Seismic Swarm S20070717.1: A July 2007 Earthquake Sequence in Greece

Seismic swarm S20070717.1 occurred in Greece between 12:47 on 17 July 2007 and 06:59 on 20 July 2007, spanning 66 hours and 12 minutes. During this interval, 58 earthquakes were recorded, with magnitudes ranging from 2.3 to 4.9 and focal depths between 0 and 32 km. The sequence exhibited typical swarm characteristics, featuring multiple events of comparable size without a single dominant mainshock.

The swarm began with a magnitude 2.7 event at 19 km depth. Activity intensified rapidly, reaching a peak magnitude of 4.9 at 18:23 on 17 July at 22 km depth. Subsequent events included a magnitude 4.0 at 21:09 and a magnitude 3.8 at 20:00, both at shallower depths. On 19 July, another significant shock of magnitude 4.6 occurred at 18 km depth. The final recorded event was a magnitude 2.6 at surface depth on 20 July.

Greece lies at the convergent boundary between the African and Eurasian tectonic plates. The region experiences ongoing subduction along the Hellenic Arc, where the African plate descends beneath the Aegean plate at rates of approximately 35–40 mm per year. This tectonic setting produces frequent shallow to intermediate-depth seismicity across the Aegean Sea and mainland Greece. The Hellenic subduction zone has generated some of Europe’s largest historical earthquakes, including the 365 AD event near Crete that triggered a major tsunami.

Earthquake swarms in Greece often occur in areas of crustal extension or near volcanic centers such as the South Aegean Volcanic Arc. While the precise epicentral area of swarm S20070717.1 remains unspecified in available records, the depth distribution—predominantly between 0 and 22 km—aligns with the brittle upper crust typical of the Aegean back-arc region. Such swarms may reflect fluid migration or stress redistribution along fault networks rather than classic mainshock-aftershock sequences.

The 2007 swarm contributed to the broader understanding of clustered seismicity in the eastern Mediterranean. Modern monitoring by the Hellenic Unified Seismic Network and international agencies has improved detection thresholds, allowing detailed catalogs of similar low-to-moderate magnitude sequences. Updated seismic hazard assessments for Greece incorporate these data to refine probabilistic models, particularly for regions with elevated swarm activity.

In summary, swarm S20070717.1 illustrates the persistent seismic productivity of the Hellenic tectonic domain. Continued observation of such sequences supports improved forecasting of ground shaking and informs building-code updates across the country.

References

• Hellenic Unified Seismic Network (HUSN) earthquake catalog
• United States Geological Survey (USGS) Earthquake Hazards Program
• McKenzie, D. (1972). Active tectonics of the Mediterranean region. Geophysical Journal International.