Science, vol. 341, no. 6152, pp. 1380-1384; doi: 10.1126/science.1242032