Earthquakes with focal depths greater than 300 kilometres are classed as deep-focus earthquakes, and scientists have no settled explanation for how they occur.

Is it true that earthquakes with focal depths greater than 300 kilometres are classed as deep-focus earthquakes, and scientists have no settled explanation for how they occur?

Yes. A Wikipedia reference states that the physical mechanism of deep-focus earthquakes is poorly understood and remains an outstanding problem, and that subducted lithosphere below 300 km should deform plastically rather than brittlely. USGS and a 1956 peer-reviewed study confirm the 300 km boundary for deep earthquakes, while other papers propose candidate mechanisms without settling the question.

Verdict: SUPPORTED · 5 sources for, 0 against · Last checked 2 Oct 2026

The private library was unavailable for some searches in this check. Missing library results are not evidence against the claim.

Deep-focus earthquake - Wikipedia. (n.d.). en.wikipedia.org. https://en.wikipedia.org/wiki/Deep-focus_earthquake

The energy of earthquakes. (n.d.). cir.nii.ac.jp. https://doi.org/10.1144/gsl.jgs.1956.112.01-04.02

Determining the Depth of an Earthquake | U.S. Geological Survey. (n.d.). usgs.gov. https://www.usgs.gov/programs/earthquake-hazards/determining-depth-earthquake

Distribution of stresses in the descending lithosphere from a global survey of focal‐mechanism solutions of mantle earthquakes. (n.d.). cir.nii.ac.jp. https://doi.org/10.1029/rg009i001p00103

Earthquake generating stresses in a descending slab. (n.d.). cir.nii.ac.jp. https://doi.org/10.1016/0040-1951(85)90175-1