Seismic Swarm S20100118.1: Analysis of a Prolonged 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 subduction zone drives intense seismic activity across the region, including the Hellenic Trench and associated fault systems in the Aegean Sea. The country experiences frequent earthquakes, with historical records documenting destructive events such as the 365 AD Crete earthquake and the 1953 Kefalonia sequence, which caused widespread damage. Modern monitoring reveals ongoing microseismicity linked to plate motion rates of approximately 35–40 mm per year.

Swarm S20100118.1 began at 08:06 on 18 January 2010 and concluded at 12:46 on 1 February 2010, spanning 340 hours and 39 minutes. During this period, 837 earthquakes were recorded across Greece. The sequence featured predominantly shallow events, with most hypocenters between 0 and 17 km depth. Magnitudes ranged from 1.9 to 5.5, with the largest shock occurring at 15:56:09 on 18 January at a reported depth of 0 km.

Examination of the first 100 events shows a rapid onset of activity on 18 January, with 30 events recorded that day alone. Early shocks included a 2.5 magnitude event at 5 km depth, followed by a 2.8 at similar depth, and then the 5.5 mainshock. Subsequent events clustered tightly in time, with many magnitudes between 2.3 and 3.7 occurring within minutes of each other. Depths remained shallow, rarely exceeding 17 km, and several events registered at 0–2 km, indicating possible near-surface faulting. By 19 January, activity continued at a steady rate, featuring repeated 2.0–3.0 magnitude tremors at depths of 5–15 km. This pattern reflects typical swarm behavior, where energy release occurs through numerous moderate events rather than a single dominant shock followed by aftershocks.

Greece has recorded 10 such swarms since 2000, with three occurring in 2008 and seven in 2009. These sequences underscore the region's persistent tectonic stress accumulation and release along active fault networks. The 2010 swarm aligns with this history, contributing to the cumulative seismic record that informs hazard assessment models used by Greek authorities.

Overall, Swarm S20100118.1 exemplifies the dynamic seismicity of the Hellenic subduction system. Continued monitoring of similar sequences supports improved understanding of fault interactions and aids in refining preparedness measures for future events.