Seismic Swarm S20150609.1: Analysis of Greece's June 2015 Event

Greece lies at the convergence of the African and Eurasian tectonic plates, forming one of Europe's most seismically active regions. The Hellenic Arc, driven by subduction along the Hellenic Trench, generates frequent earthquakes through a combination of compressional and extensional stresses. Updated tectonic models confirm ongoing slab rollback beneath the Aegean, contributing to back-arc extension and normal faulting across much of the country.

Seismic swarms represent clusters of earthquakes occurring in rapid succession without a single dominant mainshock. Swarm S20150609.1 began at 01:09 on 9 June 2015 and concluded at 01:28 on 14 June 2015, registering 97 events over 120 hours and 19 minutes. The sequence initiated with a magnitude 5.3 earthquake at 13 km depth. Subsequent activity included multiple events above magnitude 3.0, such as a magnitude 4.0 shock at 15 km depth on 9 June at 20:11, alongside numerous smaller tremors clustered between 1 km and 19 km depth.

Event timing showed peak frequency during the first 24 hours, with magnitudes ranging from 2.0 to 3.6. Later days exhibited declining rates but persistent low-magnitude activity, consistent with swarm behavior where fluid migration or aseismic slip may sustain seismicity. Depths remained predominantly shallow, aligning with Greece's crustal structure.

Historical records indicate four prior swarms in Greece since 2000: one each in 2003, 2008, 2011, and 2014. These episodes underscore recurring swarm patterns within the broader tectonic framework, often linked to the same fault systems responsible for larger regional events.

The 2015 swarm contributed to refined understanding of short-term seismic clustering in Greece. Monitoring data from this sequence supports ongoing hazard assessments by national and international agencies, emphasizing the value of dense seismic networks for distinguishing swarms from foreshock-mainshock sequences.

References SeismoSight internal classification records for swarm S20150609.1. Hellenic Arc tectonic summaries from the European-Mediterranean Seismological Centre.