Seismic Swarm PS20181026.1 Near Zakynthos, Greece: Geological Context and Event Analysis

The Ionian Islands region of western Greece lies within one of Europe’s most seismically active zones. The area southwest of Lithakiá on Zakynthos is influenced by the convergent boundary between the African and Eurasian plates, where the African plate subducts beneath the Aegean plate along the Hellenic Trench. This tectonic setting produces frequent earthquakes, often at shallow to intermediate depths, and has shaped the islands’ geology through repeated uplift, faulting, and basin formation over millions of years.

On 25–26 October 2018, a seismic swarm designated PS20181026.1 occurred approximately 33 km southwest of Lithakiá. The sequence began at 22:22 UTC on 25 October and concluded at 12:41 UTC on 26 October, lasting 14 hours and 18 minutes. Eight earthquakes were recorded during this period. The largest event reached magnitude 6.8 at a depth of 14 km, while the remaining events ranged from magnitude 4.3 to 5.1, all occurring at depths of 10 km except the mainshock.

The temporal distribution shows rapid onset followed by a gradual decline in activity. After the initial magnitude 5.1 foreshock at 22:22 on 25 October, the magnitude 6.8 mainshock occurred 32 minutes later. Subsequent events included two magnitude 5.1 shocks and three magnitude 5.0 shocks, with the final recorded event at 12:41 on 26 October. Depths remained consistently shallow, consistent with the region’s upper-crustal fault systems.

This swarm fits into a longer pattern of clustered seismicity in the Ionian Islands. Since 2000, fifteen swarms have been documented in the broader area. These occurred in 2003 (1 swarm), 2006 (1), 2007 (3), 2008 (2), 2009 (2), 2011 (2), 2014 (3), and 2015 (1). Such episodic activity reflects the ongoing accommodation of plate convergence through both large single events and distributed swarm sequences along the subduction interface and associated strike-slip faults.

The Hellenic subduction zone has produced destructive earthquakes throughout recorded history, including events that have altered coastlines and triggered tsunamis. Modern monitoring networks operated by Greek and European agencies continue to track microseismicity to improve hazard assessment. The 2018 swarm, while energetic, remained offshore and caused limited impact on land, underscoring the protective role of distance from population centers.

References

• Hellenic Seismic Network reports (Institute of Geodynamics, National Observatory of Athens)
• USGS Earthquake Catalog
• European-Mediterranean Seismological Centre (EMSC) data archives