Seismic Swarm S20210112.1: Insights into Greece's Tectonic Activity

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 frequent seismic activity across the region, including the Aegean Sea and mainland areas. The Hellenic subduction system, combined with strike-slip faulting in the Aegean and back-arc extension, produces both large destructive earthquakes and episodic swarms of smaller events. Depths typically range from shallow crustal levels of 0–15 km, reflecting brittle failure in the upper lithosphere.

The swarm designated S20210112.1 began at 00:47 on 12 January 2021 and concluded at 12:03 on 15 January 2021, lasting 83 hours and 16 minutes. During this interval, 73 earthquakes were recorded, with magnitudes spanning 2.5 to 5.2 and focal depths predominantly between 2 and 10 km. The sequence featured an initial cluster of events below magnitude 3.0, followed by a mainshock of 5.2 at 22:09 on 12 January. Subsequent activity included multiple events above magnitude 4.0, such as 4.6, 4.7, 4.5, 4.4, and 4.3, distributed over the following days. The swarm exhibited typical swarm characteristics: rapid onset, lack of a single dominant mainshock-aftershock decay pattern, and clustering of events at similar shallow depths.

Swarm seismicity in Greece often reflects fluid migration, stress triggering along fault networks, or aseismic slip episodes within the complex plate-boundary zone. The January 2021 sequence aligns with patterns observed in prior swarms, where activity concentrates in areas of high strain accumulation near the subduction interface or secondary faults. Depths consistently under 10 km suggest involvement of the upper crust rather than deeper slab processes.

Historical records indicate elevated swarm frequency in Greece since 2000. A total of 28 swarms have been documented through 2021. Notable clusters occurred in 2008 (2 swarms), 2009 (7 swarms), 2010 (8 swarms), and 2011 (11 swarms). These episodes demonstrate recurring seismic unrest linked to the ongoing convergence of the African and Eurasian plates at rates of approximately 4–5 cm per year. Such swarms contribute to long-term strain release without necessarily producing great earthquakes, though they underscore the persistent seismic hazard in the region.

Analysis of the 2021 swarm highlights the value of dense monitoring networks for distinguishing swarm behavior from foreshock sequences. The event distribution, with repeated magnitude 4+ shocks over three days, illustrates how swarms can sustain elevated seismicity rates and inform probabilistic hazard assessments. Continued observation of similar sequences aids in refining models of crustal deformation along the Hellenic Arc.

References

USGS Earthquake Catalog and Tectonic Summaries for the Eastern Mediterranean
Hellenic Arc Subduction Zone Reports, National Observatory of Athens
Global Seismicity Studies, International Seismological Centre