New high-resolution relocation of the seismicity in the Southwestern Alps (France, Italy) to improve active faults imaging: Preliminary results

The geodynamic complexity of the Southwestern Alps (France, Italy) comes from its strong tectonic inheritage due to the European-African plates convergence. The motion being currently mainly accommodated along the Maghrebides, this region of the Alps only registers small to moderate seismicity linked to low-deformation rates (convergence rates of 0.3-0.9 mm/yr). Hence until now, the geometry of the active faults in the Southwestern Alps remains unclear and imprecise. Yet, a better knowledge of these faults is a prerequisite for the establishment of a regional deformation model and the improvement of the seismic hazard assessment.
Taking advantages of a nine-year seismicity catalog (7659 earthquakes of local magnitudes ranging between -0.73 and 5.03), recorded by the French and Italian permanent national networks presenting no major evolution since 2014, a high-resolution relocation is currently ongoing. The purposes are to (1) understand how the seismic events are linked to the mapped faults, (2) highlight unknown deep seismogenic structures and (3) finally improve the overall picture of the 3D geometry of active faults in the Southwestern Alps.
We present here the preliminary analysis of the relocated catalog. The seismicity is relocated using the double-difference relative method HYPODD with both cross-correlation and catalog times. As a result, the relocation is achieved for 5828 earthquakes. The uncertainties are reduced to less than 120m in horizontal and less than 600m in vertical compared to the initial average uncertainties of less than 2 kilometers for both values, referred by previous papers.
We assess the reliability of our results by comparing, at regional scale, our new relocations with those obtained by similar methods in Ubaye region. We illustrate how the double-difference relocation refines active zones imaging at multiple scales, particularly in the swarms. In Isola region located around 60 kilometers from Nice, a swarm, active since summer 2021, initially detected by the national network as a 3-kilometerlong/1-kilometer-large shape, has been precised into a 1-kilometer-long/100-meterlarge spatial activity. This relocation improvement enabled us to detect progressive activation of fault segments. On larger scale, relation between faults that may play a key role in the present-day general dynamics of the Alpine chain and deep seismogenic structures is clarified. It is the case for the High-Durance valley (France), where the precise geometry at depth of the Crustal penninic Front and High-Durance fault is determined.