Seismic Swarm Analysis: Dodecanese Islands, Greece (October–November 2020)

The Dodecanese Islands occupy a tectonically complex segment of the southeastern Aegean Sea, situated above the Hellenic subduction zone where the African plate descends beneath the Eurasian plate. This convergence generates frequent shallow to intermediate-depth seismicity across the region. The islands themselves rest on a mosaic of continental fragments and accreted oceanic material, with active normal and strike-slip faults accommodating both extensional and shear stresses.

Between 12:45 UTC on 30 October 2020 and 05:21 UTC on 2 November 2020, a swarm comprising 47 earthquakes was recorded in the Dodecanese Islands. The sequence lasted 64 hours and 36 minutes. Magnitudes ranged from 2.0 to 4.2, with the largest event (M 4.2) occurring at 14:46 UTC on 30 October at a depth of 10 km. A second notable shock of M 4.1 followed at 22:53 UTC the same day, also at 10 km depth. Depths throughout the swarm remained predominantly shallow, concentrated between 5 km and 12 km, consistent with brittle failure in the upper crust.

Temporal clustering was evident in the first 12 hours, when 14 events exceeded M 2.5, including five above M 3.0. Subsequent activity showed a gradual decline in both rate and magnitude, although sporadic events continued until the swarm terminated early on 2 November. Depths showed no systematic migration, remaining within a narrow 2–16 km window and suggesting a localized source volume.

Historical records maintained since 1 January 2000 indicate that only one earthquake swarm has been documented in the Dodecanese Islands prior to the present sequence. That earlier swarm also occurred in 2020, underscoring the relative infrequency of swarm-type behavior in this segment of the Hellenic Arc despite the region’s overall high seismicity rate.

The 2020 swarm provides a clear example of episodic, non-mainshock-aftershock seismicity typical of fluid-influenced or aseismic-slip-triggered faulting in the Aegean. Continued monitoring remains essential given the proximity of the islands to populated centers and the potential for larger events along the same fault systems.

References
SeismoSight internal classification database (swarm S20201031.1 parameters and event list)
Hellenic Arc tectonic framework summaries from peer-reviewed geophysical literature (post-2015 updates)