Seismic Swarm S20080922.1: Analysis of the September 2008 Event in Greece

Greece lies at the convergence of the African and Eurasian tectonic plates along the Hellenic subduction zone, one of Europe's most seismically active regions. This setting produces frequent earthquakes through a combination of subduction, crustal extension in the Aegean, and strike-slip faulting. The Hellenic Arc has generated major historical events, including the 365 CE Crete earthquake and the 1999 Athens quake, underscoring the area's long-term seismic hazard.

Between 07:15 on 21 September 2008 and 09:40 on 22 September 2008, a swarm comprising 31 earthquakes was recorded in Greece. Magnitudes ranged from 1.3 to 3.2, with focal depths between 5 km and 37 km. The sequence began with events of 2.8 and 2.9 at depths of 35 km and 20 km, respectively. Subsequent activity included several magnitude-3.2 shocks at varying depths, such as those at 15 km, 28 km, 22 km, and 20 km. Smaller events dominated later stages, with magnitudes mostly below 2.0 and depths clustering around 15–25 km. The swarm concluded after 26 hours and 24 minutes without a dominant mainshock, a pattern typical of swarm-type seismicity driven by fluid migration or stress redistribution along faults.

Such swarms differ from standard aftershock sequences by lacking a clear triggering event of significantly larger magnitude. In the Greek tectonic environment, they often reflect localized adjustments within the extensional back-arc basin or along the subduction interface. Historical records since 2000 indicate only three prior swarms in the same classification system: one each in 2003, 2007, and this 2008 episode. These infrequent occurrences highlight that while background seismicity remains elevated, clustered swarm behavior is relatively uncommon.

The 2008 swarm's depth distribution, spanning both shallow crustal and mid-crustal levels, aligns with the known seismogenic thickness in the Hellenic region. Shallower events near 5–15 km likely occurred within the brittle upper crust, whereas deeper shocks reflect activity near the plate interface. No damage or felt reports were associated with these low-to-moderate magnitudes, consistent with the modest energy release.

Continued monitoring by national and international networks remains essential for distinguishing swarm activity from potential foreshock sequences in this high-hazard setting.

References

• Hellenic National Seismic Network (HNSN) bulletins, 2008–2023.
• USGS Earthquake Catalog, global events 2000–present.
• McKenzie, D. (1972). Active tectonics of the Mediterranean region. Geophysical Journal International.