Seismic Swarm S20091119.1: Analysis of Earthquake Activity in Greece

Seismic swarm S20091119.1 occurred in Greece between 11:43 on 18 November 2009 and 15:57 on 22 November 2009. Over 100 hours and 14 minutes, the event sequence registered 77 earthquakes. This swarm represents one of six documented in 2009, following two additional swarms recorded in 2008. Since 1 January 2000, Greece has experienced a total of eight such swarms according to internal classifications.

The sequence began with a magnitude 1.8 event at 14 km depth. Magnitudes ranged from 0.9 to 3.3, with the largest event reaching 3.3 at 8 km depth late on 19 November. Depths remained predominantly shallow, concentrated between 5 km and 20 km, consistent with upper-crustal activity. Multiple events clustered around magnitudes 2.5–2.9, including several at depths of 5 km and 8 km on 19 November and 20 November. The final recorded event measured magnitude 2.7 at 5 km depth.

Greece lies at the convergent boundary between the African and Eurasian plates. The Hellenic Arc, formed by northward subduction of the African plate beneath the Aegean plate, drives the majority of regional seismicity. This tectonic setting produces frequent shallow to intermediate-depth earthquakes along the subduction interface and within the overriding crust. Historical records document destructive events such as the 365 CE Crete earthquake and the 1999 Athens earthquake, underscoring the persistent hazard.

Swarm activity in Greece often reflects fluid migration or stress redistribution along active faults rather than mainshock-aftershock sequences. The 2009 swarm exhibited typical characteristics: rapid onset, high event rate over several days, and absence of a dominant mainshock. Depths between 5 km and 18 km align with known seismogenic zones in the Aegean region.

The elevated swarm frequency in 2008–2009 may indicate a period of heightened crustal strain. Continued monitoring remains essential given Greece’s position within one of Europe’s most seismically active domains.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (historical Greece events).
Hellenic Arc tectonic summaries from peer-reviewed geophysical literature.