Seismic Swarm S20100510.1: Earthquake Activity in Greece, May 2010

Greece lies at the convergent boundary between the African and Eurasian tectonic plates, where the Hellenic Arc subduction zone drives frequent seismic activity. The region experiences both shallow crustal events and deeper earthquakes linked to slab subduction, with historical records documenting destructive quakes over centuries. This tectonic setting produces episodic swarms—clusters of events without a single dominant mainshock—often tied to fluid migration or stress transfer along faults. Swarm S20100510.1 was recorded in Greece beginning at 17:48 on 9 May 2010 and concluding at 02:58 on 18 May 2010. Over 201 hours and 9 minutes, 239 earthquakes were detected. Historical statistics since 2000 indicate 17 comparable swarms in the country, distributed as follows: one in 2007, four in 2008, seven in 2009, and five in 2010. Analysis of the first 100 events reveals predominantly low-to-moderate magnitudes, ranging from 1.3 to 3.9, with the majority between 2.0 and 3.0. Depths clustered in the shallow to mid-crustal range of 2–15 km, though a few reached 19–25 km. The sequence began with a 2.7 magnitude event at 5 km depth, followed by a 3.3 magnitude shock later the same evening. Activity intensified on 13 May, including a 3.9 magnitude event at only 2 km depth and multiple events above magnitude 3.0 within hours. Temporal distribution showed sustained occurrence across day and night periods, with clusters of three to five events within short intervals on several days. Depths remained consistent overall, suggesting a localized source volume rather than progressive migration. This swarm fits the pattern of Greece’s recurrent low-magnitude seismic episodes, which rarely produce surface damage but contribute to ongoing strain release along the Hellenic Arc. Continued monitoring supports regional hazard assessment in this highly active tectonic environment. References SeismoSight internal swarm classification records Institute of Geodynamics, National Observatory of Athens USGS Earthquake Hazards Program regional tectonic summaries