In situ X-ray and acoustic observations of deep seismic faulting upon phase transitions in mantle olivine

Activity of deep earthquakes, which increases with depth from ~400 km to a peak at ~600 km and abruptly decreases to zero at 680 km, is enigmatic, because brittle failure is unlikely to occur under the corresponding pressures of 13−24 GPa. It has been suggested that pressure-induced phase transitions of olivine in subducted slabs are responsible for occurrence of the deep earthquakes, based on deformation experiments under pressure. However, most experiments were made using analogue materials of mantle olivine and at pressures below ~5 GPa, which are not applicable directly to the actual slabs. Here we report the results of deformation experiments combined with in situ X-ray observations and acoustic emission measurements on (Mg,Fe)2SiO4 olivine at 11−17 GPa and 860−1250 K, equivalent to the conditions of colder regions of the slabs subducted into the mantle transition region. We find that faulting occurs only at very limited temperatures of 1100−1160 K, accompanied by intense acoustic emissions from both inside and outside of the sample, immediately before the rupture. The formation of lenticular packets filled with nanocrystalline olivine and wadsleyite is confirmed in the recovered sample without faulting, indicating that the faulting is caused by adiabatic shear heating along the weak layer of the connected lenticular packets, where nanocrystalline olivine plays important roles. Our study suggests that the transformational faulting occurs on the isothermal surface of the metastable olivine wedge in subducted slabs, leading to deep earthquakes in limited regions and depth range.