Seismic Swarm S20151129.1 Southwest of Lefkáda, Greece

The seismic swarm designated S20151129.1 occurred approximately 20 km southwest of Lefkáda in the Ionian Islands of western Greece. Registered between 13:08 on 28 November 2015 and 02:36 on 2 December 2015, the sequence lasted 85 hours and 28 minutes and comprised 53 earthquakes. All events were of low to moderate magnitude, with the largest reaching 4.4 at a depth of 27 km on 2 December. Most events clustered at shallow depths between 1 km and 12 km, consistent with activity along regional fault structures.

This swarm exemplifies typical clustered seismicity in the Ionian domain, where numerous small earthquakes occur without a dominant mainshock. Magnitudes ranged from 2.0 to 4.4, and the temporal distribution showed peak activity on 29 November, including multiple events above magnitude 3.0. Depths remained predominantly crustal, reflecting brittle failure in the upper lithosphere.

Lefkáda lies within one of Europe’s most seismically active zones, situated along the boundary between the African and Eurasian plates. The area is influenced by the Cephalonia-Lefkada Transform Fault, which accommodates dextral strike-slip motion and connects the Hellenic subduction zone to the north. This tectonic setting produces frequent moderate earthquakes and occasional larger events, with historical records documenting destructive shocks in 1825, 1948, and 2003.

Since 2000, four swarms have been recorded in the immediate region according to internal classification data. Earlier episodes occurred in 2003 (two swarms) and 2015 (two swarms, including S20151129.1). These sequences highlight the episodic nature of background seismicity along the transform system.

The 2015 swarm produced no reported damage, consistent with its modest magnitudes and offshore location. Such events contribute to ongoing strain release and provide valuable data for refining local seismic hazard models. Continued monitoring remains essential given the region’s proximity to populated islands and its position within the broader Mediterranean tectonic framework.

References
SeismoSight internal swarm classification S20151129.1
Institute of Geodynamics, National Observatory of Athens earthquake catalog
USGS Earthquake Hazards Program tectonic summaries for the Ionian Sea