Triggering mechanisms and repeating earthquakes during the intense 2020–2021 seismic crisis in the Gulf of Corinth

SUMMARY The 2020-2021 seismic crisis in the western Gulf of Corinth was characterized by a sustained duration and the activation of multiple neighbouring fault segments. Using a high-resolution microseismicity data set we explored the possible mechanisms driving this activity, which appeared migrating in time across the local fault network. Applying the density-based clustering algorithm, DBSCAN, we identified 95 repeaters comprised in 19 multiplets, each multiplet containing at least four earthquakes, the majority of which ($\sim 78\ \mathrm{ per}\ \textrm{cent}$) occurring during the first phase of the activity. The analysis of the migration front revealed a complex interplay of triggering mechanisms. An average migration velocity of $180.24$ $\mathrm{ m}\,\mathrm{ d}^{-1}$ was estimated for the entire duration, with fluid-induced aseismic slip emerging as a potential driving force during the first period, along with intermediate faster migration rates ($\mathrm{ km}\,\mathrm{ d}^{-1}$) during rapid bursts of activity. Coulomb stress modelling further demonstrated a cascading stress transfer among the activated fault segments, progressively triggering the larger earthquakes over time. Statistical modelling based on the Epidemic Type Aftershock Sequence model highlighted the coexistence of swarm-like activity nested within mainshock-aftershock sequences, offering insights into the dynamic evolution of seismicity. These findings highlight the complex mechanisms that drive the evolution of the seismicity, where aseismic slip appears to act as the main triggering mechanism for the first major rupture of the seismic activity, followed by stress perturbations.