Passive seismic methods have become increasingly important for investigating crustal structures in tectonically active regions. The Asal Rift, located in central Djibouti, is characterized by pronounced lithospheric thinning resulting from extensional processes accommodated by major normal faults and a high geothermal flux associated with mantle upwelling beneath the region.In this study, we evaluate the contribution of passive seismology to the assessment of the geothermal potential of the Asal Rift. The dataset comes from a network of 31 short-period and broadband seismic stations deployed between 2009 and 2011 as part of the Dynamics of Rifting in Asal (DORA) Project. We performed ambient seismic noise cross-correlations, applied Frequency–Time Analysis to extract Rayleigh-wave dispersion curves within the 1–5 s period band and constructed group velocity maps of the region. From those group velocity maps we constructed a 3D shear-wave velocity model around the rift. The results reveal a significant decrease in seismic velocities within the rift zone, where several geothermal development projects are ongoing. These findings are interpreted as thermal anomalies that provide valuable insights for guiding future geothermal exploration and improving the understanding of crustal dynamics in the Asal Rift. Updated results will be presented at the meeting.

The growing volume of InSAR time series offers new opportunities to systematically detect transient aseismic deformation, but identifying low-amplitude slow slip events (SSEs) remains challenging due to noise and limited temporal resolution. Here, we adapt the geodetic matched filter, originally developed for GNSS data, to InSAR displacement time series in the context of shallow strike-slip faults. The method relies on correlations between physics-based templates and relative displacement time series constructed between pixels located on either side of the fault, enhancing the signal-to-noise ratio and mitigating atmospheric artifacts. Using synthetic experiments with realistic noise, we quantify detection thresholds and show that SSEs with magnitudes as small as Mw 4–4.5 can be reliably detected at shallow depths. A validation strategy based on spatial coherence and weighted stacking of displacement time series significantly reduces false detections. We apply this approach to the Izmit and Ismetpasa segments of the North Anatolian Fault using multi-level processed InSAR datasets. The method successfully retrieves previously documented SSEs and shows that advanced post-processing improves detection capability. Detected events have magnitudes Mw 4.0–4.3, shallow depths (< 2–4 km), and durations of days to weeks, consistent with independent geodetic observations. These results demonstrate that physics-based template matching provides a robust and scalable framework for automatic SSE detection in InSAR time series.

Flood-pulsed wetlands are characterized by significant seasonal water fluctuations, which play a critical role in the dynamics of these sensitive ecosystems. Among the growing number of existing remote sensing products, we explore the potential of interferometric (InSAR) coherence time series, derived from Sentinel-1 synthetic-aperture radar images, to characterize the hydrological dynamics of the Okavango Delta, a vast flood-pulsed wetland. Interferometric coherence reflects changes in surface conditions, making it a powerful tool for detecting flood propagation. By fitting harmonic functions, we produce parameters that quantify the seasonality of coherence time series with short isotemporal baselines (12 days). In particular, we developed a normalized seasonal index based on the ratio between the seasonal amplitude and the root-mean-square error of the fitted harmonic function, to map the seasonality of the coherence time series. A multi-annual analysis of coherence time series reveals a strong relationship between their seasonality, land cover, and flood frequency. Unsupervised clustering applied to statistical and seasonal metrics of coherence time series yields consistent classifications that map the variability of flood frequencies across wetland areas and clearly distinguish wetlands from dry zones. Similarly, thresholds applied to normalized seasonal indices delineate the year-to-year extent of flood pulses with accuracy around 79 %. We show that coherence time series in never flooded areas exhibit a pronounced seasonal pattern driven by rainfall cycle, whereas this seasonality is disrupted by flood pulses in wetlands. Building on this, we developed a change-detection approach to map the floods by identifying the date when coherence time series diverge from their seasonal pattern. The resulting flood arrival dates achieve 74-83 % accuracy compared to a reference dataset derived from optical data. Our results highlight the potential of coherence time series as a robust indicator of seasonal variations in inundation extent in flood-pulsed wetlands.

In extensional settings under Andersonian mechanics, low-angle normal faults should not form in favour of steeply dipping normal faults. However, InSAR shows that a seismic sequence including an earthquake with magnitude Mw 5.6 on August 1st, 2023 (NEIC - National Earthquake Information Center) at the northern end of the Afar rift was caused by normal faulting on a low-angle 35° dipping plane. Our best-fit InSAR model shows that the low-angle normal fault occurred on the west margin of the rift axis, it was relatively deep (6.7 km) and it slipped fully seismically, having a geodetic magnitude of Mw 5.66 in agreement with the global seismic recordings (NEIC). Temporally, the faulting occurred at the end of a one-year period (December 2022-December 2023) of increased seismicity in the northern sector of Afar, with swarms of seismicity migrating northward along the rift. The seismic characteristics, fault location and kinematics are consistent with the low-angle normal fault being triggered by fluids that locally could be released by a deep magmatic heat source along the rift axis under high extensional stresses. Our observations show that low-angle normal faults can form in rifting settings, are activated seismically and are likely fluid-induced.

Located at the southwestern terminus of the East African Rift System, the Okavango Rift System represents an opportunity to study the propagation of an active rift at its early stages (Gaudaré et al., in review). The Okavango Graben (northern Botswana) is an active half-graben of the Okavango Rift System, which shows normal to dextral strike-slip tectonic displacements of the order of 1 mm per year (Pastier et al., 2017). In addition to the impact of tectonics, large volumes of water (~10 km3 per year) brought in by the annual flood of the Okavango River generate seasonal subsidence of over 2 cm in the graben (Dauteuil et al., 2023). The prevalence of the hydrologic signal over the tectonic signal makes it challenging to provide clear interpretations of the Rift dynamics within the Okavango Graben. The previous studies are based on a network of GNSS stations, providing punctual data on displacements. To quantify the deformation field over the Okavango Graben, we analyze interferometric synthetic aperture radar (InSAR) data produced by the ForM@Ter LArge-scale multi-Temporal Sentinel-1 InterferoMetry service (FLATSIM, Thollard et al., 2021). FLATSIM automatically computes interferograms from Sentinel-1 synthetic aperture radar data and inverts them into displacement time series. The products span from April 2016 to April 2021 with a temporal resolution of 12 days, a spatial resolution of 115 x 115 m and cover the entire Okavango Rift System. We analyze and compare the seasonality of both the interferometric coherences and the InSAR displacement time series. Change detection in the interferometric coherence allows us to delineate flooded surfaces through time in the Okavango Graben, from which we deduce water loadings on the lithosphere and model the corresponding flexural response of the lithosphere. We then compare this response to the spatial distribution of annual vertical oscillations extracted from the displacement time series. Taking these seasonal signals into account, our objective is to estimate the rates of the tectonic subsidence in the Okavango Graben to better constrain the propagation of the East African Rift System at its southwestern end.Dauteuil, O., Jolivet, M., Gaudaré, L., & Pastier, A.-M. (2023). Rainfall-induced ground deformation in southern Africa. Terra Nova, 00, 1–7. https://doi.org/10.1111/ter.12650Gaudaré, L., Dauteuil, O., & Jolivet, M. Geomorphology of the Makgadikgadi Basin (Botswana): insight into the propagation of the East African Rift System. Tectonics, in review.Pastier, A.-M., Dauteuil, O., Murray-Hudson, M., Moreau, F., Walpersdorf, A., & Makati, K. (2017). Is the Okavango Delta the terminus of the East African Rift System? Towards a new geodynamic model: Geodetic study and geophysical review. Tectonophysics 712–713, 469–481. https://doi.org/10.1016/j.tecto.2017.05.035Thollard, F., Clesse, D., Doin, M.-P., Donadieu, J., Durand, P., Grandin, R., Lasserre, C., Laurent, C., Deschamps-Ostanciaux, E., Pathier, E., Pointal, E., Proy, C., & Specht, B. (2021). FLATSIM: The ForM@Ter LArge-Scale Multi-Temporal Sentinel-1 InterferoMetry Service. Remote Sensing, 13(18), 3734. https://doi.org/10.3390/rs13183734

The present-day tectonics of the northernmost Africa region is dominated by the oblique convergencebetween the Nubia and Eurasia plates initiated 35 Ma ago. GNSS-derived velocities indicate that therelative plate motion of only ~5 mm/yr is accommodated within a wide region involving inlandand offshore tectonic structures, from Morocco to Tunisia and from the Mediterranean Sea to the Saharaplatform. Due to limited and sparse geodetic measurements, the main zones of strain accommodation aswell as the strain partitioning between thrust and strike-slip faults remain unresolved. However, despitethe low strain budget, significant seismicity and destroying earthquakes have been recorded in the regionwith five M>6 events nucleated on both inland and offshore faults during the last decade. To better identify the active inland faults and constrain their interseismic behavior for a better seismichazard assessment in northernmost Africa, we used multi-temporal InSAR analysis to produce the firstregional-scale interseismic velocity map of the region. The primary data consists of up to ~8 years ofSAR imagery from the Sentinel-1 satellite constellation for 6 tracks in both the ascending and descendingorbits. The data processing and the generation of InSAR-based time series describing the spatio-temporalevolution of surface deformation were performed using the New Small Baselines Subset processing chain(NSBAS, Doin et al. 2011). We followed a three-blocks processing strategy leading to (1) interferogramsgeneration, (2) phase unwrapping, and (3) time series estimation. Within the first block, interferometricnetworks combining image pairs with short and long temporal baselines were defined to mitigate potentialbias introduced by changes in soil properties (e.g., snow, vegetation growth, dunes). Before unwrapping,interferograms flattening, multi-looking, and atmospheric phase screen (APS) corrections based on theECMWF ERA-5 atmospheric model were implemented. The resulting time series of surface displacementalong the line-of-sight direction (LOS) for the 2014-2022 period were decomposed into the near-verticaland horizontal (E-W) components and expressed into a Eurasia-fixed reference frame using the GNSSvelocities in Billi et al. (2023). Our estimated deformation maps reveal multi-scale present-day motions, with large- and small-scalesignals suggesting tectonic origin and ground response to anthropogenic activity or landslides, respectively.The oblique plate convergence involves the interseismic loading of a series of E-W oriented right-lateralstrike-slip inland faults. Between the longitudes ~3°W and ~9°E, this inland deformation is localizedwithin a 25-75 km wide zone consistent with a unique linear strike-slip fault without any clear uplift relatedto thrusting on already mapped transfer structures. The rates and directions of surface displacementssuggest that the shortening component of the Nubia-Eurasia relative plate motion is almost entirelyaccommodated by offshore tectonic structures which has an important impact on the assessment ofseismic and tsunamigenic hazards.

The northern Africa region faces significant seismic and tsunami hazards, driven by the ongoing convergence between the Nubian and Eurasian plates. Using radar interferometry (interferometric synthetic aperture radar, InSAR) analysis, we provide constraints on the interseismic behavior of tectonic structures within the region between ∼2°W and ∼11°E using the first InSAR-derived regional-scale maps of east-west and vertical velocity components. Despite the unresolved N-S−oriented motions due to inherent limitations to the InSAR technique, our millimeter-per-year level velocity maps reveal three distinct domains of strain accommodation along the plate boundary. In Western Tell, both onshore and offshore folds and thrusts accommodate frontal collision, regularly generating large reverse earthquakes, such as the Ms 7.3 El Asnam earthquake of 1981. In the Eastern Tell, deformation is widely distributed from the Saharan Platform to the northern coastlines. In the Central Tell, we identify the predominantly aseismic behavior of the 500-km-long Ghardimaou-North-Constantine fault, marked by a shallow locking depth (<5.1 ± 0.5 km) and standing out as one of the slowest (<2.5 mm/yr) continental strike-slip faults ever detected with geodetic data. Strain partitioning diverts most of the shear component of the oblique convergence into this major inland structure, thereby making the offshore thrusts and folds the principal sources of seismic hazard.

Seismic waves from large earthquakes are known to trigger slip on distant faults, but the underlying mechanisms remain unclear. Using interferometric synthetic aperture radar and local geodetic and seismic data, we show that the 1000-kilometer-distant, February 2023 Kahramanmaraş earthquakes in southeastern Türkiye triggered deformation and/or eruption at 56 mud volcanoes and centimeter-scale aseismic slip on seven faults over tens of kilometers within the fluid-rich Kura Basin in the West Caspian region. This transient deformation event, with an equivalent moment magnitude of 6.1, was coupled with local inflation below major hydrocarbon fields. We postulate that seismic waves led to a change in pore pressure at depth, which in turn triggered aseismic slip along several crustal faults crossing the basin and its surroundings.

The Caucasus and Northern Iran lie within the central part of the Alpine-Himalayan belt, where the Arabian and Eurasian plates started colliding over 100 My ago and caused the building of mountain chains associated with complex tectonics, including transform faulting systems. The region contains many tectonic features including the EW-trending Greater Caucasus and the NW-trending Lesser Caucasus thrust belt separated by the Kura basin. In the southern part of the region, the tectonics are complicated by the Anatolia-Eurasia-Arabia triple junction and the northern end of the Talysh and Alborz thrust belts. There have been several destructive earthquakes in the region, including the Shamakhi earthquake sequences in 1667(8) and 1902 at the junction of the controversial and mostly a-seismic West-Caspian Fault and the Eastern Greater Caucasus and the 1721 and 1780 earthquakes on the North Tabriz fault in NW Iran. Investigations of the few publicly available seismic catalogs of the region have been insufficient to understand the seismo-tectonic behavior of the regional structures due to sparse existing seismic networks. To better characterize the active structures in the Caucasus and Northern Iran we produced regional-scale mean line-of-sight velocity maps and time-series of the surface displacement from the north-eastern Caucasus to northern Iran. To obtain this dataset we performed Synthetic Aperture Radar interferometry using the NSBAS processing (Doin et al., 2011) of Sentinel-1 images along both ascending and descending tracks for 9-years (2015 to 2023). Main processing steps (such as atmospheric correction, multilooking and filtering) were applied to counter biases and loss of coherence due to the snow and vegetation coverage in the Greater Caucasus mountains. We produced two regional-scale interseismic velocity maps that highlight crustal motions of the large-scale tectonic structures. Moreover, we have identified coseismic deformation due to the 5.2 ml Shamakhi earthquake in the SE Caucasus mountains (Feb. 2019), the 5.9 Mw Torkamanchay earthquake in the Bozgush mountains of NW Iran (Nov. 2019), and possible aseismic strike-slip along the West Caspian fault after the large seismic events in Türkiye in February, 2023. Our results can also be used to study the local deformation of mud volcanoes in the Eastern part of Azerbaijan.

The occurrence of aseismic creep along seismogenic faults significantly impacts seismic hazard assessment by releasing accumulated stress and reducing the slip deficit. Since the 1999 M(w)7.6 Izmit earthquake on the North Anatolian Fault in Turkiye, while aseismic creep has been observed as a postseismic response to the Izmit rupture, additional slow slip events were detected in 2015 and 2016, accommodating several millimeters of relative displacement over periods of approximately one month. By automating Interferometry Synthetic Aperture Radar time series processing from 2016 to 2021 (FLATSIM project) and applying specific post-processing, we extract the tectonic signal to estimate the slip dynamics of the Izmit segment, including the detection and characterization of slow slip events. Modeling the slip distribution at depth on a 2D fault interface within a layered elastic half-space, we estimate a locking depth of 11 km and steady creep between 2 and 5 km. Above the steady creep zone, we identify two new shallow slow slip events in March 2018 and November 2019, with moment magnitudes of 4.3 and 4.4, respectively. Based on creepmeter measurements, we estimate a lateral propagation velocity of 6.4 km/day for the 2019 event. The location of these shallow slow slip events above the sedimentary-bedrock interface suggests a critical role of variations in frictional properties in the occurrence of transient slip events.

Geophysical and geological data from the North Mozambique Channel acquired during the 2020–2021 SISMAORE oceanographic cruise reveal a corridor of recent volcanic and tectonic features 200 km wide and 600 km long within and north of Comoros Archipelago. Here we identify and describe two major submarine tectono-volcanic fields: the N’Droundé province oriented N160°E north of Grande-Comore Island, and the Mwezi province oriented N130°E north of Anjouan and Mayotte Islands. The presence of popping basaltic rocks sampled in the Mwezi province suggests post-Pleistocene volcanic activity. The geometry and distribution of recent structures observed on the seafloor are consistent with a current regional dextral transtensional context. Their orientations change progressively from west to east (∼N160°E, ∼N130°E, ∼EW). The volcanism in the western part appears to be influenced by the pre-existing structural fabric of the Mesozoic crust. The 200 km-wide and 600 km-long tectono-volcanic corridor underlines the incipient Somalia–Lwandle dextral lithospheric plate boundary between the East-African Rift System and Madagascar. Supplementary Materials: Supplementary material for this article is supplied as a separate file: crgeos-159-suppl.pdf Des données géophysiques et géologiques ont été acquises lors de la campagne océanographique SISMAORE (2020–2021). Deux grands champs tectono-volcaniques sous-marins ont été découverts tout le long et principalement au nord de l’archipel des Comores : la province N’Droundé orientée N160°E au nord de Grande-Comore, et la province Mwezi orientée N130°E au nord d’Anjouan-Mayotte où des roches basaltiques de type popping-rocks suggèrent une activité volcanique possiblement actuelle à pléistocène. La géométrie et la distribution des structures récentes sont cohérentes avec un contexte régional actuel transtensif dextre. Leurs orientations évoluent d’Ouest en Est (∼N160°E, ∼N130°E, ∼EW), suggérant pour la partie occidentale, une mise en place du volcanisme influencée par la structuration crustale préexistante. Le corridor tectono-volcanique de 200 km de large et de 600 km de long dessine une limite de plaque lithosphérique Somalie-Lwandle immature en décrochante dextre entre le système du rift est-africain et Madagascar. Compléments : Des compléments sont fournis pour cet article dans le fichier séparé : crgeos-159-suppl.pdf

Ce chapitre expose le fonctionnement des frontières de plaques divergentes en s’appuyant sur la notion de cycle comme cela est fait pour les autres types de frontières. L’évolution à court-terme des rides océaniques actuelles ou futures est ainsi décrite à partir des observations géodésiques et sismologique acquises lors des différentes phases du cycle de diking.

From 10 May 2018 to 1 November 2022 (time of writing), an unprecedented seismic activity is observed east of Mayotte Island (France), related to the largest submarine eruption ever recorded with offshore geophysical studies. Using signals from regional and local seismic stations, we build a comprehensive catalog of the local seismicity for the first ten months of the sequence. This catalog includes a total of 2874 events of magnitude (Mlv) ranging from 2.4 to 6.0, with 77% of them relocated using a double difference location procedure. The hypocentral locations over this period are highly dependent on the small seismic network available. Therefore we compare the locations of later events using a similar network and those estimated from a local ocean bottom seismometer (OBS) network installed since March 2019. Based on the time space evolution and characteristics of the seismicity, five distinct phases can be identified, corresponding to the successive activation of two deep seismic swarms, related to the lithospheric-scale magma ascent up to the seafloor, along with progressive deepening of the seismicity interpreted as decompression of a 40 kmdeep reservoir.

In May 2018, the Mayotte island, located in the Indian Ocean, was affected by an unprecedented seismic crisis, followed by anomalous on-land surface displacements in July 2018. Cumulatively from July 1, 2018 to December 31, 2021, the horizontal displacements were approximately 21 to 25 cm eastward, and subsidence was approximately 10 to 19 cm. The study of data recorded by the on-land GNSS network, and their modeling coupled with data from ocean bottom pressure gauges, allowed us to propose a magmatic origin of the seismic crisis with the deflation of a deep source east of Mayotte, that was confirmed in May 2019 by the discovery of a submarine eruption, 50 km offshore of Mayotte ([Feuillet et al., 2021]). Despite a non-optimal network geometry and receivers located far from the source, the GNSS data allowed following the deep dynamics of magma transfer, via the volume flow monitoring, throughout the eruption.