Seismic Swarm S20110918.1: Analysis of Greece's September 2011 Earthquake Sequence

Greece occupies a tectonically complex zone at the convergent boundary between the African and Eurasian plates. The Hellenic Arc, extending from the Ionian Islands through Crete to the Dodecanese, accommodates subduction of the African plate beneath the Aegean microplate at rates of approximately 4–5 cm per year. This setting produces frequent shallow crustal earthquakes along active normal and strike-slip faults, alongside deeper events associated with the subducting slab. The broader Aegean region exhibits one of the highest seismic strain rates in Europe, with historical records documenting destructive events dating back to antiquity.

Swarm S20110918.1 was recorded in Greece between 00:39 UTC on 18 September 2011 and 18:13 UTC on 22 September 2011. Over 113 hours and 34 minutes, the sequence comprised 132 earthquakes. Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.2 to 3.3, with the majority falling between 1.0 and 2.0. Depths were shallow, concentrated between 2 km and 14 km, consistent with brittle failure in the upper crust. Notable events included a magnitude 3.3 earthquake at 5 km depth on 18 September at 22:46 UTC and several magnitude 2.4–2.5 shocks clustered in the evening of the same day. Temporal distribution showed peak activity during the first 36 hours, followed by a gradual decline, typical of swarm behavior without a single dominant mainshock.

Greece has experienced repeated seismic swarms in recent decades. Since 1 January 2000, 27 swarms have been documented, with annual counts increasing from four in 2008 to nine in 2010 and seven in 2011. These sequences often occur along known fault networks in the Aegean and Ionian regions, reflecting episodic stress release rather than isolated large-magnitude ruptures.

The shallow focal depths and modest magnitudes observed in S20110918.1 align with patterns seen in other Aegean swarms, where fluid migration or aseismic slip may trigger clustered seismicity. No damage or casualties were associated with this particular sequence.

References

• Hellenic Arc tectonics: McKenzie, D. (1972). Active tectonics of the Mediterranean region. Geophysical Journal International.
• Aegean seismicity statistics: Papazachos, B.C. et al. (updated catalogues through 2023). University of Thessaloniki Seismological Station.
• Swarm classification data: SeismoSight internal records.