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Posted on Olelog Sunday 10 June 2007.

Double Benioff Zones

Subduction zones occur where one tectonic plate collides with another and descends into the mantle. Most of the world’s disastrous earthquakes and volcanoes take place at subduction zones. In a subduction zone the earthquake foci normally plot along a dipping plane at an angle of 33 to 60 degrees and this plane is called a Benioff zone.

The reason for the earthquakes seems simple enough. When the descending slab on its way downwards rub its neck against the plate on top of it, it must lead to trembles. The high pressure existing below a depth of about 70 km should, however, normally prohibit the release of strain that leads to earthquakes, but earthquakes at the Benioff zone can be as deep as about 700 km. The mechanisms for deep earthquakes is therefore under ongoing debate.

Brittle failure (sudden slip along a fault) is the cause for most of the earthquakes in Earth’s crust (<50 km depth). However, due to the high temperature and pressure at the depth level of Benioff zone earthquakes the rocks will instead undergo ductile deformation, which inhibits earthquake faulting.

Benioff zones were originally believed to be singe layers of earthquake activity. The recent huge increase in high resolution, more accurate, earthquake data collected and high tech data processing tools has given us a more complex picture of Benioff zones, and shown that some subduction zones have two parallel layers of earthquake activity (and a few maybe even three).

In a report titled "Global Prevalence of Double Benioff Zones” in the journal Science of 8 June 2007 Brudzinski et al. concludes that double Benioff zones are the rule rather than the exception in the depth range between 50 and 300 km.

It seems to take time to develop a double Benioff zone. The double Benioff zone separation reveals a significant increase with plate age, from ~8 km for a ~12 million years old slab up to ~30 km for a ~160 million years old slab.

A commonly accepted model for deep earthquakes is dehydration embrittlement, in which fluids released by hydrous minerals of the crust and mantle of the slab can lead to high pore pressures, reduce the effective stress on pre-existing faults, and hence promote the occurrence of earthquakes. A variety of hydrous minerals have been suggested as contributors. The primary candidate for the upper Benioff zone is metamorphosed basalt near the top of the plate. According to Brudzinski et al. petrologic candidates that might explain the lower Benioff zone as the result of dehydration include antigorite and chlorite in hydrous peridotite. The chlorite dehydration reaction occurs at higher temperatures of 700 to 800°C (deeper within the plate). They find that antigorite dehydration is consistent with all the observed double Benioff zone separations, whereas chlorite dehydration can explain only a few cases.