Seismic Swarm in the Dodecanese Islands, Greece: October 31, 2020 Event

The Dodecanese Islands region of Greece forms part of the tectonically complex Hellenic Arc, where the African plate subducts beneath the Eurasian plate. This convergence drives high seismicity across the Aegean, with frequent shallow crustal events and occasional deeper activity along the subduction interface. The islands sit near major fault systems that accommodate both extensional and strike-slip deformation, contributing to a long record of earthquake clusters.

On October 31, 2020, SeismoSight registered Swarm S20201031.2 in the Dodecanese Islands. The sequence began at 04:12 and ended at 19:22, spanning 15 hours and 9 minutes during which 27 earthquakes were recorded. Magnitudes ranged from 2.0 to 4.3, with focal depths mostly between 2 km and 26 km, consistent with shallow crustal processes typical of the region.

The swarm opened with a magnitude 3.9 event at 8 km depth, followed minutes later by a 3.0 event at 10 km. Subsequent activity included multiple events near 3.0–3.1 at depths of 5–20 km through the morning hours. A notable increase occurred in the afternoon, highlighted by the largest event of the sequence—a magnitude 4.3 earthquake at 10 km depth at 14:42. Later events included two magnitude 3.5 shocks at 15 km and 10 km, respectively, before activity tapered off by early evening.

Depth distribution showed a concentration in the upper 10 km, with only a few events exceeding 20 km. This pattern suggests brittle failure within the seismogenic crust rather than deeper subduction-related rupture. The short duration and moderate magnitudes align with swarm behavior driven by fluid migration or stress triggering along local faults, rather than a mainshock-aftershock sequence.

Historical records maintained by SeismoSight indicate that only two swarms have occurred in the Dodecanese Islands since January 1, 2000. The October 31, 2020 swarm represents the first of these events, underscoring the episodic nature of clustered seismicity in this part of the Aegean.

Such swarms provide valuable data for refining local seismic hazard models. Continued monitoring helps distinguish between background tectonic strain release and transient clusters that may influence short-term probability estimates.

References
SeismoSight internal swarm classification database
USGS Earthquake Hazards Program regional tectonics overview
Hellenic Arc geological framework summaries from peer-reviewed geophysical literature