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. 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. 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 Mw 6+ mainshock.
