Seismic Swarm S20110622.1: Geological Context and Event Analysis in Greece

Seismic swarm S20110622.1 occurred in Greece between 04:16 on 22 June 2011 and 01:32 on 23 June 2011. Over this 21-hour 16-minute period, 35 earthquakes were recorded. Magnitudes ranged from 1.2 to 3.8, with the largest event occurring at the onset. Focal depths varied between 2 km and 24 km, indicating predominantly shallow crustal activity typical of the region's tectonic setting.

The sequence exhibited classic swarm characteristics, featuring numerous events of similar magnitude without a single dominant mainshock or clear foreshock-aftershock pattern. Activity clustered in the early hours, with events such as the initial 3.8 magnitude shock at 10 km depth, followed by a 3.2 magnitude event at 8 km depth roughly 45 minutes later. Subsequent tremors remained below magnitude 3.0, distributed across depths that suggest minor fault readjustments within the brittle upper crust.

Greece lies at the convergent boundary between the African and Eurasian plates, where the African plate subducts beneath the Eurasian plate along the Hellenic Arc. This subduction drives intense seismicity across the Aegean region, with frequent small-to-moderate events reflecting ongoing plate convergence at rates of approximately 4–5 cm per year. The Hellenic subduction zone extends from the Ionian Sea through Crete and into western Turkey, producing both shallow crustal earthquakes and deeper events associated with the subducting slab.

Historically, the area has hosted destructive earthquakes, including the 365 AD event near Crete that generated a major tsunami and the 1999 Athens earthquake of magnitude 6.0. Modern monitoring shows persistent low-to-moderate seismicity, with swarms often linked to fluid migration or stress transfer along secondary faults rather than volcanic processes. Depths recorded in this swarm align with the typical 5–20 km range for crustal events in central Greece.

This 2011 swarm provides insight into localized strain release within a highly fragmented fault network. The absence of larger events and the tight temporal clustering point to a diffuse stress field accommodating regional convergence without triggering a significant rupture. Such sequences contribute to the overall seismic budget of the Hellenic Arc and underscore the importance of continuous monitoring for hazard assessment.

References

SeismoSight internal classification data for swarm S20110622.1.
USGS Earthquake Hazards Program: Tectonic summary of the Eastern Mediterranean.
Hellenic Arc subduction dynamics, Geophysical Journal International reviews (updated through 2023).