Seismic Swarm S20110127.2 in Greece: Event Analysis and Regional Context

A seismic swarm designated S20110127.2 occurred in Greece, commencing at 09:28 on 27 January 2011 and concluding at 08:05 on 28 January 2011. Over this 22-hour-36-minute interval, 44 earthquakes were recorded. Swarm activity consists of clustered events without a dominant mainshock, typically reflecting localized stress adjustments, fluid migration, or minor fault interactions within the crust.

Event magnitudes ranged from 1.9 to 3.1, with the largest shock reaching 3.1 at 01:36 on 28 January. Focal depths varied between 5 km and 26 km, indicating shallow to mid-crustal sources. The sequence began with a 2.6-magnitude event at 7 km depth and featured repeated occurrences near 12 km depth, alongside shallower activity at 5–8 km. Later events included a 2.7-magnitude shock at 12 km depth that marked the swarm’s termination.

Greece occupies a tectonically complex zone at the convergent margin between the African and Eurasian plates. The Hellenic Arc subduction system drives regional deformation, with the African plate descending beneath the Aegean microplate at rates of approximately 4–5 cm per year. This setting produces frequent shallow and intermediate-depth seismicity, accompanied by active normal and strike-slip faulting throughout the Aegean back-arc region.

Historical records document recurrent seismic episodes across Greece, including destructive events such as the 365 CE Crete earthquake and the 1999 Athens quake. Instrumental monitoring since the late twentieth century has captured numerous low-to-moderate magnitude sequences, consistent with the ongoing plate-boundary dynamics. Within the SeismoSight classification framework, only one prior swarm has been identified since 1 January 2000, occurring in 2009.

The 2011 swarm’s modest magnitudes and shallow depths align with background crustal seismicity in the Aegean, where small events commonly cluster without escalating to larger ruptures. Such patterns contribute to refined understanding of local stress fields and aid in distinguishing swarm behavior from foreshock–mainshock sequences.

References
SeismoSight internal swarm classification records
USGS Earthquake Hazards Program – regional tectonics summaries
Hellenic Arc subduction parameters from peer-reviewed geophysical literature