A seismic swarm designated S20210201.2 was recorded in the Aegean Sea between 22:02 on 31 January 2021 and 23:23 on 4 February 2021. Over 97 hours and 21 minutes, 86 earthquakes were registered. The events occurred within a tectonically complex region where the African plate subducts beneath the Eurasian plate along the Hellenic Arc, producing frequent shallow to moderate-depth seismicity.
The swarm initiated with a magnitude 2.9 event at 5 km depth. Activity intensified on 1 February, featuring multiple events of magnitude 4.0–5.2 at depths of 5–10 km. Notable shocks included three magnitude 4.8 events and a peak magnitude 5.2 earthquake, all clustered around 10 km depth. Subsequent days showed a gradual decline, with events ranging from magnitude 2.2 to 3.6 and depths extending to 20 km. The final recorded event on 4 February reached magnitude 3.0 at 5 km depth.
This sequence exemplifies swarm behavior, characterized by numerous events of similar magnitude without a single dominant mainshock. Depths remained predominantly shallow (under 15 km), consistent with crustal faulting in the Aegean extensional regime. Magnitudes stayed below 6.0, limiting surface damage potential while illustrating ongoing tectonic strain release.
The Aegean Sea lies within one of Europe’s most seismically active zones. Convergence rates of 4–5 cm per year drive normal and strike-slip faulting across the Hellenic subduction system and associated back-arc basins. Historical records document destructive events, including the 365 CE Crete earthquake and the 1956 Amorgos quake, underscoring the region’s long-term hazard.
Since 1 January 2000, only three prior swarms have been identified in the same sector: one each in 2008, 2011, and 2017. The 2021 swarm therefore represents a relatively infrequent clustering pattern within an otherwise steady background of isolated events.
Analysis of the 86 events reveals a rapid onset followed by an exponential decay in frequency and magnitude after the first 24 hours. Most activity concentrated between 5–10 km depth, with occasional deeper outliers to 20 km. This distribution aligns with known brittle failure zones in the Aegean crust.
Continued monitoring remains essential given the area’s tectonic setting. The 2021 swarm provides additional data for refining probabilistic models of swarm recurrence and associated ground-motion forecasts.
SeismoSight internal swarm classification records
USGS Earthquake Catalog (historical Aegean events)
Hellenic Arc tectonic summaries, Geophysical Journal International