Dike‐Induced Earthquakes as Probes of the Local Stress Field Prior to the 1998 Eruption of the Piton de la Fournaise Volcano, La Réunion

Abstract Volcano‐tectonic (VT) earthquakes generated during dike propagation provide useful constraints on stress changes in volcanic systems. During the 1998 pre‐eruptive unrest at Piton de la Fournaise, ∼600 VT earthquakes were recorded. Using manually picked first‐motion polarities and corrected station metadata, we calculated more than 400 high‐quality fault plane solutions. These show systematic depth‐dependent changes in dominant P‐ and T‐axis orientations. Below ∼1–2 km below sea level, the dominant P‐axes are mostly vertical, consistent with strong edifice‐loading control, whereas at shallower depths they become mainly horizontal and commonly dike‐perpendicular, consistent with increasing influence from tectonic compression and dike‐induced stress perturbation. A 2D damage‐based dike propagation model combined with 3D elastic models of tectonic stress and edifice loading suggests that these variations reflect a depth‐variable local stress field during magma migration. Our results provide a qualitative framework, with limited semi‐quantitative constraints, for interpreting dike‐induced seismicity before eruption.