Seismic Swarm S20111220.2: 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 Eurasian plate along the Hellenic Arc. This subduction zone drives intense seismic activity across the region, including the Aegean Sea and surrounding areas. The Hellenic subduction system has produced some of Europe’s largest historical earthquakes, with documented events exceeding magnitude 7.0. Shallow crustal faults and volcanic centers, such as those near Santorini and Methana, further contribute to frequent microseismicity and occasional swarms.

Swarm S20111220.2 was recorded over a 30-hour-35-minute period beginning at 20:24 on 19 December 2011 and concluding at 03:00 on 21 December 2011. During this interval, 27 earthquakes were detected. Magnitudes ranged from 0.7 to 3.3, with the majority below 2.5. Depths remained shallow, concentrated between 2 km and 10 km, consistent with activity along upper-crustal structures in the Aegean extensional regime.

The temporal distribution showed an initial cluster of events on the evening of 19 December, followed by a peak in activity during the early hours of 20 December. A magnitude 3.3 event at 2 km depth occurred at 04:47 on 20 December, representing the largest shock of the sequence. Subsequent events declined in both frequency and magnitude, with the final recorded tremor at 03:00 on 21 December. The shallow focal depths suggest rupture within brittle crustal layers rather than deeper subduction-related processes.

Statistical context places this swarm within a broader pattern of seismic clustering observed since 2000. A total of 27 swarms have been identified in the Greek catalog through the end of 2011. Annual counts indicate a marked increase in swarm occurrences from 2008 onward, with two swarms in 2008, seven in 2009, nine in 2010, and nine in 2011. These episodes typically involve low-to-moderate magnitudes and short durations, reflecting fluid migration or stress transfer along segmented faults rather than mainshock-aftershock sequences driven by a single large rupture.

Such swarms contribute to ongoing hazard assessment by highlighting zones of elevated microseismicity. Although individual events in S20111220.2 remained below damaging thresholds, their occurrence underscores the persistent tectonic loading along the Hellenic plate boundary. Continued monitoring of similar clusters supports refined models of crustal deformation and improves probabilistic forecasts for future seismic activity in Greece.

References

United States Geological Survey, Earthquake Hazards Program – Hellenic Arc Tectonics
European-Mediterranean Seismological Centre – Regional Seismicity Reports
Greek Institute of Geodynamics – National Earthquake Catalog Summaries