Seismic Swarm S20110202.1: A February 2011 Event in Greece
Greece occupies a tectonically active zone at the convergent boundary between the African and Eurasian plates. Subduction along the Hellenic Arc generates frequent earthquakes, with the region experiencing both large mainshock-aftershock sequences and episodic earthquake swarms. Swarm activity clusters in areas of crustal extension and fluid migration, often producing hundreds of small events over days without a dominant mainshock.
Swarm S20110202.1 began at 19:52 on 1 February 2011 and concluded at 17:58 on 3 February 2011, lasting 46 hours and 6 minutes. During this interval, 29 earthquakes were recorded across Greece. Magnitudes ranged from 0.8 to 3.3, with the largest event occurring at the outset (magnitude 3.3 at 2 km depth). Subsequent events remained predominantly below magnitude 2.0, though two reached 2.6 and 2.1 on 2 February. Focal depths varied between 5 km and 16 km, indicating shallow crustal sources typical of Greek swarm sequences.
The temporal pattern showed an initial energetic phase followed by sustained low-level activity. After the opening magnitude-3.3 shock, magnitudes dropped rapidly, with most events between 1.0 and 1.8. A secondary peak included the magnitude-2.6 event at 5 km depth late on 2 February. Depths fluctuated modestly, rarely exceeding 15 km, consistent with fluid-driven swarm behavior rather than deeper subduction-related seismicity.
Since 1 January 2000, Greece has recorded 20 swarms. Annual counts increased toward the end of the decade: one swarm in 2007, four in 2008, seven in 2009, and eight in 2010. This upward trend aligns with improved detection networks and may reflect episodic tectonic or hydrothermal forcing within the Hellenic system.
The 2011 swarm fits the established pattern of short-duration, low-magnitude clusters that release strain without producing destructive shaking. Such events contribute to long-term seismic hazard assessment by highlighting zones of persistent microseismicity along Greece’s active fault networks.
References
SeismoSight internal swarm classification database (S20110202.1 event parameters).
USGS Earthquake Hazards Program – Tectonic summary of the Aegean region (updated regional framework).
Hellenic Arc seismicity reviews in Journal of Geophysical Research (plate-boundary characteristics).