Seismic Swarm S20110724.1: Analysis of Greece's 2011 Earthquake Sequence

Greece lies at the convergent boundary between the African and Eurasian tectonic plates, where the African plate subducts northward beneath the Aegean region along the Hellenic Arc. This subduction zone drives intense seismic activity, with frequent earthquakes resulting from crustal deformation, faulting, and volcanic processes in the Aegean Sea. The region has a long history of both large destructive events and episodic swarms of smaller tremors, reflecting the dynamic interplay of these plates.

Swarm S20110724.1, registered between 10:20 on 23 July 2011 and 08:55 on 17 August 2011, exemplifies this activity. Lasting 598 hours and 34 minutes, it produced 421 earthquakes. Analysis of the first 100 events reveals predominantly shallow focal depths between 2 and 23 km, with the majority clustered around 5–15 km. Magnitudes ranged from 0.4 to 4.5, indicating a swarm dominated by low-energy releases punctuated by occasional moderate shocks.

The sequence began with events of magnitude 1.2–2.1 at depths of 5–11 km on 23 July. Early activity remained modest until 24 July, when a magnitude 3.7 event occurred at just 2 km depth, followed by several magnitude 2+ tremors. Depths fluctuated but stayed generally shallow, consistent with upper-crustal faulting. Subsequent days saw a gradual increase in event frequency and size, culminating in the swarm’s largest recorded shock of magnitude 4.5 on 28 July at 10 km depth. Other notable events included magnitudes of 3.4, 3.2, and 2.8, all at depths of 5–10 km.

This pattern aligns with Greece’s documented swarm behavior since 2000, during which 25 swarms have been identified. Yearly counts show variability: four in 2008, seven in 2009, nine in 2010, and five in 2011. Such swarms typically arise from fluid migration or stress transfer along pre-existing faults rather than a single mainshock-aftershock sequence.

The shallow depths observed in S20110724.1 suggest activation of near-surface structures within the extensional tectonic regime of the Aegean. While individual events posed limited hazard, the cumulative energy release and prolonged duration underscore the importance of continuous monitoring in this high-seismicity zone.

References

SeismoSight internal swarm classification records.
Hellenic Arc tectonic framework descriptions from standard geological literature.