Seismic Swarm S20111119.1: Insights into Greece's November 2011 Activity

Greece lies at the convergence of the African and Eurasian tectonic plates along the Hellenic Arc, a subduction zone responsible for intense seismic activity. The region experiences frequent earthquakes due to the northward subduction of the African plate beneath the Aegean plate, combined with extensional tectonics in the back-arc area. This setting produces both shallow crustal events and deeper subduction-related seismicity, with historical records documenting destructive earthquakes dating back to antiquity, including the 365 AD Crete quake that generated a major tsunami.

The seismic swarm designated S20111119.1 was recorded between 12:47 on 18 November 2011 and 13:20 on 22 November 2011. Over this 96-hour-and-32-minute period, 77 earthquakes were detected. Magnitudes ranged from 0.4 to 2.6, with the majority below 2.0, indicating low-energy events typical of swarm behavior rather than a mainshock-aftershock sequence. Depths clustered between 5 and 13 km, suggesting activity within the upper crust consistent with Greece's active normal-fault systems.

The sequence began with a 1.7-magnitude event at 10 km depth, followed by numerous small tremors. Peak activity included a 2.2-magnitude shock on 18 November and a 2.6-magnitude event on 21 November, both at shallow depths of 5 km. Later events on 22 November remained below 1.5 magnitude. This pattern of clustered, low-to-moderate magnitudes over several days aligns with fluid-driven or stress-transfer mechanisms often observed in Greek swarms.

Since 1 January 2000, Greece has recorded 28 such swarms. Annual counts show variability, with 4 in 2008, 7 in 2009, 9 in 2010, and 8 in 2011. These episodes highlight the episodic nature of seismicity in the Hellenic region, where swarms frequently occur without leading to larger mainshocks.

This swarm exemplifies the background seismic noise that characterizes Greece's tectonics. Ongoing monitoring by national networks helps distinguish such clusters from potential precursors, supporting hazard assessment in one of Europe's most earthquake-prone countries.

References

• Hellenic Seismic Network reports on regional tectonics.
• USGS Earthquake Hazards Program documentation for Greece.