Seismic Swarm S20070325.1: Analysis of Activity Near Lixouri, Greece

A significant seismic swarm occurred northwest of Lixouri on the island of Cephalonia, Greece, from 13:57 on 25 March 2007 to 21:56 on 11 April 2007. In 415 hours and 58 minutes, the swarm produced 255 earthquakes. Cephalonia lies within the tectonically complex Ionian Sea region, where the African plate subducts beneath the Eurasian plate along the Hellenic Arc. This setting is further influenced by the Cephalonia Transform Fault, a major dextral strike-slip structure that accommodates differential motion between the Apulian and Aegean microplates.

The island’s geology features Mesozoic carbonates thrust over Cenozoic flysch, creating a landscape prone to both seismic shaking and associated hazards such as coastal uplift or subsidence. Historical records document repeated destructive events, including the catastrophic 1953 earthquakes that devastated much of the island. Since 2000, only one prior swarm has been identified in the immediate area, occurring in 2003, underscoring the episodic nature of clustered seismicity in this segment of the fault system.

The 2007 swarm initiated with a magnitude 5.7 event at 15 km depth. Subsequent activity included 99 additional events within the first several days, with magnitudes ranging from 2.2 to 4.6 and focal depths predominantly between 0 and 22 km. Shallow hypocenters (less than 10 km) were common, consistent with brittle failure along the upper crustal portion of the Cephalonia Fault Zone. The sequence displayed classic swarm characteristics: an abrupt onset, absence of a single dominant mainshock-aftershock decay pattern, and sustained elevated rates over more than two weeks.

Magnitudes clustered between 3.0 and 4.0, with occasional events exceeding 4.0. Depth distribution showed no strong migration, suggesting distributed failure across a fault network rather than simple unilateral propagation. The largest events occurred early in the sequence, after which activity gradually declined while maintaining a steady background rate until termination on 11 April.

Such swarms are typical of the Ionian Islands, where fluid involvement or stress transfer along intersecting faults can trigger prolonged clusters without producing a single large rupture. The 2007 activity did not escalate into a major mainshock, illustrating the region’s capacity for moderate, distributed seismicity that nonetheless poses risks to infrastructure and communities.

References

USGS Earthquake Catalog
Institute of Geodynamics, National Observatory of Athens
Greek Seismological Network reports
Papazachos, B.C. et al., 2000. Seismological and tectonic features of the Ionian Islands.