Seismic Swarm S20220217.1: Analysis of Earthquake Activity in Greece

Greece lies at the convergent boundary between the African and Eurasian tectonic plates, where the African plate subducts beneath the Aegean Sea along the Hellenic Arc. This subduction zone drives frequent seismic activity, including shallow crustal earthquakes and occasional deeper events associated with the descending slab. The region experiences both individual large-magnitude quakes and episodic swarms, reflecting complex fault interactions within the overriding plate and along the subduction interface.

Seismic swarm S20220217.1 occurred in Greece between 17:38 on 16 February 2022 and 20:58 on 17 February 2022, spanning 27 hours and 19 minutes. During this interval, 33 earthquakes were recorded. The sequence featured four events of magnitude 4.0 or greater, with the strongest reaching 4.4 at a depth of 10 km. Most hypocenters clustered between 2 km and 16 km depth, indicating primarily shallow crustal sources.

The swarm began with a magnitude 4.3 event at 10 km depth, followed by numerous smaller shocks. Subsequent notable events included another magnitude 4.3 at 10 km and the peak magnitude 4.4 at 10 km. Depths remained consistently shallow, with several events at or near 2 km, suggesting activation of near-surface fault structures. Activity showed temporal clustering, with higher rates in the first 12 hours after onset before gradually declining.

Historical records since 2000 indicate that only five swarms have occurred in the monitored area. Earlier episodes took place in 2003 (two swarms), 2015 (two swarms), and 2016 (one swarm). Swarm S20220217.1 therefore represents a relatively infrequent phenomenon in this setting, consistent with the episodic nature of swarm activity along the Hellenic margin.

Swarm sequences such as this one typically arise from fluid migration or slow slip along fault networks rather than from a single mainshock-aftershock cascade. The predominance of events below magnitude 3.5 alongside a few moderate shocks aligns with this mechanism. Depths mostly under 15 km further support involvement of the brittle upper crust, where pore-pressure changes can trigger distributed failure without producing a dominant rupture.

The February 2022 swarm did not exceed magnitudes capable of widespread damage, yet it underscores the persistent seismic hazard in Greece. Ongoing monitoring by national and international networks remains essential for distinguishing swarms from foreshock sequences that might precede larger events.

References

SeismoSight internal swarm classification records.
Hellenic Arc tectonic framework, USGS Earthquake Hazards Program.
Aegean seismicity summaries, European-Mediterranean Seismological Centre.