[S-VC25] International Volcanology

[SVC25-05] InSAR evidence of small-scale faults activation during rift zone intrusions at Piton de la Fournaise volcano

*Quentin Dumont¹, Jean-Luc Froger², Lydie Gailler³, Anthony Finizola^4,5, Stéphanie Barde-Cabusson⁶ (1.Solid Earth Physics Laboratory, Department of Geophysics, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan, 2.Université Jean Monnet - Faculté des Sciences et Techniques, Laboratoire de géologie de Lyon : Terre, Planètes, Environnement - UMR CNRS 5276 LGL-TPE, 42023 Saint-Etienne, France, 3.Université Clermont Auvergne, CNRS, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France, 4.Laboratoire GéoSciences Réunion, Université de La Réunion, Saint Denis, France, 5.CNRS, Institut de physique du globe de Paris, Université de Paris, Paris, France, 6.Geociencias Barcelona (GEO3BCN), CSIC, Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain)

Keywords:Magmatic intrusion, Fault detection, InSAR, Stress Modelling, Self-Potential, Magnetic Anomaly

Volcanic edifices result from the complex spatio-temporal succession of construction phases, elasto-plastic and fragile deformations, and destructive phases. Their concurrent effects lead to complex internal structure including discontinuities at different scales from regional faults to microscopic fractures in the rocks. These discontinuities can represent important weakness zones within the volcanoes which are strongly associated with the eruptive activity, hydrothermal fluid or gas migration and alteration, flank stability and their related hazards. It is therefore necessary to image and detect these discontinuities and to monitor them. Recognition of the discontinuities on the field can be a hard task for meter-to-centimeter scale discontinuities, especially for buried structures. Interferometric Synthetic Aperture Radar (InSAR) can help detecting and mapping their location thanks to its sensitivity to differential motion down to millimeter scale. At Piton de la Fournaise volcano, the InSAR observations have highlighted small scale fringe shifts interpreted as faults during the 11–18th September 2016 eruption. In this study we investigate further in depth the presence of small scale faults on the flanks of Piton de La Fournaise, their motion and their implications for volcanic activity. We have carried out a careful analysis of the InSAR data acquired on 29 eruptions spanning between October 2010 and July 2023. By filtering out long wavelength displacements we have revealed small scale differential motion of 1-3 cm indicative of faults on the northern and southern flank of the summit cone of Piton de la Fournaise. 151 fault structures were identified, whose average length is about 200 m. 87% of them are located along the axis of the main NE-SE rift zone of the volcano. Their motion is characterized by an east-west opening associated to inflation along the fault trace and sometimes a very narrow subsidence at the centre of the inflation. Stress modelling indicates that faults are inflation-induced tensile fracture activated due to intrusion-induced dilatational stress. Moreover, faults distribution delimits areas of deflation similar to graben-like structures following the rift zone axes, whose dimension are of 200-300 m wide for 400-600 m long. In addition to the previous faults, InSAR data have also allowed to highlight on the northwestern flank semi-circular to circular nested faults delimiting areas of centripetal subsidence. The location of these ring-shaped faults is in agreement with the historical record of past crater collapse location now buried. This supports the idea that the structures we image are remnants of the past crater collapse ring-faults. Finally, based on self-potential and magnetic anomaly measurements previously acquired at the volcano, we show that faults participate to hydrothermal fluid circulation leading to rock alteration weakening the edifice along the rift zones and potentially affecting the stability of the flanks.