Active deformation within the slow-straining northernmost Africa region constrained by InSAR time-series

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.