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Slow-slip events are earthquake-like events only with much lower slip rates. While peak coseismic velocities can reach tens of meters per second, slow-slip is on the order of 10 −7±2 m/s and may last for days to weeks. Under the rate-and-state model of fault friction, slow-slip is produced only when the asperity size is commensurate with the critical nucleation size, a function of frictional properties. However, it is unlikely that all subduction zones embody the same frictional properties. In addition to friction, plastic flow of antigorite-rich serpentinite may significantly influence the dynamics of fault slip near the mantle wedge corner. Here, we show that the range of frictional parameters that generate slow slip is widened in the presence of a serpentinized layer along the subduction plate interface. We observe increased stability and damping of fast ruptures in a semi-brittle fault zone governed by both brittle and viscoelastic constitutive response. The rate of viscous serpentinite flow, governed by dislocation creep, is enhanced by high ambient temperatures. When effective viscosity is taken to be dynamic, long-term slow slip events spontaneously emerge. Integration of rheology, thermal effects, and other microphysical processes with rate-and-state friction may yield further insight into the phenomenology of slow slip.

The constitutive behavior of faults intervenes in virtually every aspect of the seismic phenomenon but is poorly understood, particularly regarding how effective normal stress affects the boundaries of the seismogenic zone. Here, we explore the mechanical properties of Pelona schist, Westerly granite, phyllosilicate‐rich gouge, gabbro, hornblende, lawsonite blueschist, montmorillonite, and smectite in hydrothermal conditions at various confining pressures and explain the laboratory observations with a physical model of fault friction. The thermobaric activation of healing and deformation mechanisms explains the boundaries of unstable slip as a function of slip‐rate, temperature, and effective normal stress for a given lithology. The constitutive law affords extrapolation of laboratory data in the conditions relevant to seismic cycles throughout the crust, explaining the focus of large earthquakes in collision, subduction, and continental and oceanic transform settings. Plain Language Summary An important goal of earthquake physics involves predicting the failure of rocks under the various physical conditions encountered during the seismic cycle. Here, we analyze mechanical data for Pelona schist, Westerly granite, phyllosilicate‐rich gouge, gabbro, hornblende, lawsonite blueschist, montmorillonite, and smectite that reveal how normal stress, temperature, and slip‐rate affect the frictional properties of rocks. We capture these effects consistently at constant coefficients with a physics‐based constitutive friction law. The boundaries of the seismogenic zone follow a thermobaric activation, whereby the transition temperature is a function of pressure. Increasing confining pressure may induce or inhibit velocity‐weakening behavior, depending on the constitutive properties controlling the healing and deformation mechanisms. The constitutive model provides an increasingly realistic representation of fault behavior during seismic cycles applicable to a wide range of tectonic contexts. Key Points The temperature boundaries of the seismogenic zone depend on confining pressure, implying a thermobaric activation of fault friction The model explains schist, granite, gabbro, hornblende, clays, and natural gouge friction evolution with velocity, temperature, and pressure The constitutive model provides a realistic representation of fault behavior during seismic cycles applicable to all tectonic contexts

The role of upper‐plate faulting in the seismic cycle of large megathrust earthquakes remains poorly understood. We use quasi‐dynamic numerical simulations of seismic cycles to analyze the interaction between crustal faulting and the foreshock sequence of the 2014 Iquique (Mw 8.2) earthquake in Northern Chile. Multi‐cycle models incorporating upper‐plate faulting align better with coseismic displacements, replicating events akin to the Iquique earthquake. Upper‐plate faulting significantly influences foreshock seismicity and deformation patterns. By calibrating the average hydraulic state—varying the effective normal stress—along the megathrust with pre‐earthquake seismicity, we find that lower pore pressure ratios result in more seismicity before the mainshock. This implies that the hydraulic state of the megathrust is critical for foreshock activity. This comprehensive modeling approach underscores the importance of the mechanical interplay between the megathrust and upper‐plate faults in precursory sequences of large subduction zone earthquakes. Plain Language Summary Active upper‐plate faults in subduction zones worldwide have shown activity before, during, and after large earthquakes, suggesting a link with megathrust behavior. Advancing our understanding of these mechanisms is essential for enhancing seismic hazard assessment. The 2014 Iquique earthquake (Mw 8.2) in Northern Chile is probably the best‐documented earthquake in terms of pre‐earthquake upper‐plate activity, characterized by an intense foreshock sequence 15 days before the main event. Here, we use numerical simulations of the seismic cycle over several thousand years to analyze the interaction between crustal faulting and foreshocks preceding events like the 2014 Iquique earthquake. Upper‐plate faults in our models significantly influence the seismic activity prior to the mainshock and are key to explaining the geodetic observations of coseismic displacements. Additionally, we find that the hydraulic state at the plate interface strongly affects foreshock activity. This research highlights the importance of fault interactions and fluid dynamics in the seismic cycles of subduction zones. Key Points Offshore upper‐plate faulting controls precursory activity of partial megathrust ruptures in Chile The hydraulic state at the plate interface impacts precursory seismic activity before megathrust earthquakes

The forthcoming SWOT altimetric missions aim to resolve the mesoscale with an unprecedented spatial resolution and swath. However, high-frequency processes, such as tides, are undersampled in time and aliased onto lower frequencies, so they need to be corrected properly. Unlike barotropic tides, internal tides (ITs) are not completely stationary and have significant temporal variability due to their interactions with the ocean circulation and the stratification variability. Stratification changes impact both the generation and the propagation of ITs. The present study proposes a methodology to quantify the impacts of background stratification using a clustering method for the classification of a broad range of stratification and idealized modeling of ITs in the frequency domain. The methodology is successfully tested in the western equatorial Atlantic and in the Bay of Biscay. For the western equatorial Atlantic, a single pycnocline is observed and only the two first vertical modes of ITs have significant amplitudes. With no variation in the stratification intensity, the variation in the depth of this single pycnocline linearly impacts the elevation amplitude, energy fluxes and surface wavelength of the two modes. In the Bay of Biscay, there is a permanent deep pycnocline and secondary seasonal pycnoclines near the surface. No proxy have been found to describe the changes in ITs, so a seasonal climatology is explored. The seasonality of the stratification strongly affects the elevation amplitudes as well as the energy fluxes of modes 1, 2 and 3. The distribution of the modes vary with the background stratification, changing the horizontal scales of the ITs.

The Amazon shelf break is a key region for internal tide (IT) generation. It also shows a large seasonal variation in circulation and associated stratification. This study, based on a high-resolution model (1/36∘) explicitly forced by tide, aims to better characterize how the ITs vary between two contrasted seasons. During the season from March to July (MAMJJ) the currents and mesoscale eddies are weak while the pycnocline is shallower and stronger. From August to December (ASOND) mean currents and mesoscale eddies are strong, and the pycnocline is deeper and weaker than in MAMJJ. For both seasons, semi-diurnal M2 ITs are generated on the shelf break mainly between the 100 and 1000 m isobath in the model. South of 2∘ N, the conversion from barotropic to baroclinic tide is more efficient in MAMJJ than in ASOND. Local dissipation of the coherent M2 at the generation sites is higher in MAMJJ (30 %) than in ASOND (22 %), because higher modes are favourably generated (modes 2 and 3), making the internal wave packet more dispersive. The remaining fraction (70 %–80 %) propagates away from the generation sites and mainly dissipates locally every ∼ 100 km, which corresponds to the mode 1 reflection beams. About 13 %, 30 %, and 40 % of the M2 coherent IT dissipates at the first, second, and third beams. M2 coherent baroclinic flux propagates more northward during MAMJJ while it seems to be blocked at 6∘ N during ASOND. There is no intensified dissipation of the coherent M2 that could explain the disappearance of the coherent flux. In fact, the flux at this location becomes more incoherent because of strong interaction with the currents. This has been shown in the paper using 25 h mean snapshots of the baroclinic flux that shows branching and stronger eastward deviation of the IT when interacting with mesoscale eddies and stratification during ASOND. Finally, we evaluated sea surface height (SSH) frequency and wavenumber spectra for subtidal (f<1/28 h−1), tidal (1/28 h−1 < f<1/11 h−1), and supertidal (f>1/11 h−1) frequencies. Tidal frequencies explain most of the SSH variability for wavelengths between 250 and 70 km. Below 70 km, the SSH is mainly incoherent and supertidal. The length scale at which the SSH becomes dominated by unbalanced (non-geostrophic) IT was estimated to be around 250 km. Our results highlight the complexity of correctly predicting IT SSH in order to better observe mesoscale and sub-mesoscale eddies from existing and upcoming altimetric missions, notably the Surface Water Ocean Topography (SWOT) mission.

The M2 MIOST24 (Multivariate Inversion of Ocean Surface Topography 2024) internal tide atlases described in this study are available in annual (MIOST24a) and monthly (MIOST24m) regional versions for modes 1 and 2 in the Indo-Philippine archipelago and the region off the Amazon shelf. They are derived from 25 years (1993–2017, period 1) of sea level anomalies (SLAs) from altimetry observations. Compared to the MIOST22 (Multivariate Inversion of Ocean Surface Topography 2022) internal tide atlases, MIOST24 incorporates M2 mode-1 and mode-2 wavelengths based on monthly stratification profiles from GLORYS12v1 (1993–2020). The differences between MIOST24a and MIOST22a lead to a RMSD (root mean square difference) of up to 3 cm, reflecting amplitude changes, while the RMSD between MIOST24a and the internal tide atlas HRET (High-Resolution Empirical Tide) indicates both amplitude and phase differences. MIOST24m highlights significant monthly variability of M2 internal tides in the Indo-Philippine archipelago and off the Amazon shelf. In the Amazon region, the internal tide propagates far offshore from March to June but is blocked closer to the coast from August to December. In both regions, the RMSDs between the MIOST24a annual and MIOST24m monthly mode-1 atlases are mainly governed by the phase contribution term. This suggests that the monthly variability of the mode-1 internal tide is more affected by changes in its propagation direction than by fluctuations in amplitude. In contrast, for mode 2, the monthly variability is predominantly associated with amplitude variations, particularly near internal tide generation sites. Variance reductions of SLAs show that MIOST24m outperforms MIOST24a, MIOST22a, and HRET in period 1 in the two regions of interest and in the Amazon region in period 2 (2018–2023). Monthly atlases are therefore recommended to correct the internal tides of the SLAs used to derive them. All these results support the development of MIOST24 global atlases.

Thanks to the autonomous Argo floats of the OUTPACE cruise (Oligotrophy to UlTra-oligotrophy PACific Experiment) and of the THOT (TaHitian Ocean Time series) project, some features of intermediate-flow dynamics, at around 1000 m depth, within the central and western South Pacific Ocean (around 19∘ S, 156∘ E–150∘ W) are described. In the Coral Sea, we highlight minima in dissolved oxygen of 140 µmol kg−1 that are associated with the signature of a southward transport of waters between two zonal jets: from the North Vanuatu Jet to the North Caledonia Jet. This transport takes place in the core of a cyclonic eddy or via the path between a cyclonic eddy and an anticyclonic one, highlighting the importance of mesoscale dynamics in upper thermocline and surface layers. Further east, we observe a strong meridional velocity shear with long-term float trajectories going either eastward or westward in the lower thermocline. More interestingly, these trajectories also exhibit some oscillatory features. Those trajectories can be explained by a single Rossby wave of 160-day duration and 855 km wavelength. Considering the thermohaline context, we confirm the meridional shear of zonal velocity and highlight a permanent density front that corresponds to the interface between Antarctic intermediate waters and North Pacific deep waters. Hence both circulation and thermohaline contexts are highly prone to instabilities and wave propagation.

Off the Amazon shelf, internal solitary waves (ISWs) generated by internal tides interact with mesoscale eddies, leading to significant modifications in their propagation and structure. For the first time, these interactions are directly observed from repeated high-resolution satellite measurements, notably those provided by the newly launched SWOT (Surface Water and Ocean Topography) mission. The study investigates ISWs detection and the modifications in ISW's characteristics resulting from interactions with eddies across three contrasting scenarios: ISW propagation in the absence of mesoscale eddies, ISW refraction by a cyclonic eddy, and ISW diffraction by an anticyclonic eddy. ISW crests are extracted using a band-pass filtering approach, enabling precise tracking of essential geometric and dynamical features, including propagation direction, crest-to-crest spacing, and wavefront shape. Prior to any interaction with eddies, three mode-1 ISW packets propagate in a steady and coherent manner, exhibiting planar wavefronts and consistent direction, with Absolute Dynamic Topography (ADT) amplitudes ranging from 8 to 14 cm. The results highlight the diversity of ISW responses depending on eddy conditions. In the absence of eddies, interaction with a seamount induces energy transfer from mode-1 to mode-3 ISWswaves (13 crests detected), while propagation direction is maintained and the ADT signature weakens to below 8 cm. In the presence of a cyclonic eddy above the seamount, ISW trajectories are refracted westward by roughly 50 degrees (7 crests detected), with increased wavecrest curvature and intensification of mode-3 generation, while ADT amplitudes decrease below 6 cm. Conversely, near the western boundary of an anticyclonic eddy, ISWs split into two distinct pathways: a western branch is refracted with flatter wavefronts, reduced crest spacing, and ADT amplitudes below 0.2 cm (9 crests detected), while an eastern branch follows the eddy's edge, displaying enhanced curvature, recognizable surface signatures of wave packets, and ADT values exceeding 9 cm (6 crests detected). Together, these findings demonstrate the ability of SWOT-based monitoring to capture the complexity of ISW dynamics and provide new insight into their nonlinear response to interactions with mesoscale and submesoscale oceanic structures.