Seismic Swarm S20110211.1: Analysis of Greece's February 2011 Event

Greece lies at the convergent boundary between the African and Eurasian tectonic plates, where the African plate subducts northward beneath the Aegean plate 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 ongoing deformation, with the Hellenic trench marking one of Europe's most active seismic belts. Historical records document destructive events such as the 365 AD Crete earthquake, underscoring the long-term hazard. SeismoSight internal classification identifies Swarm S20110211.1 as occurring in Greece from 16:39 on 11 February 2011 to 05:42 on 17 February 2011. Over 133 hours and 3 minutes, 148 earthquakes were registered. Analysis of the first 100 events reveals predominantly shallow focal depths between 0 and 16 km, with the majority clustered at 5–14 km. Magnitudes ranged from 0.5 to 4.5, featuring an early peak of M4.5 at 17:56 on 11 February followed by numerous events in the 1.0–3.5 range. Later notable shocks included M3.5, M3.2, and M3.1, indicating sustained energy release without a single dominant mainshock-aftershock sequence typical of isolated events. Temporal distribution shows highest activity in the first 48 hours, with events occurring at intervals of minutes to hours. Depths remained consistently shallow, consistent with crustal faulting in the Aegean extensional regime. The swarm pattern aligns with fluid migration or stress triggering along pre-existing faults common in this tectonic setting. Since 1 January 2000, Greece has recorded 20 seismic swarms according to SeismoSight records. Annual counts include two in 2008, seven in 2009, nine in 2010, and two in 2011, reflecting episodic clustering amid background seismicity driven by plate convergence rates of approximately 35–40 mm per year. This swarm exemplifies the distributed nature of Aegean seismicity, where swarms often occur without producing surface rupture yet contribute to regional strain accumulation. Monitoring such sequences aids in refining probabilistic hazard models for the Hellenic Arc. References SeismoSight internal swarm classification and event catalog (S20110211.1 parameters and historical swarm statistics). USGS Earthquake Hazards Program regional tectonic summaries for the Hellenic subduction zone.