Seismic Swarm S20140112.1: Analysis of Activity in Southern Greece

Southern Greece lies within one of Europe’s most tectonically active zones, shaped by the northward subduction of the African plate beneath the Aegean microplate along the Hellenic Arc. This convergent margin produces frequent shallow to intermediate-depth seismicity, often expressed as earthquake swarms—clusters of events without a single dominant mainshock. The region’s geology features a complex network of normal and strike-slip faults that accommodate both subduction-related compression and back-arc extension in the Aegean Sea.

Swarm S20140112.1 began at 04:12 on 11 January 2014 and concluded at 03:54 on 13 January 2014, spanning 47 hours and 41 minutes. During this interval, 33 earthquakes were recorded. The sequence opened with a magnitude 5.0 event at the surface, followed by numerous smaller shocks whose magnitudes ranged from 2.0 to 3.3. Depths remained predominantly shallow, concentrated between 0 and 17 km, consistent with brittle failure in the upper crust of the Hellenic forearc.

The temporal pattern showed two peaks of activity on 11 January, with events clustered in the early morning and afternoon, then a gradual decline on 12 January punctuated by a magnitude 3.3 shock near midnight. The final recorded event on 13 January registered magnitude 2.8 at 5 km depth. Such rapid succession of similar-sized events without a clear aftershock decay curve is characteristic of swarm behavior driven by fluid migration or aseismic slip rather than static stress transfer from a mainshock.

Historical records maintained by SeismoSight indicate that only two prior swarms have occurred in the same sector since 1 January 2000: one in 2006 and another in 2011. This low frequency underscores that swarm-type sequences remain relatively uncommon compared with typical mainshock-aftershock sequences in southern Greece, yet they provide valuable windows into the underlying stress regime and possible hydrothermal influences along the plate interface.

The shallow focal depths observed throughout S20140112.1 align with the regional velocity structure, where the seismogenic zone is limited to the upper 15–20 km because of elevated geothermal gradients associated with Aegean extension. Continued monitoring of similar swarms can improve understanding of how fluids and slow slip modulate seismic hazard in this densely populated region.

References
SeismoSight internal swarm catalogue (S20140112.1 parameters).
USGS Earthquake Hazards Program – Hellenic Arc tectonics overview (updated 2023).
Papazachos, B.C. et al., “Seismicity and tectonics of the Aegean region,” Tectonophysics (2022 update).