Seismic Swarm S20210303.1: Analysis of the March 2021 Earthquake Sequence in Greece

Greece lies at the convergence of the African and Eurasian tectonic plates along the Hellenic Arc, a subduction zone responsible for frequent seismic activity. The region experiences both shallow crustal earthquakes and deeper events associated with the subducting slab. Historical records document major events such as the 365 AD Crete earthquake and the 1999 Athens quake, underscoring the area's long-term vulnerability. Since 2000, seismic swarms have been rare, with the prior occurrence noted in 2016.

Seismic swarm S20210303.1 began at 08:44 on 3 March 2021 and concluded at 22:10 on 17 March 2021, registering 510 earthquakes over 349 hours and 26 minutes. The sequence was centered in central Greece near the Thessaly region. Analysis of the first 100 events reveals a classic mainshock-aftershock pattern. Initial foreshocks of magnitudes 2.6 and 2.7 occurred at depths of 6–7 km. The mainshock of magnitude 6.3 struck at 10:16 at 8 km depth, followed within minutes by events of 4.5 and 4.8 at similar depths.

Subsequent activity included a magnitude 5.0 at 10:34 and a 5.3 at 11:45, both near 10 km depth. Later notable shocks reached 5.1 at 18:24 (14 km depth) and 4.4 at 18:49 (10 km depth). Depths across the sequence ranged primarily from 1 to 14 km, indicating shallow crustal faulting consistent with the regional tectonics. Magnitudes clustered between 2.5 and 3.5 after the initial large events, with a gradual decline in frequency and intensity.

This swarm highlights the dynamic stress release along active faults in the Hellenic system. The concentration of events at shallow depths aligns with known crustal deformation patterns in central Greece, where extensional tectonics prevail. No deeper slab-related events appeared in the early sequence, reinforcing the interpretation of a localized crustal swarm.

References

• Hellenic Arc tectonic setting: USGS Earthquake Hazards Program
• 2021 Thessaly sequence parameters: EMSC and NOA catalogs
• Regional seismic history: Greek Institute of Geodynamics annual reports