Volcano‐Magnetic Signal Reveals Rapid Evolution of the Inner Structure of Piton de la Fournaise.

Near‐real time analysis of magnetization can provide important information for the imaging of volcano systems and their spatiotemporal evolution. This study focuses on the contribution of volcano‐magnetic signals from reiterations of ground magnetic measurements to investigate the evolution of active structures at the Piton de la Fournaise volcano from 2017 to 2020. Changes are demonstrated by magnetic anomalies along a reference profile by means of the reiteration periods. These variations are first modeled qualitatively in 2D using electrical resistivity constraints in order to investigate the evolution of magnetization at depth through time, and the model is subsequently compared with the 3D intrusive activity from depth up to the surface from Interferometric Synthetic Aperture Radar (InSAR) inverse modeling. The shallow areas of demagnetization modeled from one reiteration to another are consistent with the geometry and location of the underlying intrusions revealed by the 3D InSAR models, suggesting strong thermal, stress, and electrokinetic effects due to magmatic activity not only at the surface but also at depth, along the main magmatic paths. It also raises a question as to the extent of the associated thermal diffusion processes at the scale of individual magma injections. This study confirms that detecting resistivity and magnetization anomalies, and quantifying their spatiotemporal evolution, can provide powerful tools for imaging volcanic systems at various scales and for providing warning of associated hazards. It also highlights the necessity for 4D monitoring of volcanic edifices using this method to provide greater precision, an important issue that is now made possible by the use of Unmanned Aerial Vehicle measurements. Plain Language Summary: Volcanic activity (through volcano‐tectonic, magmatic, and hydrothermal processes) undeniably disturbs the magnetic field, especially at a local scale. Recent studies have evidenced the potential of magnetic field measurements in imaging thermal anomalies (strong influence of temperature on magnetic measurements) and mechanical heterogeneities (fracturing‐alteration). Piton de la Fournaise (La Réunion Island, Indian Ocean) is an outstanding target to address such issues, being highly active and closely monitored. In this study, we used repeated magnetic measurements conducted over 3 years along a 3,780‐m‐length profile close to the summit of Piton de la Fournaise. We used the complementarities between magnetization and electrical resistivity parameters in a 2D magnetic modeling approach, in order to image the consequence of the eruptive dynamics on the edifice structure at depth. The temporal evolution of the magnetic anomalies appears strongly correlated to the depths and volumes of magmatic intrusions along the main active area. These results open new perspectives in studying the spatiotemporal changes in magmatic, hydrothermal, and mechanical changes and alteration processes within volcanic edifices. Key Points: Volcano‐magnetic variations at Piton de la Fournaise volcano (magmatic and mechanical changes, hydrothermal alteration processes)2D modeling of magnetic anomalies evolution under electrical resistivity constraints consistent with Interferometric Synthetic Aperture Radar 3D intrusions modelsImaging the main paths of weakness at depth that could be associated with potential future magmatic transfers [ABSTRACT FROM AUTHOR]