Knowing the elasticity of ferropericlase across the spin transition can help explain seismic and mineralogical models of the lower-mantle including the origin of seismic heterogeneities in the middle to lowermost parts of the lower mantle1234. However, the effects of spin transition on full elastic constants of ferropericlase remain experimentally controversial due to technical challenges in directly measuring sound velocities under lower-mantle conditions12345. Here we have reliably measured both VP and VS of a single-crystal ferropericlase ((Mg0.92,Fe0.08)O) using complementary Brillouin Light Scattering and Impulsive Stimulated Light Scattering coupled with a diamond anvil cell up to 96 GPa. The derived elastic constants show drastically softened C11 and C12 within the spin transition at 40–60 GPa while C44 is not affected. The spin transition is associated with a significant reduction of the aggregate VP/VS via the aggregate VP softening because VS softening does not visibly occur within the transition. Based on thermoelastic modelling along an expected geotherm, the spin crossover in ferropericlase can contribute to 2% reduction in VP/VS in a pyrolite mineralogical model in mid lower-mantle. Our results imply that the middle to lowermost parts of the lower-mantle would exhibit enhanced seismic heterogeneities due to the occurrence of the mixed-spin and low-spin ferropericlase.

Intermediate‐depth earthquakes (focal depths 70–300 km) are enigmatic with respect to their nucleation and rupture mechanism and the properties controlling their spatial distribution. Several recent studies have shown a link between intermediate‐depth earthquakes and the thermal‐petrological path of subducting slabs in relation to the stability field of hydrous minerals. Here we investigate whether the structural characteristics of incoming plates can be correlated with the intermediate‐depth seismicity rate. We quantify the structural characteristics of 17 incoming plates by estimating the maximum fault throw of bending‐related faults. Maximum fault throw exhibits a statistically significant correlation with the seismicity rate. We suggest that the correlation between fault throw and intermediate‐depth seismicity rate indicates the role of hydration of the incoming plate, with larger faults reflecting increased damage, greater fluid circulation, and thus more extensive slab hydration.

We perform numerical experiments of damped quasi-dynamic fault slip that include a rate-and-state behavior at steady state to simulate earthquakes and a plastic rheology to model permanent strain. The model shear zone has a finite width which represents a natural fault zone. Here we reproduce fast and slow events that follow theoretical and observational scaling relationships for earthquakes and slow slip events (SSEs). We show that the transition between fast and slow slip occurs when the friction drop in the shear zone is equal to a critical value, Δμc. With lower friction drops, SSEs use nearly all of mechanical work to accumulate inelastic strain, while with higher friction drops fast slips use some of the mechanical work to slip frictionally. Our new formulation replaces the state evolution of rate and state by the stress evolution concurrent with accumulation of permanent damage in and around a fault zone.

Geological observations show that fault zone composition varies and often accommodates a mixture of brittle and ductile deformation. There is growing evidence that the nature of this mixture may play an important role in determining whether the fault creeps steadily or slides in slow slip events (SSEs) and/or fast earthquakes. Using numerical experiments of slip events in a fault zone of finite thickness, we explore how the ratio of brittle to ductile material and the absolute friction change resulting from a variation in slip velocity, |b−a|, affect energy partitioning and slip behavior in brittle‐ductile mixtures. We treat brittle material as Mohr‐Coulomb elastoplastic and ductile material as Maxwell viscoelastic. We simulate velocity‐weakening (a−b<0) behavior in the brittle part of the mixture and velocity‐strengthening (a−b≥0) behavior in the ductile part using a rate‐and‐state formulation dependent on plastic strain accumulation. We show that: (1) mixtures can exhibit multiple slip behaviors including earthquakes and slow slip, (2) highly brittle mixtures do not tend to generate SSEs while weakly brittle mixtures can generate slow slip over a wider range of compositions, (3) structural features formed during simulated creep, SSEs, and earthquakes share notable similarities with structures observed in natural fault zones. We find that slip‐synchronous strengthening in the ductile portion of the mixture controls whether a rupture propagates as SSEs of yearlong durations. Shorter duration SSEs occur when the length of the plastic shear segments formed during slip is similar to the characteristic weakening distance for an earthquake.

In electrocatalytic hydrolysis, the oxygen evolution reaction (OER) reaction involves a four-electron transfer process. The complex transfer process reduces the rate of hydrolysis. Therefore, the electrocatalyst with good OER performance is desirable for not only fundamental research but also further application. Transitionmetal electrocatalysts, as one of the alternatives to noble-metal catalysts, have abundant reserves and unique d orbital electrons. In particular, transition-metal molybdates undergo dynamic reconstruction at oxidation potentials, and the hydroxyl oxides formed after reconstruction are the main active species for oxygen-related reactions. In this work, we prepared self-supported Fe-doped NiMoO 4 •nH 2 O@NiOOH electrocatalysts by hydrothermal reaction and electrochemical oxidation. Porous NiOOH was generated on the surface of NiMoO 4 •nH 2 O by electrooxidation, and Fe doping was realized in this process. The porous structure of the surface is conducive to the penetration of the electrolyte, which can accelerate the ion transport rate. The doping of Fe was used to modulate the electronic structure and improve the electrocatalytic activity. The overpotential was only 227 mV at 10 mA/cm 2 in the 1 M KOH electrolyte. In addition, the electrocatalyst exhibited high stability at a current density of 20 mA/cm 2 .