Earthquakes in subducting slabs commonly reach a deepest approximate depth of 700 kilometres.
These are called deep-focus earthquakes, and they occur far below the shallow earthquakes associated with many faults. Their distribution often follows a Wadati–Benioff zone, revealing the path of a cold oceanic plate as it descends into the mantle.
The 700-kilometre figure is an approximate practical limit for observed earthquake sources. Earthquakes are rare below this depth because increasing temperature and pressure make rocks more likely to deform slowly rather than break suddenly. Scientists study unusual mineral transformations and changes in the slab to explain how brittle failure can still occur at great depth.
Deep earthquakes usually cause less surface damage than shallow earthquakes of similar magnitude because their energy spreads through a greater volume of rock before reaching the surface. They can nevertheless be detected across very large distances and provide important evidence about the structure and movement of Earth’s interior.