Seismic Swarm S20070813.1 Near Gáïos, Greece: Geological Context and Event Analysis

The seismic swarm S20070813.1 occurred in the Ionian Sea region of western Greece, centered 9 km north-northeast of Gáïos on the island of Paxos. The sequence began at 02:04 on 12 August 2007 and concluded at 02:28 on 17 August 2007, spanning 120 hours and 24 minutes. During this period, 59 earthquakes were recorded, with magnitudes ranging from 2.5 to 4.2 and focal depths between 0 and 48 km. No single dominant mainshock was evident, consistent with swarm-type activity characterized by clustered events of similar magnitudes over a short interval.

The swarm initiated with a 3.1 magnitude event at 7 km depth, followed rapidly by additional shocks including a 3.3 magnitude event at the surface. Peak activity on 12 August included a 4.2 magnitude earthquake at 10 km depth and a 4.0 magnitude event at 8 km. Subsequent days showed sustained moderate seismicity, with events such as a 3.7 magnitude shock at 2 km depth on 15 August. Depths varied widely, indicating activation across multiple crustal levels without clear migration patterns.

Paxos lies within the tectonically complex Hellenic subduction system, where the African plate converges with the Eurasian plate at rates of approximately 5–10 mm per year. The Ionian Islands occupy a transitional zone influenced by both subduction along the Hellenic Arc and strike-slip faulting associated with the Cephalonia Transform Fault to the north. This setting produces frequent shallow to intermediate-depth seismicity. Paxos itself consists primarily of Mesozoic carbonate platforms overlying older basement rocks, with limited sedimentary cover that can amplify local ground motion during events.

Seismic swarms in this region reflect fluid migration or stress transfer along pre-existing faults rather than classic mainshock-aftershock sequences. Historical records since 2000 indicate only two such swarms in the immediate area, with one occurring in 2003. The 2007 sequence aligns with the broader pattern of episodic unrest in the Ionian Sea, where moderate-magnitude clusters occur periodically without producing destructive surface ruptures.

The 2007 swarm produced no reported damage or casualties, likely due to its moderate magnitudes and offshore location. Monitoring by regional networks continues to track similar activity, underscoring the importance of preparedness in this high-seismicity corridor of the Mediterranean.

References

SeismoSight internal swarm classification data for S20070813.1
USGS Earthquake Hazards Program regional tectonic summaries for the Hellenic Arc
Hellenic National Observatory of Athens seismic bulletins (historical swarm records)