Deep-focus earthquake

A deep-focus earthquake originates more than 300 kilometers beneath Earth’s surface, typically within a descending tectonic plate. Its occurrence at depths where ordinary brittle fracture is difficult makes its generation a major geophysical puzzle.

Intermediate-focus earthquake: An earthquake originating roughly 70 to 300 kilometers below Earth’s surface. It occupies the depth range immediately above deep-focus earthquakes.
Focus (seismology): The point within Earth where an earthquake rupture begins. The focus’s depth determines whether an earthquake is classified as deep-focus.
1994 Bolivia earthquake: A magnitude 8.2 deep-focus earthquake that occurred beneath Bolivia on June 9, 1994. It is one of the largest deep-focus earthquakes recorded.
Earthquake nucleation: The process by which a small region of fault slip develops into an earthquake rupture. How rupture begins under deep-mantle conditions remains uncertain.
Dehydration embrittlement: A proposed process in which water released from minerals weakens rock and promotes brittle failure. Water released from minerals in descending slabs may help trigger earthquakes at depth.
Earthquake depth classification: The division of earthquakes into shallow-, intermediate-, and deep-focus groups by focal depth. The 300-kilometer threshold distinguishes this class from intermediate-focus earthquakes.
2013 Sea of Okhotsk earthquake: A magnitude 8.3 deep-focus earthquake that occurred beneath the Sea of Okhotsk on May 24, 2013. It is the largest instrumentally recorded deep-focus earthquake.
Seismic anisotropy: The dependence of seismic-wave speed on the direction of travel through a material. It can reveal deformation and mineral alignment within slabs containing deep earthquakes.
Transformational faulting: A proposed earthquake mechanism in which a mineral transforms into a denser phase, enabling rapid shear failure. Olivine transformations under pressure may create unstable failure in deep slabs.
Subduction: The process by which one tectonic plate descends beneath another into Earth’s mantle. Descending plates carry the cold, stressed rock in which most deep-focus earthquakes occur.

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