Seismic Swarm S20111230.1: A Case Study of Earthquake Activity in Greece

Greece lies at the convergent boundary between the African and Eurasian tectonic plates, where the African plate subducts beneath the Eurasian plate along the Hellenic Arc. This subduction zone drives the region’s high seismicity, producing both shallow crustal events and deeper earthquakes associated with the subducting slab. The Hellenic subduction zone extends from the Ionian Sea through the Aegean Sea, contributing to frequent seismic swarms and moderate-to-large earthquakes. Historical records document destructive events such as the 365 CE Crete earthquake and the 1956 Amorgos earthquake, underscoring the long-term tectonic activity that continues today.

Seismic swarms, sequences of earthquakes occurring in a localized area over a short period without a dominant mainshock-aftershock pattern, are common in Greece. Since 1 January 2000, twenty-nine such swarms have been recorded nationally. Their annual frequency has shown an upward trend, with three swarms in 2008, seven in 2009, nine in 2010, and ten in 2011. These clusters typically involve low-to-moderate magnitude events at shallow depths, reflecting fluid migration or stress redistribution along active faults within the broader plate-boundary system.

Swarm S20111230.1 began at 06:31 on 29 December 2011 and concluded at 18:24 on 30 December 2011, spanning 35 hours and 52 minutes. During this interval, thirty-five earthquakes were registered across Greece. The sequence commenced with a magnitude 1.9 event at 2 km depth, followed by numerous smaller shocks ranging from magnitude 0.3 to 2.6. The largest event reached magnitude 4.7 at 15:19 on 29 December at 9 km depth. Depths throughout the swarm remained predominantly shallow, between 2 km and 17 km, consistent with upper-crustal deformation along Hellenic Arc-related structures.

The temporal distribution showed the highest activity concentrated in the afternoon and evening of 29 December, with twenty events recorded before midnight. Activity continued at a lower rate through 30 December, tapering to a final magnitude 0.8 event at 7 km depth. Magnitudes clustered below 2.0 for most events, with only two exceeding magnitude 2.0 besides the peak shock. This pattern illustrates the diffuse, swarm-like character typical of Greek seismicity, where energy release occurs through many small ruptures rather than a single large earthquake.

Such swarms provide valuable data on local stress conditions and fault interactions within Greece’s complex tectonic framework. Continued monitoring by national and international networks supports improved understanding of subduction-driven deformation and aids in refining seismic hazard assessments for the region.

References: USGS Earthquake Hazards Program – Hellenic Arc tectonics and historical seismicity summaries.
Hellenic National Meteorological Service – National seismic swarm statistics and catalog descriptions.