Seismic Swarm S20080717.1: Analysis of a July 2008 Earthquake Sequence 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 tectonic setting produces frequent seismic activity, including earthquake swarms, across the region. The Hellenic subduction zone has shaped the country's geology for millions of years, contributing to the formation of the Aegean Sea and numerous volcanic features such as those on Santorini and Methana. Seismic swarm S20080717.1 was recorded in Greece beginning at 00:30 on 17 July 2008 and concluding at 18:01 on 31 July 2008. Over 353 hours and 30 minutes, a total of 297 earthquakes were detected. Analysis of the first 100 events reveals a sequence dominated by low-magnitude tremors, with depths predominantly between 0 and 17 km. Magnitudes ranged from 0.7 to 3.2, indicating a typical swarm pattern without a single dominant mainshock. The initial events on 17 July clustered around magnitudes of 1.5 to 2.6 at depths near 10 km. Subsequent activity on 18 and 19 July included several events reaching magnitude 3.0 to 3.2, often at shallower depths of 5 to 8 km. Later events through 20 July maintained similar characteristics, with many occurring at depths of 1 to 11 km. This distribution suggests activity within the upper crust, consistent with regional fault systems influenced by extensional tectonics in the Aegean. Such swarms are common in Greece due to the complex interplay of subduction, back-arc extension, and strike-slip faulting. Historical records document comparable sequences, including swarms near Corinth and the Ionian Islands, often linked to fluid migration or stress transfer along active faults. Updated monitoring by national and international networks confirms ongoing seismicity in these zones, with no significant long-term changes to the fundamental tectonic framework since 2008. The swarm's temporal evolution shows an initial burst followed by sustained lower-level activity, typical of swarm behavior rather than a foreshock-aftershock sequence. Depths remained shallow throughout, aligning with the brittle upper crust in this part of the Mediterranean.

References

• Hellenic Seismic Network reports on Aegean tectonics
• United States Geological Survey tectonic summaries for the Mediterranean region
• SeismoSight internal classification data for swarm S20080717.1