Seismic Swarm S20110901.1: Analysis of Seismic Activity in Greece

Greece occupies one of Europe's most seismically active zones, situated at the convergent boundary between the African and Eurasian tectonic plates. The Hellenic Arc, formed by the subduction of the African plate beneath the Aegean plate, drives frequent earthquakes across the region. This tectonic setting produces both shallow crustal events and intermediate-depth seismicity, with the Hellenic Trench serving as the primary locus of activity. Historical records document destructive earthquakes dating back millennia, including events that reshaped coastal landscapes and influenced ancient civilizations.

Seismic swarms represent clusters of earthquakes occurring in rapid succession without a dominant mainshock, a phenomenon observed repeatedly in Greece. Since 2000, 26 such swarms have been documented nationally. Annual counts include four in 2008, seven in 2009, nine in 2010, and six in 2011, underscoring the episodic nature of this activity amid ongoing plate convergence.

Swarm S20110901.1 began at 14:05 on 31 August 2011 and concluded at 21:04 on 11 September 2011, spanning 270 hours and 58 minutes. During this interval, 114 earthquakes were recorded across Greece. Analysis of the first 100 events reveals predominantly low-magnitude activity, with values ranging from 0.3 to 3.4. Depths varied between 2 km and 29 km, consistent with shallow crustal processes typical of the Hellenic system. The largest events reached 3.4 on 4 September and 3.3 on 1 September, both at depths of 5 km or less. Temporal distribution showed clustering on 1–4 September, followed by a gradual decline.

Such swarms often arise from fluid migration or stress redistribution along faults within the extensional regime of the Aegean. Depths predominantly under 20 km align with the brittle upper crust where most Greek seismicity concentrates. No damage or felt reports were associated with this swarm, reflecting its modest energy release compared to major regional events.

Ongoing monitoring by national networks continues to refine understanding of these patterns, aiding hazard assessment in this tectonically dynamic environment.

References SeismoSight internal classification data for swarm parameters and event lists. USGS Earthquake Hazards Program for regional tectonic framework.