Seismicity decreases with depth because elevated confining pressure prevents frictional sliding of faults. However, seismicity tends to increase with depth in the mantle transition zone (depths of 410−600km). It has been believed that pressure-induced phase transitions of olivine in the ‘cold’ subducted slabs is the cause of high seismicity in the mantle transition zone.
The mechanism of deep earthquake has been studied for four decades. It has been considered that the pressure-induced phase transition of olivine induces shear localization to spinel-filled lens, followed by a deep earthquake. This ‘transformational faulting’ model has been experimentally tested by researchers using germanite olivine or mantle olivine. However, the olivine-ringwoodite transition is too sluggish to induce deep earthquakes in the cold core of the deep subducted slabs (600ºC), such as the Mariana slab, if we assume the diffusion-controlled nucleation of ringwoodite on olivine grain boundaries. A plausible explanation for the olivine-ringwoodite transition in the cold deep slabs may be the diffusionless pseudo-martensitic transition (i.e., shear transition) of olivine to ringwoodite. Recent studies showed that an intermediate structure of poirierite needs to be formed when the diffusionless shear transition of olivine-ringwoodite proceeds.
We conducted deformation experiments on metastable olivine under the pressure-temperature conditions of deep subducted slabs. We carefully observed the faulted olivine samples, which were obtained in our experiments, and we found poirierite grains in the fault gouge. The observed crystallographic orientations of poirierite and olivine grains are consistent with a theoretical model. The poirierite grains transform to ringwoodite as a result of shear deformation. Release of quite high latent heat via the poirierite-ringwoodite transition can induce a significant weakening of the fault gouge, without the aid of grain-size-sensitive creep, resulting in the occurrence of faulting. The diffusionless phase transition of olivine to ringwoodite via poirierite is effective not only at high temperatures but at low temperatures. Our findings provide a natural explanation for the cause of high seismicity in the strongly deformed areas of ‘cold’ deep subducted slabs.