Seismic Swarm S20070629.1: Analysis of Activity Near Paxos, Greece

The seismic swarm S20070629.1 occurred in the Ionian Sea region of Greece, centered 12 km north of Gáïos on the island of Paxos. It began at 18:09 on 29 June 2007 and concluded at 16:18 on 6 July 2007, spanning 166 hours and 9 minutes. During this interval, 138 earthquakes were recorded, marking the second such swarm in the area since 2000, following an earlier event in 2003. Paxos lies within the tectonically active Ionian Islands, part of the broader Hellenic arc system. This zone results from the convergence between the African and Eurasian plates, producing frequent seismic events through a combination of strike-slip faulting and thrust mechanisms. The crust in this sector exhibits moderate to high seismicity, with many events occurring at shallow depths due to the regional fault networks. Analysis of the first 100 events reveals a sequence dominated by moderate-magnitude shocks. The largest was a magnitude 5.4 earthquake at 10 km depth on 29 June at 18:09:11 UTC. Subsequent activity included multiple events between magnitude 3.0 and 4.8, such as a 4.8 shock at 0 km depth later that evening and a 4.4 event at 16 km depth. Depths ranged widely from 0 km to 58 km, though the majority clustered between 0 and 10 km, indicating predominantly shallow crustal involvement. Smaller events (magnitude 2.4–3.6) occurred at irregular intervals, with some deeper occurrences exceeding 30 km, including several at 50–58 km on 30 June and 1 July. The temporal distribution showed peak activity in the first 48 hours, followed by a gradual decline. Magnitudes generally decreased after the initial mainshock, consistent with swarm behavior rather than a classic mainshock-aftershock sequence. Depths remained variable throughout, suggesting activation across multiple fault segments at differing crustal levels. This swarm underscores the ongoing seismic hazard in the Paxos vicinity, where historical patterns indicate episodic clustering of events within an otherwise diffuse background of low-to-moderate activity. Continued monitoring supports improved understanding of strain accumulation along the Ionian margin.

References

SeismoSight internal swarm classification records. USGS Earthquake Hazards Program regional tectonics summaries for the Ionian Sea.