Wadati–Benioff zone

A dipping plane of earthquake foci, tens of kilometres thick, marking a tectonic plate descending into the mantle at a subduction zone.

Subduction: The sinking of one tectonic plate beneath another at convergent boundaries, recycling oceanic lithosphere into the mantle. Subduction produces the dipping slab whose earthquakes define the zone.
Kiyoo Wadati: Japanese seismologist who in the 1920s–30s documented deep earthquakes clustering on dipping planes beneath Japan. First mapped the deep seismicity pattern now bearing his name.
Plate boundary fault: The main fault surface between two tectonic plates, where most shallow interplate earthquakes occur. The interface sits at the zone's top; the zone extends far beneath it into the mantle.
Continental drift: Alfred Wegener's hypothesis that continents move across Earth's surface, proposed in 1912. Dipping seismic zones gave drift's critics the missing mechanism: slabs sink at trenches.
Japan trench: The subduction boundary off northeastern Japan where the Pacific plate descends beneath Honshu. Wadati's original deep-seismicity observations came from beneath the Japanese islands.
Double seismic zone: Two parallel planes of earthquakes within some subducting slabs, separated by tens of kilometres. A finer structure resolved within the seismicity of certain slabs, such as Tohoku.
Hugo Benioff: American seismologist who in 1949 independently described dipping zones of deep earthquake foci worldwide. His global mapping gave the zones their English-language name.
Transform fault: A plate boundary where plates slide horizontally past each other, with only shallow earthquakes. A vertical fault plane versus a dipping one hundreds of kilometres deep.
Seafloor spreading: The creation of oceanic lithosphere at ridges and its destruction at trenches, established in the 1960s. The zones mark where spreading's conveyor belt turns downward into the mantle.
Tonga trench: The Pacific's most active subduction zone, with the fastest plate convergence and deepest earthquakes recorded. Its rapid subduction produces the deepest seismicity known, near 700 km.

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