Precursory patterns, evolution and physical interpretation of the 2025 Santorini-Amorgos seismic sequence

Abstract

The 2025 Santorini-Amorgos seismic sequence marked a significant episode of seismic unrest in the Hellenic Volcanic Arc, offering a unique opportunity to investigate precursory patterns and the dynamic evolution of seismicity in a complex volcano-tectonic setting. Here, we analyze the preparatory phase of the crisis using a publicly available manually revised catalog, anomaly detection, and statistical modeling. We identify four distinct stages of seismic activity: 1) an initial volcano-driven phase starting in the summer 2024 with slightly accelerating moment release and focusing towards the Amorgos region; it was followed by 2) a progressive onset of the seismic sequence during January associated with stronger clustering, steady b-value and rapidly increasing magnitude variability captured by higher entropy and by some features of the multifractal spectrum. 3) A successive very strong five-days-long chaotic phase in early February, with evident breakdown of the Gutenberg-Richter law and multifractality, decreasing b-value and rapid anomalous diffusion of seismicity. Finally, 4) a slowly diffusive phase dominated by aftershocks occurring after mid-February. Physical and statistical analyses reveal significant premonitory changes marked by progressive migration of seismic activity towards the area that hosted the sequence. Our findings suggest that the 2025 crisis was promoted by a multiscale crustal weakening process. It was likely triggered by a magmatic-tectonic interaction and governed by the strong segmentation of the Santorini-Amorgos normal-faulting system which reduced the probability of a $M_w$ 6+ mainshock.