Alexandre Schubnel

Transformational faulting in olivine has been widely invoked as a possible triggering mechanism for deep‐focus earthquakes, yet its dynamics remain debated. It is unclear whether it requires the combined effects of heat release, volume change and mechanical strength contrast, or whether it can be triggered by any one of these alone. Here, we tested if transformational faulting can be caused solely by a large volume change, through deformation experiments on novaculite during the quartz‐coesite transition. Under sluggish reaction kinetics, acoustic emissions and microstructural observations indicate strain localization and fault plane formation. Under fast kinetics, deformation was distributed and associated with fewer, lower‐magnitude events. b ‐values of ∼1 and ∼3 reflect these two regimes. Our results provide the first evidence of transformational faulting during the quartz‐coesite transition, suggesting that volume change is the primary driver of brittle failure at high pressure, broadening the range of mineral transitions generating deep and intermediate‐depth earthquakes.

Tidal stress is a globally acting perturbation driven primarily by the gravitational forces of the Moon and the Sun. Understanding how tidal stresses can trigger seismic events is essential for constraining tectonic environments that are sensitive to small stress perturbations. Here, employing a spring–block model with rate‐and‐state friction, we investigate tidal triggering on velocity‐weakening stable sliding faults with stiffness slightly exceeding the critical stiffness. We first apply a step and a boxcar with finite duration normal stress perturbation to demonstrate a resonance‐like amplification of slip velocity for specific boxcar durations. Next, we perform nondimensional analyses and numerical simulations with harmonic perturbations to identify the key parameters controlling tidal triggering and their admissible ranges. Triggered slip events are further characterized using physically observable quantities, including radiation efficiency and tidal phase. Our results show that even small stress perturbations can trigger periodic as well as temporally complex slip events on stable sliding faults. The triggering behavior is primarily controlled by the normalized perturbation period and the normalized perturbation amplitude. An increase in the normalized period shifts event timing from the peak of tidal stress toward the peak of stress rate, whereas increasing the normalized amplitude promotes a transition from slow to fast events. This framework helps explain the period‐dependent sensitivity and the observed phase preference between tidal stress and maximum slip velocity. Comparison between observed and model‐predicted tidal correlation patterns may therefore help constrain the instantaneous frictional strength of the interface, as well as the characteristic slip distance for frictional weakening.

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The interplay between the strength of crystalline rocks and fluid diffusion is fundamental to the dynamics of the crust and that of fault systems. We measured the strength of thermally cracked Westerly granite as a function of strain‐rate, confining pressure and fluid pressure. Experiments were conducted at the same initial pore fluid pressure with either nominally drained or undrained boundary conditions. The samples internal pore pressure was measured with pressure transducers attached to the surface of the samples. A systematic decrease in the internal pore pressure was observed when approaching peak stress, leading to an increase in the effective stress and strength of the samples. Although peak brittle strength increased with strain‐rate, it remained consistent with a simple Mohr‐Coulomb failure criterion when calculating the effective stress from the internal pore pressure. Performing additional experiments with undrained boundary conditions revealed that the Mohr‐Coulomb envelope could be determined from a single experiment, demonstrated by the stress trajectory being almost tangential to the failure criterion during the final stage of the experiment. Finally, we demonstrate that the onset of dilatant strengthening (and pore pressure drop) was reached for a critical volumetric (dilatant) strain rate. Because of the competition between fluid diffusion and dilatancy, this critical volumetric strain‐rate, derived through dimensionless analysis, is proportional to permeability, and consequently, exhibits pressure dependence. Overall, our results are summarized with a simple conceptual model, where the brittle strength is bounded by the drained (lower) and undrained (upper) limits, the transition between both being permeability and strain‐rate dependent.

The entire editorial board of the Journal of Geophysical Research‐Solid Earth would like to sincerely thank all our colleagues who reviewed manuscripts for us in 2025. The hours they spent reading in order to provide insightful comments on manuscripts not only help improve the quality of these manuscripts but also ensure the scientific rigor of our reviewing process and eventually, of the research published in the field of Solid Earth Geophysics by our journal. With the advent of open science and AGU's data policy, the reviewing process now also encompasses checking the accessibility and availability of data and developed software. This is a key objective of AGU's FAIR (Findable, Accessible, Interoperable and Reusable) policy, for which many reviewers have provided suggestions that helped to improve the data presentation and availability, and which also fed the editorial board's reflection on the matter. Of course, we particularly appreciate timely reviews, particularly in light of the growing demands imposed by the increase of manuscripts submitted to Journal of Geophysical Research‐Solid Earth . We received 2,090 submissions in 2025, and 1,789 reviewers contributed to their evaluation by providing 3,016 reviews in total. We are deeply thankful for all of their contributions.

The origin of tectonic tremors, low-amplitude and long-duration seismic signals, observed at depth greater than 20-30km at some of the world’s tectonic plate boundaries, remains enigmatic. Here, olivine + antigorite mineral assemblages, containing up to 75 vol.% of the hydrous bearing phase, thought as analogues for dry and water-rich subducting lithologies, were experimentally compressed along pressure-temperature (P-T) paths typical of hot subduction zones. During all experiments, ultrasonic acoustic monitoring of the compression was performed. At PT conditions below 1GPa and 500°C, earthquake-like signals were recorded, with a peak activity at 0.5GPa and 250°, ie. conditions coresponding to the base of the megathrust . Above these PT conditions, spectral analysis (corner frequency, stress drop, duration vs. moment) revealed that the recorded acoustic emissions (AE) signals shared striking similarities with natural tectonic tremors. In particular, stress drops of few kPa and linear moment release vs. duration scalings were observed. While nominally dry experiments confirmed that these tremor-like AEs originated from the viscous deformation of the dry matrix, hydrous mineral bearing experiments demonstrated that experimental tremors could be triggered in bursts at the onset of dehydration reactions, probaly via a mechanism compatible with dehydration stress transfer.

In fluid-rich faults, thermal pressurization (TP) is theoretically predicted to induce rapid fault weakening and facilitate large earthquakes, whereas dilatant strengthening (DS) can counteract this process through pore-space expansion. This study experimentally investigates the relative efficiency and condition of TP and DS using triaxial stick-slip tests with on-fault pore pressure (Pp) measurements. Dynamic stick-slip events were generated on four saturated, saw-cut, thermally cracked Westerly granite samples under varying effective confining (30–60 MPa) and pore pressures (25–45 MPa). Results reveal a systematic rupture transition from co-seismic Pp rise (TP-type) at low shear stress to Pp drop (DS-type) at higher shear stress, accompanied by a coupled fast–slow spectrum: fast TP → slow TP → slow DS → fast DS. Fast events reach slip velocities up to three orders of magnitude higher (0.5–10 mm s-¹) than slow ones (0.001–0.5 mm s-¹). The TP model under undrained, adiabatic conditions reproduces the measured Pp evolution, indicating a progressive shear-zone widening (0.05–0.5 mm, as overestimated values) that reduces TP efficiency and promotes slow events. For DS sequences, an increasing dilatancy coefficient is inferred, consistent with enhanced Pp drops. Breakdown energy shows no clear difference between TP and DS events, suggesting similar rupture energetics despite opposite pore-pressure evolution. Overall, this study provides the first direct experimental evidence of TP–DS transitions, demonstrating that TP governs early-stage weakening but diminishes as the shear zone widens, allowing DS to dominate. These results imply that in mature fault zones, after several seismic cycles, fault weakening may be mainly governed by co-seismic dilatancy, although strong, fast ruptures can still occur when the dilatant strengthening is not sufficient to stop the on-going rupture.

Tidal stress is a periodic stress acting globally on the Earth, driven primarily bythe gravitational forcing of the Moon and the Sun. Understanding how tidalstress can trigger seismic events is essential for constraining tectonicenvironments that are sensitive to small, periodic stress perturbations.Here we investigate tidal triggering on stable sliding, velocity-weakening (VW)rate-and-state frictional (RSF) faults using a spring-block framework. We firstapply idealized step-like and boxcar normal stress perturbations todemonstrate a resonance-like amplification of slip rate when the perturbationduration approaches the intrinsic RSF time scale. Building on this observation,we perform non-dimensional analyses and numerical simulations withsinusoidal tidal-like perturbations to identify the key parameters controllingtidal triggering and their admissible ranges. We further characterize thetriggered events through observable quantities, including radiation efficiencyand tidal phase. Our results show that resonance effects allow tidal stress to trigger bothregular periodic and complex temporal slip events on otherwise stable slidingVW faults. The triggering behavior is primarily controlled by two non-dimensional parameters: the normalized perturbation period and thenormalized perturbation amplitude. Increasing the normalized period shiftsevent timing from peak tidal stress toward the maximum stress rate, whereasincreasing the normalized amplitude promotes a transition from slow to fastslip events. The parameter space permitting tidally triggered slip eventssuggests that the RSF parameter,$a\sigma$, which characterizes theinstantaneous frictional strength of an interface, should not exceed tens tohundreds of kilopascals, and that the characteristic slip distance for frictionalweakening is likely on the order of micrometers.

Shallow tectonic tremors along the northeastern Japan subduction zone show regional differences in their spatiotemporal evolution, raising the question of whether their response to tidal stressing also varies along strike. We analyze the tremor catalogue obtained by Sagae et al. (JGR, 2025, e2025JB031348) for the period from August 2016 to August 2024. Based on their spatial distribution, tremor activity can be divided into three major regions: the southern end of the Kuril Trench (40.8–42°N; northern region), the northern Japan Trench (38.8–40.5°N; central region), and the southern Japan Trench (35–36.8°N; southern region). Here, we investigate the tidal sensitivity of tectonic tremors in these three regions. Our statistical analysis shows that tidal sensitivity is highest in the northern area, where tremors are clustered and occur in recurrent along-strike propagating bursts. Cluster-scale analyses in this northern region indicate that tidal sensitivity increases during the later stages of tremor clusters, consistent with the characteristics reported for deep tectonic tremors. Tidal sensitivity is intermediate in the southern area, where tremors appear more scattered. In the central region, where tremor activity has declined gradually since 2011 Mw 9.0 Tohoku-Oki earthquake, tidal sensitivity is lowest. In this region however, tremors and fast earthquakes occur in close spatial proximity. There, we further examine the relationship between tremor activity, fast earthquakes and tidal stress to explore potential interactions between slow and fast earthquakes.