Seismic Swarm S20220831.1 in the Dodecanese Islands, Greece

The Dodecanese Islands form part of the southeastern Aegean Sea and lie within one of Europe’s most seismically active zones. This archipelago sits above the Hellenic subduction zone, where the African plate descends beneath the Eurasian plate at rates of approximately 4–5 cm per year. The resulting crustal extension and strike-slip faulting produce frequent earthquake swarms and moderate mainshock-aftershock sequences. Historical records document destructive events in 1481, 1926, and 1956, while instrumental monitoring since the late twentieth century has recorded hundreds of events annually across the broader Aegean.

Seismic swarm S20220831.1 began at 09:56 UTC on 31 August 2022 and concluded at 07:49 UTC on 2 September 2022, spanning 45 hours and 53 minutes. During this interval, 61 earthquakes were recorded. The sequence initiated with a magnitude 4.7 event at 10 km depth, rapidly followed by the largest event of the swarm—a magnitude 5.4 earthquake, also at 10 km depth. Subsequent activity consisted predominantly of small-magnitude events (M 2.0–3.3) occurring at shallow focal depths between 1 km and 18 km. Notable later shocks included a magnitude 4.6 event at 14:02 UTC on 31 August and several magnitude 3+ events distributed through the first day.

The temporal distribution showed highest productivity in the first 12 hours, with 34 events recorded by 22:00 UTC on 31 August. Depths remained consistently shallow, consistent with the extensional faulting typical of the Aegean back-arc region. No event exceeded magnitude 5.4, and the swarm decayed gradually, ending with isolated magnitude 2.0 events on 2 September.

Since 1 January 2000, only two earthquake swarms have been identified in the Dodecanese Islands according to SeismoSight classification. The earlier swarm occurred in 2020, indicating that such clustered, moderate-magnitude sequences remain relatively infrequent in the instrumental record for this specific sector of the Hellenic Arc.

The 2022 swarm occurred within a tectonic setting dominated by north-south extension and east-west shear. Focal mechanisms for regional events typically reveal normal and strike-slip faulting on planes oriented roughly east-west and northeast-southwest. Shallow hypocenters reflect brittle failure within the upper 15–20 km of crust, above the deeper subduction interface.

Continued monitoring by national and international networks remains essential for understanding swarm recurrence and potential links to fluid migration or stress transfer along Aegean faults. The 2022 sequence did not produce reported damage, consistent with its moderate magnitudes and offshore location.

References

SeismoSight internal swarm classification S20220831.1
Hellenic National Seismic Network (HL) event catalogue
USGS Earthquake Hazards Program regional tectonics summary
Papazachos, B.C. et al., “Seismicity and tectonics of the Aegean,” Tectonophysics (updated compilations through 2023)