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.

Deep seismic faulting triggered by nano-crystallization of wadsleyite from 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 transformations 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 960−1250 K. We find that shear cracking followed by rapid formation of nano-crystalline wadsleyite on the crack surface is essential for the occurrence of faulting, which is observed only at temperatures around 1160 K. The faulting is accompanied by intense acoustic emissions and partial melting, which is likely to be induced by rapid sliding and adiabatic shear heating along the weak layer of nano-crystalline wadsleyite. In contrast, the olivine to ringwoodite transformation in (Mg,Fe)2SiO4 olivine would not cause such faulting because of the slow diffusion creep of ultrafine-grained ringwoodite. Our findings suggest the transformational faulting occurs on the surface of the metastable olivine wedge in subducted slabs, leading to deep earthquakes in the limited depth range.

Spatiotemporal Seismotectonic Implications for the Izu–Bonin–Mariana Subduction Zone from b-Values

Studies on the physical properties of the entire Izu–Bonin–Mariana (IBM) subduction zone contribute to comprehensive seismotectonic understanding and earthquake potential assessment, especially given previous controversial conclusions. Determining seismic b-value is a method that has been used for other regions and is adopted here to study the spatiotemporal variations along the IBM system. Based on the frequency–magnitude distribution relation log10(N)=a−bM, b-values are mapped within the subduction zone using earthquakes with Mw≥2 after 2005. The b-value anomalies in cross sections indicate detailed seismotectonic characteristics against the regional geological background. The common characteristics from north to south: (1) regional high b-values at shallow depths in the overriding are associated with relatively low temperatures in thermal model, the bottom half of which correspond with highly serpentinized mantle wedge; and (2) low b-values at intermediate depths are associated with high temperatures along the primarily heated hydrated slab. In the Izu–Bonin segment, low b-values around the slab deflection at deep depths respond to stress buildup and shearing instability of metastable olivine in primarily heated hydrated slabs. In the Mariana segment, high b-values beneath the volcanic region at depths from the surface to 50 km and between 50 and 100 km are associated with extension and volcanism and the melting region, respectively. Temporal b-value variations indicate regional changes before and after large events for further seismic risk analysis. Stress drops of large intermediate and deep earthquakes are negligible to local stress state in strong flexure of the incoming slab. The rupture zone around the Pagan region at an approximate depth of 200 km and the region around the rifting–spreading transition in the northern Mariana trough at depths between 180 and 350 km are areas for potential large earthquakes.

A Metastable Fo-III Wedge in Cold Slabs Subducted to the Lower Part of the Mantle Transition Zone: A Hypothesis Based on First-Principles Simulations

The metastable olivine (Ol) wedge hypothesis assumes that Ol may exist as a metastable phase at the P conditions of the mantle transition zone (MTZ) and even deeper regions due to inhibition of the phase transitions from Ol to wadsleyite and ringwoodite caused by low T in the cold subducting slabs. It is commonly invoked to account for the stagnation of the descending slabs, deep focus earthquakes and other geophysical observations. In the last few years, several new structures with the forsterite (Fo) composition, namely Fo-II, Fo-III and Fo-IV, were either experimentally observed or theoretically predicted at very low T conditions. They may have important impacts on the metastable Ol wedge hypothesis. By performing first-principles calculations, we have systematically examined their crystallographic characteristics, elastic properties and dynamic stabilities from 0 to 100 GPa, and identified the Fo-III phase as the most likely metastable phase to occur in the cold slabs subducted to the depths equivalent to the lower part of the MTZ (below the ~600 km depth) and even the lower mantle. As disclosed by our theoretical simulations, the Fo-III phase is a post-spinel phase (space group Cmc21), has all cations in sixfold coordination at P < ~60 GPa, and shows dynamic stability for the entire P range from 0 to 100 GPa. Further, our static enthalpy calculations have suggested that the Fo-III phase may directly form from the Fo material at ~22 GPa (0 K), and our high-T phase relation calculations have located the Fo/Fo-III phase boundary at ~23.75 GPa (room T) with an averaged Clapeyron slope of ~−1.1 MPa/K for the T interval from 300 to 1800 K. All these calculated phase transition pressures are likely overestimated by ~3 GPa because of the GGA method used in this study. The discrepancy between our predicted phase transition P and the experimental observation (~58 GPa at 300 K) can be explained by slow reaction rate and short experimental durations. Taking into account the P-T conditions in the cold downgoing slabs, we therefore propose that the Fo-III phase, rather than the Ol, highly possibly occurs as the metastable phase in the cold slabs subducted to the P conditions of the lower part of the MTZ (below the ~600 km depth) and even the lower mantle. In addition, our calculation has showed that the Fo-III phase has higher bulk seismic velocity, and thus may make important contributions to the high seismic speeds observed in the cold slabs stagnated near the upper mantle-lower mantle boundary. Future seismic studies may discriminate the effects of the Fo-III phase and the low T. Surprisingly, the Fo-III phase will speed up, rather than slow down, the subducting process of the cold slabs, if it metastably forms from the Ol. In general, the Fo-III phase has a higher density than the warm MTZ, but has a lower density than the lower mantle, as suggested by our calculations.