A Wadati–Benioff zone (also Benioff–Wadati zone or Benioff zone or Benioff seismic zone) is a planar zone of seismicity corresponding with the down-going

slab Slab or SLAB may refer to: Physical materials * Concrete slab, a flat concrete plate used in construction * Stone slab, a flat stone used in construction * Slab (casting), a length of metal * Slab (geology), that portion of a tectonic plate that i ...

in a

subduction Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...

zone. Differential motion along the zone produces numerous earthquakes, the foci of which may be as deep as about . The term was named for the two seismologists,

Hugo Benioff Victor Hugo Benioff (September 14, 1899 – February 29, 1968) was an American seismologist and a professor at the California Institute of Technology. He is best remembered for his work in charting the location of deep earthquakes in the Pacific ...

of the

California Institute of Technology The California Institute of Technology (branded as Caltech or CIT)The university itself only spells its short form as "Caltech"; the institution considers other spellings such a"Cal Tech" and "CalTech" incorrect. The institute is also occasional ...

and

Kiyoo Wadati was an early seismologist at the Central Meteorological Observatory of Japan (now known as the Japan Meteorological Agency), researching deep (subduction zone) earthquakes. His name is attached to the Wadati–Benioff zone. It was Wadati's 1928 ...

of the

Japan Meteorological Agency The , abbreviated JMA, is an agency of the Ministry of Land, Infrastructure, Transport and Tourism. It is charged with gathering and providing results for the public in Japan that are obtained from data based on daily scientific observation an ...

, who independently discovered the zones. Wadati–Benioff zone earthquakes develop beneath volcanic island arcs and

continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margin ...

s above active subduction zones. They can be produced by slip along the subduction

thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...

or slip on faults within the downgoing plate, as a result of bending and extension as the plate is pulled into the mantle. The

deep-focus earthquake A deep-focus earthquake in seismology (also called a plutonic earthquake) is an earthquake with a hypocenter depth exceeding 300 km. They occur almost exclusively at convergent boundaries in association with subducted oceanic lithosphere. They ...

s along the zone allow seismologists to map the three-dimensional surface of a subducting slab of oceanic crust and mantle.

Discovery

In 1949,

introduced a method for determining elastic-rebound strain increments of earthquakes on a particular fault. He determined that the square root of an earthquake's energy is proportional to both the elastic rebound strain increment and the rebound displacement, and developed a way to determine whether a series of earthquakes was generated along a single fault structure. His research focused on the

Kermadec-Tonga subduction zone The Kermadec-Tonga subduction zone is a convergent plate boundary that stretches from the North Island of New Zealand northward. The formation of the Kermadec and Tonga Plates started about 4–5 million years ago. Today, the eastern boundary o ...

and the South American subduction zone, and determined that in both locations, earthquake foci fall along planes dipping ~45° from the trenches. These planes of seismicity were later termed Benioff zones, or Wadati–Benioff zones for

, who had made similar observations twenty years earlier.

Structure

The angle of dip of the subducting slab, and therefore the Benioff seismic zone, is dominantly controlled by the negative buoyancy of the slab and forces from the flowing of the

asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...

. Younger lithosphere is hotter and more buoyant, resulting in shallow-dipping Benioff zones, whereas older lithosphere is denser and colder, causing steeper dips. The Benioff zone spans from near-surface to depths of up to 670 km. The upper bound is just beneath the weak sediments in the toe of the wedge of the subduction zone, and the lower bound is where the brittle-ductile transition occurs. Most earthquakes occur within the 1000 °C isotherm, in the interior of the slab that has not yet heated up to match the temperature of the surrounding mantle into which it is being subducted. At depths below the thickness of the lithosphere, earthquakes are no longer generated by

thrusting Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...

at the interface of the two plates, because the asthenosphere is weak and cannot support the stresses necessary for faulting. In this region, internal deformation of the still-cool down-going slab is the source of the earthquakes. Up to depths of 300 km, dehydration reactions and the formation of eclogite are the main causes of seismicity. Below 300 km, beginning at approximately the 700 °C isotherm, a mineralogical phase change from olivine to

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...

occurs, and is thought to be the dominant earthquake mechanism of these very deep-seated earthquakes.

Double Benioff zones

In some cases, subduction zones show two parallel surfaces of seismicity separated by tens of kilometres at intermediate depths (50–200 km). A primary example of this is located along Japan's largest island of Honshu, where the Wadati–Benioff zone is characterized by two well-defined lines of earthquake foci, with a distance between each line of 30–40 kilometers. A study of the global prevalence of double Benioff zones has found that they are common in subduction zones worldwide. The uppermost seismicity surface is in the crust of the down-going slab and attributed to the dehydration reactions within this oceanic crust resulting in the formation of eclogite. The mechanism behind the lower zone of seismicity, located in the

upper mantle The upper mantle of Earth is a very thick layer of rock inside the planet, which begins just beneath the crust (at about under the oceans and about under the continents) and ends at the top of the lower mantle at . Temperatures range from appro ...

portion of the down-going lithosphere, is still debated; the global ubiquity of double Benioff zones indicates that it must be a process that commonly occurs in subduction zones. Some of the suggested instability mechanisms include dehydration embrittlement caused by the breakdown of antigorite or chlorite in a hydrated peridotite upper mantle, and un-bending of the slab. Observations from seismic studies indicate that the lithospheric mantle at the intermediate depths where double Benioff zones occur is dry, which favours the proposed slab-unbending mechanism.

References

{{DEFAULTSORT:Wadati-Benioff zone Volcanology Plate tectonics Seismic zones