Seismic Swarm S20110205.1: Analysis of February 2011 Activity in Greece

Greece lies at the convergent boundary between the African and Eurasian plates, where the African plate subducts beneath the Aegean microplate along the Hellenic Arc. This tectonic setting produces frequent shallow to intermediate-depth earthquakes, with the highest seismic hazard concentrated in the southern Aegean, western Greece, and the Gulf of Corinth. The region experiences both large mainshock-aftershock sequences and episodic seismic swarms, the latter characterized by numerous events of similar magnitude without a dominant shock.

Seismic swarm S20110205.1 was recorded between 06:15 UTC on 4 February 2011 and 12:35 UTC on 8 February 2011, spanning 102 hours and 20 minutes. During this interval, 122 earthquakes were detected. The first 100 events, spanning the initial days of the swarm, exhibited magnitudes from 0.3 to 3.9 and focal depths between 2 km and 14 km, indicating predominantly shallow crustal activity.

The sequence began with a magnitude 1.8 event at 11 km depth. Magnitudes increased modestly through the first day, reaching 3.0 at 20:19 UTC on 4 February. A magnitude 3.9 earthquake at 2 km depth on 5 February at 02:52 UTC marked the largest event in the analyzed subset. Subsequent activity included multiple events above magnitude 2.5, with notable occurrences at 03:13 UTC (3.1), 18:57 UTC on 6 February (3.0), and 22:42 UTC on 6 February (2.9). Depths remained consistently shallow, clustering between 5 km and 10 km for the majority of events.

Temporal distribution showed the highest rate of occurrence on 4–5 February, with a gradual decline thereafter. The swarm terminated after a final cluster of events on 6–7 February. No events exceeded magnitude 4.0, consistent with the swarm classification rather than a typical aftershock sequence.

Greece has recorded 19 seismic swarms since 1 January 2000. Annual counts include two in 2008, seven in 2009, nine in 2010, and one in 2011. These swarms commonly occur along active fault systems in the Aegean and western Hellenic regions, often linked to fluid migration or stress transfer within the extending continental crust.

The S20110205.1 swarm illustrates the characteristic behavior of Aegean seismicity, where clusters of small-magnitude events reflect ongoing tectonic extension and localized stress release. Continued monitoring of such swarms contributes to refined seismic hazard assessment in one of Europe’s most active tectonic domains.

References

• Hellenic Arc tectonics and seismicity summaries, USGS Earthquake Hazards Program
• Hellenic Seismic Network bulletins and swarm catalogs, National Observatory of Athens
• SeismoSight internal swarm classification records for event S20110205.1