Seismic Swarm S20110220.1: Analysis of Greece's February 2011 Earthquake Sequence
Greece lies at the convergent boundary between the African and Eurasian tectonic plates, where the African plate subducts beneath the Eurasian plate along the Hellenic Arc. This subduction zone drives the region's high seismicity, with frequent shallow and intermediate-depth earthquakes resulting from plate collision, slab rollback, and associated crustal extension in the Aegean Sea. The Hellenic subduction system has shaped Greece's geological history for millions of years, producing active faults, volcanic arcs, and a landscape prone to both tectonic and volcanic hazards.
Earthquake swarms, characterized by clusters of events without a single dominant mainshock, occur periodically in Greece due to fluid migration along faults or stress perturbations in the crust. Swarm S20110220.1 began at 01:22 on 20 February 2011 and concluded at 23:32 on 26 February 2011, spanning 166 hours and 10 minutes. During this period, 125 earthquakes were recorded across Greece.
Analysis of the first 100 events reveals predominantly low-magnitude activity, with most events registering between 0.8 and 2.3. Depths typically ranged from 5 to 19 km, indicating shallow crustal origins consistent with Aegean extensional tectonics, though a few reached 27–32 km. Notable larger events included a magnitude 3.8 quake at 19:24 on 20 February at 5 km depth, a magnitude 3.5 at 21:57 the same day at 5 km depth, and a magnitude 3.1 at 00:12 on 21 February at 5 km depth. These shallow, higher-magnitude shocks clustered early in the sequence, followed by a gradual decline in both frequency and intensity. The swarm exhibited classic swarm behavior, with events distributed over time without clear foreshock-mainshock-aftershock progression.
Historical records since 2000 show 22 such swarms in Greece, with increasing frequency noted in later years: one in 2007, three in 2008, seven in 2009, eight in 2010, and three in 2011. This pattern aligns with Greece's ongoing tectonic activity, where swarms often precede or accompany larger regional events along major fault systems.
The 2011 swarm underscores the value of continuous seismic monitoring for understanding stress accumulation in subduction-influenced crust. No significant damage or casualties were associated with this sequence, reflecting its modest energy release compared to major Hellenic Arc events.
References
SeismoSight internal swarm classification records for S20110220.1.
USGS Earthquake Hazards Program: Tectonic summary of the Eastern Mediterranean.
Hellenic Arc subduction dynamics literature from peer-reviewed geophysical journals.