The problem
Earthquakes release energy within subducted plates roughly 300–700 kilometres deep, where pressure suppresses ordinary brittle fracture. What lets a slow-deforming rock switch to rapid rupture?
Olivine in a cold slab can retain a crystal structure that is no longer the favored one. In a 2022 model, deformation promotes a phase transformation to a denser structure within a shear band. Transformation, further deformation and heating reinforce one another, accelerating the localized motion. This is a proposed feedback mechanism, not a direct observation of a natural rupture.[1] [2]
Scope
Mechanisms of deep-focus earthquakes at roughly 300–700 km in subducted slabs.
What remains open
A mineral changing its crystal structure could help concentrate deformation into a narrow zone. Models explain possible routes, but they do not identify one mechanism for all deep earthquakes.[2]
The model must explain rapid initiation and continued rupture, not just a change of mineral structure. A 2025 analysis of seven deep events beneath northeast China connects differences in seismic signals with a modeled cold, bent slab. Experiments reported in 2026 support nearly dry transformation conditions in cold slab interiors. Regional models and laboratory conditions constrain candidates without uniquely identifying every event’s mechanism.[1] [2] [3]
Why it matters
These events challenge the usual division between sudden brittle failure and slow rock flow. They let us test how a deeply buried slab can deform abruptly.[2]
Definitions
3 terms
olivine
A major mineral in upper-mantle rocks. In sufficiently cold sinking slabs, its original crystal structure can persist to depths where a denser structure is favored.[1] [2]
phase transformation
A change in a material’s crystal structure. In the cited deep-earthquake model, olivine transforms to a denser structure and the change couples to deformation and heating.[2]
shear band
A narrow region where deformation concentrates as neighboring material moves past one another. The cited model couples transformation and heating within such a band.[2]
Sources
- Deep-Focus Earthquakes Under Northeast China—An Imprint of the Complex Tectonic History of Pacific Plate Subduction
Notes on Deep-Focus Earthquakes Under Northeast China—An Imprint of the Complex Tectonic History of Pacific Plate Subduction
Slab models and earthquake observations examine transformational faulting as a candidate explanation.
- Resolving puzzles of the phase-transformation-based mechanism of the strong deep-focus earthquake
Notes on Resolving puzzles of the phase-transformation-based mechanism of the strong deep-focus earthquake
A quantitative transformation model addresses several difficulties but does not identify every natural deep earthquake.
- Water exchange process and bulk composition regulate slab dynamics and deep earthquakes
Notes on Water exchange process and bulk composition regulate slab dynamics and deep earthquakes
High-pressure experiments test mineral water transfer and propose dry transformational faulting in cold slab interiors.