InSAR Evidence of Small‐Scale Faults Activated During Rift Zone Intrusions at Piton de la Fournaise Volcano

Volcanic edifices show complex internal structures including discontinuities at different scales. These discontinuities can act as zones of weakness and are closely linked to volcanic activity distribution and associated hazards, making their detection and monitoring essential. In this study we investigate the presence of small‐scale faults on the flanks of Piton de la Fournaise volcano, and their implications for volcanic activity. We have carried out a careful analysis of the Interferometric Synthetic Aperture Radar (InSAR) data acquired on 30 magma intrusions occurring 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 flanks of the summit. We detect 151 fault structures, 87% of which are tensile fractures located along the main NE‐SE rift zone axis and activated by intrusion‐induced dilation. Fault distribution outlines areas of deflation similar to graben‐like structures, approximately 200–300 m wide and 400–600 m long following the rift zone axes. Moreover, on the north‐western flank, nearly circular nested faults delineate areas of centripetal subsidence, and are interpreted as relict ring‐faults of past crater collapses. Electromagnetic measurements previously acquired at the volcano, indicate these faults participate in hydrothermal fluid circulation leading to rock alteration and edifice weakening along fault traces, potentially affecting the stability of the flanks. InSAR data can thus effectively reveal active structures on volcanic edifices, offering critical insights into structural weaknesses that help hazard assessment and risk mitigation.