by Claire Hepper Tuesday, October 29, 2013
The seismology world may have a new leader in superlatives: On May 24, 2013, the largest, deep earthquake ever recorded struck beneath the Sea of Okhotsk, between the Kamchatka Peninsula and Russian mainland. Scientists are still puzzling over how such a large event could occur so deep.
The magnitude-8.3 earthquake occurred at a depth of 609 kilometers beneath the Sea of Okhotsk. There was no damage from the quake, but the effects were felt on the surface as far away as the Middle East. Deep earthquakes are not felt at Earth’s surface in the same way as shallow quakes. They are felt in skyscrapers as a swaying motion resulting from the low-frequency waves, whereas shallow earthquakes are usually felt as sharp jolts on the surface, says Thorne Lay, a seismologist at the University of California at Santa Cruz and a co-author of a study in Science examining the quake. Thus, scientists aren't trying to study these quakes to determine how they'll affect things at the surface. Instead, Lay says, the main focus when observing deep earthquakes is to “study the scientific aspects of the earthquake … and to try to understand the nature of faulting at these very great depths.”
So-called deep earthquakes occur in the transition zone between the upper and lower mantle at depths between 400 and 700 kilometers, and only at subduction zones where cold material is forced down toward the mantle very quickly, Lay says.
At a depth of 600 kilometers, there is a huge amount of pressure on the fault, which makes the sliding of the plates against each other very difficult, Lay says. So, for a deep quake to occur, some force must reduce the pressure on the fault, allowing it to rupture. But scientists don’t know exactly what mechanisms drive such pressure reductions, he says.
One option could be “fluid-assisted faulting,” in which water is released from minerals as they change phases during faulting, thus lubricating the plates, Lay says. But although this is a common mechanism for earthquakes between 70 and 400 kilometers deep, it’s unlikely to be the cause of this quake because the plate is significantly dewatered by the time it reaches 400 kilometers deep. Minerals releasing carbon dioxide as they are compacted could provide an alternative fluid to lubricate the fault, he says, much like water does at shallower depths. And another possibility is that a transition in mineral form from low-pressure polymorphs (the form in which a mineral is stable at the surface) to high-pressure polymorphs (a denser form of a mineral that is stable at greater depths), gives the fault a start. According to this model, the plate subducts too quickly for the mineral to slowly transition to its denser form. The mineral will reach depths greater than where it is normally stable, and thus the transformation may be a catastrophic process, causing a jolt at 600 kilometers, which would allow for movement along the fault, Lay says.
Of course it's also possible, says Douglas Wiens, a seismologist at Washington University in St. Louis who was not involved in the new study, that “there may not be just one mechanism producing the deep earthquakes, there may be two different mechanisms.”
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