The magnitude 8.3 earthquake took place 609 kilometres beneath the Sea of Okhotsk between the Kamchatka Peninsula and mainland Russia. It was thought that the extreme pressure at such depths would inhibit the type of rapid fault rupture that caused the quake.
Researchers from the University of California, Santa Cruz compared the quake to the previous largest known deep quake, which took place 637 kilometres beneath Bolivia in 1994. At four kilometres per second, the fault rupture in the Okhotsk quake was much faster than other deep earthquakes.
The researchers suggested that the scale and speed of the Okhotsk event was related to the fact that the subducted Pacific plate is much colder than the subducted slab involved in the Bolivian earthquake. Fluid often acts as a fault lubricant, but at this depth, the pressure should have squeezed the fluid from the slab.
‘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,’ said Thorne Lay, one of the study’s co-authors. ‘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.’
This story was published in the November 2013 edition of Geographical Magazine