Deep-focus earthquakes (DFEs) present an interesting scientific challenge in that they occur at depths where brittle failure should be impossible. The fact that their occurrence is confined to locations where subducting lithospheric slabs are crossing through the transition zone suggests that olivine phase transformations may be involved in the production of these earthquakes. Experimental studies have shown that olivine can persist metastably in subducting slabs and that olivine phase transformations can lead to faulting at high pressures. However, it has been argued that large DFEs are too large to be contained within a metastable olivine wedge preserved in the interior of subducting slabs. We demonstrate, using experiments on olivine-analog materials, that transformational faulting can continue to propagate via shear-enhanced melting into the stable high-pressure phase. We also show that transformational faulting is controlled by the ratio between strain rates and the olivine-ringwoodite transformation rates, and extrapolate this relationship to the natural conditions of DFEs. Counterintuitively, these results imply that cold and fast-subducting slabs produce transformational faulting at higher temperatures, which results in more numerous DFEs.

As it descends into the Earth’s mantle, the olivine that constitutes the lithosphere of subducting slabs transforms to its high-pressure polymorphs, wadsleyite and ringwoodite, in the so-called transition zone. These transformations have important rheological consequences, since they may induce weakening, strain localization, and, in some cases, earthquakes. In this study, germanium olivine (Ge-olivine) was used as an analogue material to investigate the rheology of samples undergoing the olivine–ringwoodite transformation. Ge-olivine adopts a ringwoodite structure at pressures ~14 GPa lower than its silicate counterpart does, making the transformation accessible with a Griggs rig. Deformation experiments were carried out in a new-generation Griggs apparatus, where micro-seismicity was recorded in the form of acoustic emissions. A careful analysis of the obtained acoustic signal, combined with an extensive microstructure analysis of the recovered samples, provided major insights into the interplay between transformation and deformation mechanisms. The results show that significant reaction rates cause a weakening via the implementation of ductile shear zones that can be preceded by small brittle precursors. When kinetics are more sluggish, mechanical instabilities lead to transformational faulting, which stems from the unstable propagation of shear bands localizing both strain and transformation. The growth of these shear bands is self-sustained thanks to the negative volume change and the exothermic nature of the reaction, and leads to dynamic rupture, as attested by the acoustic emissions recorded. These micro-earthquakes share striking similarities with deep focus earthquakes, which may explain several seismological observations such as magnitude frequency relations and the occurrence of deep repeating earthquakes and foreshocks.