Seismic Swarm S20090217.2 in the Central Mediterranean Sea

Seismic swarm S20090217.2 was recorded in the Central Mediterranean Sea, commencing at 23:36 on 16 February 2009 and concluding at 02:27 on 20 February 2009. Over this 74-hour and 51-minute interval, a total of 67 earthquakes were registered. The events exhibited magnitudes ranging from 2.3 to 3.4, with the majority occurring at shallow depths between 2 and 15 km, though several reached depths up to 41 km. This pattern is characteristic of earthquake swarms, where seismic energy is released through numerous events of similar magnitude without a dominant mainshock, often linked to fluid migration or crustal stress adjustments along active faults.

The swarm initiated with a 3.3 magnitude event at 7 km depth, followed rapidly by additional shocks including 3.0 at 8 km and multiple 3.1–3.2 events clustered at 2 km depth during the early hours of 17 February. Activity persisted with intermittent peaks, such as a 3.4 magnitude event at 6 km on 17 February at 10:11 and deeper occurrences like 3.1 at 41 km later that day. On 18 February, events continued with magnitudes up to 3.3 at depths varying from 2 km to 33 km. The sequence extended into 19 February, featuring several 3.4 magnitude shocks at 8–13 km, before tapering with a final 3.1 event at 7 km on 20 February.

Geologically, the Central Mediterranean Sea occupies a tectonically complex zone at the convergent margin between the African and Eurasian plates. This region encompasses the Ionian Basin and surrounding continental margins, where ongoing convergence drives compressional and strike-slip deformation. The area's seismicity is influenced by the subduction remnants of the Tethyan Ocean and the presence of major structural features such as the Malta Escarpment and the Sicilian-Maltese fault system. Historical records document recurrent moderate seismic activity here, with events typically associated with the broader Mediterranean seismic belt extending from the Hellenic Arc westward.

Insights from the swarm reveal a predominance of shallow crustal events, suggesting activation along upper crustal faults possibly facilitated by hydrothermal processes or pore-pressure changes. The temporal clustering, with over 40 events in the first 24 hours, underscores the swarm's diffuse nature rather than a typical foreshock-mainshock-aftershock sequence. Depths predominantly under 10 km align with brittle failure in the seismogenic layer of the continental crust in this setting.

This episode contributes to understanding regional strain accumulation, where swarms like S20090217.2 may indicate episodic release along secondary faults without producing surface rupture. Updated monitoring in the Mediterranean continues to highlight similar low-to-moderate swarms as common expressions of the plate boundary dynamics.

References SeismoSight internal classification for swarm S20090217.2 USGS Earthquake Catalog for Central Mediterranean tectonics Mediterranean seismicity reviews from the European-Mediterranean Seismological Centre