Analysis of the July 2021 Seismic Swarm 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 subduction drives frequent seismic activity across the region, including the Aegean Sea and mainland areas. The Hellenic subduction zone, combined with strike-slip faulting in the North Aegean Trough and extensional tectonics in the Gulf of Corinth, produces both large earthquakes and episodic swarms. Updated tectonic models confirm ongoing convergence rates of approximately 35–40 mm per year, sustaining elevated seismicity.
Seismic swarms consist of clustered earthquakes without a dominant mainshock, often linked to fluid migration or stress redistribution along faults. Swarm S20210720.2 began at 05:05 on 20 July 2021 and concluded at 04:23 on 22 July 2021, spanning 47 hours and 17 minutes. During this interval, 38 earthquakes were recorded, with magnitudes ranging from 2.5 to 4.3 and focal depths between 1 km and 13 km.
The sequence opened with a magnitude 4.3 event at 10 km depth. Subsequent activity included multiple events near 3.0–3.1 magnitude, notably a 3.1 quake at 13 km depth on 21 July at 03:54 and two magnitude 3.0 shocks at shallow depths on 20 and 21 July. The majority of events clustered between 1 km and 10 km depth, indicating shallow crustal involvement typical of Aegean extensional regimes. Activity tapered after the final recorded event of magnitude 2.7 at 2 km depth.
Historical records since 2000 document four seismic swarms in the region: one in 2009, two in 2011, and one prior occurrence in 2021. This distribution reflects the intermittent nature of swarm-type seismicity amid Greece’s persistent tectonic loading.
Such events provide valuable data for refining seismic hazard assessments in a country where moderate swarms can precede or accompany larger tectonic releases. Continued monitoring supports improved understanding of fault interactions within the Hellenic system.
References
SeismoSight internal swarm classification S20210720.2
USGS Earthquake Catalog (Greece regional data)
Hellenic Arc tectonic summaries from peer-reviewed geophysical literature