Baptiste Rousset

The deformation regime that accommodates tectonic motion at plate boundaries changes as pressures and temperatures increase with depth, transitioning from the shallow frictional sliding of seismic ruptures to deep, viscous flow. We use recurring swarms of low-frequency earthquakes in this transition zone to measure the recurrence intervals and durations of accompanying slow slip across four plate boundaries. We find these time scales systematically decrease with depth and linearly scale with one another. Assuming a transition zone governed by episodic faulting, the observed time scales produce an average slow slip rate of 7 ± 2 mm/d across all four plate boundaries that is independent of depth. Thermal models place these slow slip dynamics within a common, narrow range of temperatures. Our results suggest deep slow slip is the result of the episodic unjamming of the plate boundary by frictional heterogeneities whose relaxation is modulated by surrounding temperature-dependent viscous material.

L'avènement de la géodésie spatiale ces dernières décennies a permis la découverte de la diversité des modes de glissement sur les failles. Dans cette thèse, nous nous intéressons au glissement asismique à la fois en contexte de subduction et en contexte décrochant. Nous étudions l'ensemble de la subduction mexicaine au niveau des régions de Guerrero et Oaxaca, où plusieurs séismes lents ont été répertoriés, ainsi que le segment en glissement asismique d'Ismetpasa le long de la faille décrochante Nord-Anatolienne en Turquie. La carte de couplage (estimé entre les grands séismes lents) entre les plaques Cocos et Nord Américaine indique un couplage élevé et relativement homogène en base de zone sismogénique. Cependant, des variations latérales de couplage importantes sont remarquables dans la zone sismogénique. En particulier, une zone de fort couplage à Oaxaca est localisée à l'emplacement de la rupture sismique de 1978 et a accumulé un déficit de glissement de 5 cm en 9 ans. Ces variations latérales de couplage sont corrélées avec les distances fosse-côte et des zones à l'état critiques sont localisées aux transitions entre faible et fort couplage. Ces observations suggèrent une pérennité au long-terme des motifs spatiaux de couplage. Nous proposons un mécanisme de déformation durant la phase co-sismique, basé sur l'existence de transitions frictionnelles sur le plan de subduction, qui permet de réconcilier les vitesses intersismiques mesurées sur quelques décennies et la morphologie long-terme. Ces vitesses intersismiques sont généralement considérées comme constantes sur plusieurs années. Cependant, une analyse plus fine montre une riche dynamique temporelle, avec en particulier la présence d'évènements transitoires finis de magnitudes et durées variables. Nous avons suivi deux approches afin de détecter et caractériser des glissements transitoires dans nos deux zones d'étude. (i) L'analyse d'un réseau dense d'interférogrammes, obtenus à partir de données InSAR des satellites CosmoSky-Med sur le segment de faille d'Ismetpasa, a permis de détecter un évènement de glissement transitoire d'un mois. Enregistré en Novembre 2013, aucun autre glissement n'a été détecté durant les 9 autres mois de l'analyse. Cet évènement d'une magnitude de 5.2 à 5.5 a relâché une énergie équivalant à 1,5 à 2 ans de glissement asismique continu à la vitesse moyenne précédemment estimée. Cette découverte remet en question le modèle mécanique de glissement de ce segment. (ii) Le développement d'une méthode de corrélation entre des évènements de glissements synthétiques et des séries temporelles GPS préalablement traitées permet la détection de glissements transitoires d'amplitude équivalente à celle du bruit. Appliquée à un jeu de données synthétiques sur la subduction mexicaine, cette méthode montre que l'on peut caractériser avec précision des évènements de Mw > 6. Appliquée à des données réelles au niveau de la lacune sismique de Guerrero entre Février 2005 et Mai 2007, cette méthode a permis de détecter 15 nouveaux événements transitoires. Ces évènements coïncident temporellement avec de fortes activités de trémors et LFEs et sont localisés aux pourtours du séisme lent de Mw 7.5 de 2006. Ces détections permettent de mieux contraindre la loi d'échelle des glissements lents et d'éclairer la dynamique spatiale et temporelles des évènements transitoires en base de zone sismogénique.

A better understanding of aseismic slip dynamics throughout the seismic cycle is essential to refine seismic hazard estimates. Analysis of the Interferometry Synthetic Aperture Radar (InSAR) time series in the last decades has proved its efficiency to detect and characterize slow slip events (SSE), especially on strike-slip segments. However, the implementation of automatic SSE detection methods is needed to overcome the large incoming flow of data. Here, we adapted the geodetic matched filter approach developed for GNSS time series by Rousset et al. (2017) to InSAR time series. The method is computing physics-based dislocation slip models corresponding to synthetic reconstructions of SSEs, that are correlated with InSAR time series, taking advantage of the high spatial density of InSAR observations. By comparing true and false detections on synthetic tests including InSAR realistic noise, we derive probabilistic estimates of the true detections as a function of SSE magnitudes and depths. We show that this method enables the detection with ≥ 90 % confidence of shallow SSEs with magnitudes larger than 4.5 using horizontal east-west InSAR time series. And it can detect events with magnitude larger than 4.25 with ≥ 45 % confidence. We applied this method along both creeping segments of the North Anatolian Fault - Izmit and Ismetpasa, by using the InSAR time series from 2016 to 2021 automatically processed in the framework of the FLATSIM project between CNES and FormaTerre by using Sentinel-1 SAR images and based on the NSBAS processing chain (Doin et al., 2011; Thollard et al., 2021). It detected without any prior knowledge three transient events already reported by previous studies along the Izmit segment (Aslan et al., 2019; Neyrinck et al., 2024), and two transient events also already reported by previous studies along the Ismetpasa one (Jolivet et al., 2023; Özdemir et al., 2025). Based on a weighted stacked time series associated with the detections, we estimate a magnitude for these events ranging from 4.0 to 5.0, also compatible with previous estimates. Applying this method on worldwide strike-slip fault segments may allow a rapid detection and a first order characterization of transient slip events.

The Main Marmara Fault lies at the western termination of the North Anatolian Fault. While the North Anatolian Fault ruptured from its eastern termination to the Izmit segment over the 20th century through a westward‐propagating sequence of Mw ≥ 7 earthquakes, the underwater Marmara segment has not experienced a large earthquake in recent times. However, due to its proximity to the megacity of Istanbul, this segment represents one of the most hazardous fault systems in the Middle East. In particular, no historical earthquake has been identified on the Central Basin segment since at least 1766, potentially making it a major seismic gap.To better assess seismic hazard along the Main Marmara Fault, we estimate the slip rate by jointly using a dense GNSS velocity field and four Sentinel-1 InSAR tracks. The GNSS velocity field consists of 111 measurements, including newly acquired densified sites along the northern shore of the Marmara Sea. The InSAR velocity field was processed automatically within the framework of the FLATSIM project, covering the period from October 2016 to April 2021. InSAR velocities are referenced to a Eurasia-fixed plate using the GNSS velocity field. We then perform a joint Bayesian inversion of slip rates using both datasets, allowing us to quantify uncertainties on the estimated slip rates.Our results indicate that the Main Marmara Fault is predominantly creeping between longitudes 27.5 and 28.6, implying that the Central Basin segment is largely aseismic. Uncertainty estimates and forward modeling demonstrate that our datasets are capable of resolving slip behavior on this segment with good accuracy. However, the shallow portion of the Central Basin segment is still accumulating up to ~10 mm/yr of slip deficit, which could permit earthquakes of up to Mw 6.0 every few decades, similar to the 2025 sequence. West of longitude 27.5 and east of longitude 28.6, including the Prince Islands segment, the fault appears to be mostly locked down to 12 km depth. On the Prince Islands segment, close to Istanbul, the accumulated strain has the potential to generate an earthquake with Mw > 7.

Earthquake cycle modeling has enabled to reproduce the full spectrum of slip rates observed along fault segments, and refine our understanding of seismic cycle dynamics. However, key parameters controlling the occurrence of shallow slow slip events (SSE) such as those observed along strike-slip fault segments remain unclear, due to rare worldwide observations and the lack of long-lasting observations covering all phases of the seismic cycle. Here, we apply rate and state friction quasi-dynamic 1D models to explain the ensemble of observations along the Izmit segment of the North Anatolian Fault in Türkiye. This fault segment ruptured in 1999 with the magnitude 7.6 Izmit earthquake, and has been since then widely studied, providing constraints on most of the phases of the seismic cycle, from mainshock amplitude and recurrence times to afterslip logarithmic decay, and the occurrence of shallow SSEs. GNSS, InSAR and creepmeters geodetic data associated with seismological and paleoseismological data enable to describe the cumulative displacement during all phases of the seismic cycle. The comparison between model predictions and the observational time scales led to an optimal set of frictional models. First, the mainshocks maximum slip of ~6 m and return times of about ≥200 yrs are explained by an unstable seismogenic layer below 5.5 km depth with a thickness of 9.5 km and with frictional parameters a-b of about -0.004. The decadal afterslip, well constrained by a pair of campaign GNSS stations located on both sides of the fault, is mostly due to a stable layer located between 5.5 and 1.3 km depth, the lower limit being compatible with the aftershocks sequence limit. We compared model slip predictions and GNSS time series by computing Green's functions for a layered elastic half space medium. Model parameters for this intermediate layer explaining the observed relaxation time have frictional parameters a-b and critical distance of about 0.005 and 8 km, respectively. Finally, a shallow layer from the surface to 1.3 km depth with either a gradient of frictional parameters with depth or constant negative frictional parameters is needed to generate shallow SSEs 20 yrs after the main earthquakes. The shallow layer depth extent being compatible with the Izmit Quaternary sedimentary basin may suggest a key role of the sediments frictional properties to allow a velocity weakening behavior. Models with a gradient of apparent frictional properties throughout the basin may suggest the importance of pore-pressure variations as a function of the fault gouge depth.

Slow-slip events (SSEs) release an important part of the accumulated strain at plate boundaries and can interact with large earthquakes. It is thus crucial to analyse in detail their temporal dynamics. While GNSS observations robustly capture the cumulative, static displacements associated with SSEs, their noise level limits the temporal resolution of transient, short-timescale potential variations in slip rate. In contrast, co-occurring tectonic tremor, sampled at much finer temporal resolution, reveals pronounced short-term intermittency within SSEs. Despite this, tremor-derived temporal variability has not yet been incorporated into kinematic SSE models, leaving the short-timescale dynamics largely unresolved. In the Mexican subduction zone, large SSEs persist for several months,recur every few years, and GNSS-based kinematic models resolve only first-order spatiotemporal evolution at these long timescales. Here we investigate the spatio-temporal evolution of the 2009–2010 SSE sequence in Guerrero, Mexico. This sequence is of particular interest because it consists of two distinct sub-events, with the onset of the second coinciding with the occurrence of the distant Maule earthquake. A detailed kinematic analysis of this SSE, combining geodetic observations and tremor activity, therefore provides a unique opportunity to assess the potential role of dynamic stress perturbations during large SSEs.We construct a tremor catalog covering the SSE sequence, using the temporary mini-array seismic network GGAP and a beamforming method at the tremor frequency band.In parallel, we use GNSS time series from the local network, to jointly analyze the SSE crustal displacement signal with the resulting tremor catalog to observe the finer dynamics of the SSE sequence. We develop two kinematic slip modeling schemes based on a least-squares formulation with regularization. In the first scheme, GNSS positions on fixed time windows are inverted sequentially as independent time steps. In the second scheme, the full GNSS time series are inverted simultaneously, which improves the recovery of displacement amplitudes and allows the incorporation of tremor-derived temporal constraints. Tremor burst timings are defined based on events clustering properties, and introduced as prior information in the kinematic inversion, allowing larger slip rates during tremor dense periods. Our results show that the studied 2009-2010 SSE sequence includes 7 major tremor bursts that are accompanied by a slip acceleration. In 2009, two major episodes of tremor and slow-slip occurred in the westernmost part of Guerrero. Immediately following the 2010 Maule earthquake, a persistent and energetic tremor and slip episode was triggered, extending the slipping region eastward along strike, where multiple additional tremor and slip episodes were subsequently observed. Although aseismic slip releases the largest moment, the accompanying tremor provides a high-resolution temporal proxy for fault slip. This enables improved temporal resolution in kinematic SSE models, and allows the identification of short-term slip accelerations that coincide with tremor timings. The complex 2009–2010 Guerrero slow-slip and tremor sequence analyzed here highlights the sensitivity of SSE slip rates and migration to far-field dynamic stress perturbations.

On February 6, 2023, two earthquakes of magnitude Mw7.8 and Mw7.6 struck in South Türkiye. The first mainshock occurred along the East Anatolian fault, at the boundary between the Anatolian and Arabian plates, and was followed 9 hours later by a second one on a secondary fault system to the North. The importance of such continental earthquakes and the relatively good data coverage of the region present an unique opportunity to investigate post-seismic deformation.To study afterslip, corresponding to post-seismic transient aseismic slip, we use a combination of ground deformation measurements, including Sentinel-1 InSAR timeseries (6 tracks covering almost 2 years after the earthquakes) and GNSS (more than 40 permanent stations and 60 campaign sites). The cities of Hassa and Gölbaşı, located on the East Anatolian fault, are investigated in detail using 8 continuous GNSS stations installed across the fault 6 months after the earthquakes.While the large surface imprint of the surface deformation, with significant displacements more than 200 km away from the fault, and our inability to model it with fault slip points toward the dominance of a visco-elastic processus, clear markers of shallow afterslip are visible. In the Pütürge segment, located at the tip of the first earthquake’s coseismic rupture, InSAR data reveals a cumulative surface offset 20 months after the earthquake of about 10 cm due to shallow afterslip. Other segments affected with afterslip have been identified in the eastern part of the rupture of the second earthquake, accounting for several centimeters of slip over 20 months. Our local GNSS networks in Hassa and Gölbaşı reveal the smaller scale complexity of post-seismic surface deformation near the fault. In Gölbaşı, subsidence of more than 2 cm/year is highlighted in the pull-apart basin, while horizontal GNSS displacements suggest possible shallow aseismic slip happening at the southern end of the basin.We model afterslip on the fault by jointly inverting InSAR and GNSS data, minimizing the least squares criterion. Afterslip is concentrated around the coseismic rupture zone, accompanied by important aftershock activity. The Pütürge segment appears as a seismic barrier, having stopped both Mw6.8 2020 Elazığ earthquake to the East and Mw7.8 2023 Kahramanmaraş earthquake to the West, possibly because of the fault geometry and/or heterogeneous coupling. Future efforts will be directed towards the evolution of afterslip with time and its interplay with aftershocks, including visco-elastic relaxation models. These results help us better understand the relationship between the different phases of the seismic cycle.

Kinematic coupling models inverted from geodetic data are widely used to evaluate how slip deficit is distributed along subduction megathrusts during the interseismic period, and are central to earthquake and tsunami hazard assessment. Yet, existing coupling models differ widely in methodology and inputs, lack common community standards, and are scattered across publications and repositories. Here, we introduce the "Coupling Cloud" (https://couplingcloud.ucsd.edu), an open, extensible, community-driven platform that curates, standardizes, documents, and disseminates more than 96 kinematic coupling models from 55 publications across 21 subduction margins. The platform provides interactive 2D and 3D plate-interface viewers to inspect coupling models together with associated information such as slab geometry, uncertainty estimates and metadata. All datasets can be downloaded directly in standardized formats: surface-projected coupling values as NetCDF, plate-interface dislocation geometries as VTU, and model metadata as YAML files. We demonstrate the advantages of centralized and standardized coupling data through a Cascadia subduction zone example, where synthesizing eight full-margin models reveals along-strike patterns that are not apparent when models are examined individually. Consolidating coupling models within a coherent, version-controlled framework enables systematic cross-margin comparison and FAIR-compliant data sharing, opening the door to more comprehensive assessment of megathrust mechanics.

Active faults exhibit a broad spectrum of slip modes, primarily governed by depth-dependent pressure and temperature conditions. These transitions manifest as fast earthquake ruptures at shallow, seismogenic depths, gradually evolving into transient slow slip and steady creep with increasing depth. In most subduction zones, the transition zone—situated between the updip seismogenic zone and the downdip steadily creeping region—is the locus of slow slip events, tectonic tremors, and Low-Frequency Earthquakes (LFEs). Within this transition zone, the recurrence patterns of tremors and LFEs display depth-dependent variations. Recurrence times decrease with depth, transitioning from low-recurrence, long-lasting bursts near the seismogenic zone to high-recurrence, short-duration bursts near the steadily creeping limit (e.g. Wech & Creager, 2011).In this study, we perform a comparative analysis across multiple plate boundaries, focusing on the along-dip spatio-temporal clustering of tremors and LFEs in Cascadia, Nankai, Mexico, and the San Andreas Fault. We developed a robust method to systematically analyze LFE catalogs from Mexico (Frank et al., 2014), Nankai (Kato et al., 2020), Cascadia (Sweet et al., 2019), and Parkfield (Shelly, 2017).Our method consists in examining the autocorrelation function of LFE occurrence time series to estimate the periodicity and duration of LFE bursts at various depths. Across all studied regions, we observe a consistent trend: recurrence intervals and burst durations of LFE activity decrease with increasing depth. Finally, we connect these depth-dependent behaviors to the thermodynamic conditions specific to each region, and to the plate convergence rates, providing insights into the rheological properties governing LFE activity within the transition zone.ReferencesWech, A.G., Creager, K.C.: A continuum of stress, strength and slip in the Cascadia subduction zone. Nature Geoscience 4(9), 624–628 (2011) https://doi.org/10.1038/ngeo1215Frank, W.B., Shapiro, N.M.: Automatic detection of low-frequency earthquakes (LFEs) based on a beamformed network response. Geophysical Journal International 197(2), 1215–1223 (2014) https://doi.org/10.1093/gji/ggu058Kato, A., Nakagawa, S.: Detection of deep low-frequency earthquakes in the Nankai subduction zone over 11 years using a matched filter technique. Earth, Planets and Space 72(1), 128 (2020) https://doi.org/10.1186/s40623-020-01257-4Shelly, D.R.: A 15 year catalog of more than 1 million low-frequency earthquakes: Tracking tremor and slip along the deep san andreas fault. Journal of Geophysical Research: Solid Earth 122(5), 3739–3753 (2017) https://doi.org/10.1002/2017JB014047

Active faults present a wide spectrum of slip behaviors, from fast earthquakes, transient slow slip events to steady creep. Although each fault has its own specificities, the distribution of these behaviors is dictated by the evolution of depth-dependent pressure and temperature conditions to first order. In most of the subduction zones, transient slow slip events, accompanied by tectonic tremors and Low Frequency Earthquakes (LFEs), are located in the transition zone, between the updip seismogenic zone and the downdip steadily creeping zone. When present, tremors and LFEs are unique tools to monitor in detail the slip behavior in the transition zone. Previous studies analyzed the spatio-temporal clustering of tremors and LFEs (Wech et Creager, 2011, Rubin et Armbruster, 2013, Frank et al., 2013), and revealed that the properties of tremors/LFE bursts evolve with increasing depth: long recurrence intervals and long-lasting bursts occur close to the seismogenic zone while short recurrence intervals and short-lasting bursts happen deeper, close to the continuously creeping zone. These observations were made in various regions and tectonic contexts, including Cascadia (Wech and Creager, 2011), Nankai (Obara et al., 2011) and Mexico (Frank et al., 2013) for subduction zones, and the San Andreas strike-slip fault (Shelly et al., 2017).In this study, we aim to gain insight on the rheological properties of the transition zone by a systematic analysis of the LFEs clustering properties in different regions and tectonic contexts. To do so, we analyze with similar methods LFE catalogs obtained with template matching, for Mexico (Frank et al., 2013), Nankai (Kato et al., 2020), Cascadia (Sweet et al., 2019) and Parkfield (Shelly et al., 2017).We analyze the clustering properties of the LFE families (asperities), by computing the auto-correlation spectra of LFE catalogs for single families, and stacking them for bins along depth. This spectrum shows the density of recurrence of LFE bursts, and we observe a clear variation of maximum recurrence intervals between 100 and 10 days along depth for all regions. Additionally, we calculated and compared LFE burst recurrences using the cumulative LFE time series derivative, from which we retrieved prominent bursts, and using other clustering methods such as DBSCAN. Finally, we compile the results from different regions and compare them with the along depth pressure-temperature conditions of these faults (Behr and Bürgmann, 2021) to have a rheological insight on these observations made using LFEs.