GNSS velocities and kenematic block model in Tunisia : quantifying present-day active deformation along the Africa-Eurasia plate boundary

Tunisia occupies a critical position at the eastern termination of the Atlas system, within the central Mediterranean plate boundary zone. The oblique convergence between the Nubian and Eurasian plates is here accommodated by a network of strike-slip and thrust faults that characterize transpressio tectonics. Moderate but persistent seismicity marks the occurrence of destructive historical earthquakes (Utique 408 AD, Kairouan 859 AD) and damaging instrumental events such as Metlaoui 2023 (Mw 5)  and Meknassy 2025  (Mw 4.8)  underscoring the need to quantify present-day deformation for seismic hazard assessment.

To address this need, the collaborative ONM-ITES project has built a multi-scale GNSS network. It integrates data from 21 stations of the OTC (Office de Topographie et Cadastre) network with 6 days of record per year from 2012 to 2019, an expanded national campaign grid of 24 TU stations (with 3 days of record per campaign 2019, 2021, 2023 and 2025), five new permanent stations strategically installed since June 2023, and two dense temporary networks of 16 stations each on the Gafsa-Metlaoui (TG) and Kairouan (TK) fault zones (with 3 days of record per campaign each year from 2021 to 2025) totaling 82 GNSS stations with known precise velocities in Tunisia. Processing is done in ITRF2020 with respect to a fixed Eurasia reference frame yields a robust horizontal velocity field.

Our velocity field reveals a non-linear south-to-north decreasing gradient, with rates ranging from 5.8 mm/yr in the south to as low as 0.8 mm/yr in the far north. This pattern reflects the partitioning of Nubia-Eurasia convergence across Tunisia's distinct tectonic domains. The derived strain rate field shows a striking spatial correlation between areas of high strain concentration and zones of intense historical and instrumental seismicity. The strain pattern provides independent validation and precise location of major deformation boundaries.

Building on this, we present a first order block model developed to interpret the observed velocity field and active tectonics. This model delineates the main tectonic blocks and strain distribution of Tunisia based on residual velocity analysis and quantifies the slip rates along their bounding faults. It provides the first geodetically-derived estimates of long-term slip rates in agreement with key seismogenic fault systems in Tunisia.

This integrated analysis synthesizes the geodetic deformation and related seismic cycle from the derived slip rates and localized strain concentrations. It provides critical constraints to assess the seismic potential and seismic hazard evaluation in Tunisia.