Seismic Swarm in Southern Greece, April 2011

A seismic swarm designated S20110420.1 occurred in southern Greece from 03:08 on 20 April 2011 to 04:39 on 22 April 2011. In approximately 49.5 hours, 49 earthquakes were recorded. Magnitudes ranged from 1.5 to 3.5, with the majority of events occurring at shallow depths near 2 km and others extending to 28 km. The sequence featured repeated low-magnitude shocks clustered in time, including events of magnitude 3.2 and 3.1 on 20 April, a magnitude 3.3 on 22 April, and a peak magnitude 3.5 also on 22 April. Depths showed a bimodal pattern, with many events at or near 2 km and others distributed between 10 km and 28 km.

Southern Greece occupies a highly active tectonic setting at the convergent boundary between the African and Eurasian plates. The Hellenic subduction zone, where the African plate descends beneath the Aegean plate, generates frequent seismic activity across the Peloponnese, Crete, and surrounding regions. This tectonic framework produces both large mainshock-aftershock sequences and episodic swarms driven by fluid migration or crustal stress adjustments along faults. Historical records document recurrent moderate seismicity, with notable swarms previously identified in 2008.

The April 2011 swarm aligns with the region's established pattern of clustered, shallow seismicity. No single large mainshock dominated the sequence; instead, energy release occurred through numerous small events over two days. Such swarms are characteristic of the extensional and strike-slip regimes that accommodate plate motion in the Aegean domain.

SeismoSight internal classification
USGS Earthquake Hazards Program
Hellenic National Seismic Network reports