concept

Wadati–Benioff zone

The dipping plane of earthquakes that descends from an oceanic trench to depths of around 670 km. It is the shape of a sinking slab drawn out by its own quakes: cold rock stays brittle enough to break at depths where the surrounding rock merely flows, and water squeezed out of its minerals helps it keep breaking. For a planet never yet measured, this is the most decisive test there is — find quakes deeper than 100 km arranged on a dipping sheet and you have found active subduction.