Deciphering the complex spatiotemporal evolution of the 2025 Santorini - Amorgos seismic sequence

At the end of January 2025 a unique seismic sequence, in terms of intensity and earthquake magnitudes, started to evolve in the offsore area between Santorini and Amorgos. At the same time, GNSS timeseries recorded at Santorini showed the initiation of a rapid surface deformation phase with subsidence motion, precisely succeeding the six-months inflation period in the intra-caldera region of Santorini. Starting from February 1st and for the next 12 days seismicity increased sharply in rates and magnitudes reaching or exceeding 5.0, registering more than 210 events of M≥4.0 in a period of two weeks, more than twice fold the annual rate in the broader area of Greece. Herein, we use a high-resolution relocated catalogue to decipher the complex spatiotemporal evolution of the sequence and the physical process at play. The catalogue consists of more than 22,500 events, including 8,200 events detected by the routine analysis of the National Observatoty of Athens (NOA) complemented by several more obtained with a machine learning detection algorithm. Phase data for all the detected earthquakes were manually picked by the scientific staff of NOA and then processed with NonLinLoc and a local velocity model to obtain initial locations. All events were then relocated using the hypoDD algorithm, further constraining their location solutions. The bulk of relocated seismicity is constrained in an elongated SW-NE zone between Santorini and Amorgos, consistent with regional tectonics, mainly at depths of 5-15 km. The sequence evolved as a swarm displaying some unique migration patterns classified in distinct main phases of onward and backward propagation episodes. Examining more closely the spatiotemporal evolution of seismicity indicates a step-like propagation pattern characterized by secondary seismicity fronts trailed by aseismic backfronts. The main propagation front of seismicity is consistent with a model simulating lateral dyke propagation away from an over-pressurized magma chamber, while migration velocities versus duration for the various secondary fronts are comparable to the ones reported in a global dataset of various dyke-induced swarms that we compiled. The b-value spatial variations further indicate distinct high-low zones that can be induced by high differential stresses combined with higher crustal heterogeneities and increased pore-pressures at shallower depths. Overall, the analysis and results integrated with further observations are consistent with a lateral dyke intrusion in the Santorini-Amorgos rift zone facilitated by the regional tectonic fabric and inducing pronounced seismicity, with its finer details elucidating the dyke emplacement process.

Acknowledgements

We would like to thank all the personnel of the Institute of Geodynamics, National Observatory of Athens and especially the seismic analysis team and the technical staff for their tireless efforts in monitoring and responding to the Santorini - Amorgos seismic crisis.