Seismic Swarm Analysis: Dodecanese Islands, Greece (Swarm S20201030.2)

The Dodecanese Islands occupy a tectonically active segment of the Aegean Sea where the African plate subducts beneath the Eurasian plate along the Hellenic Arc. This convergent boundary generates frequent moderate-to-large earthquakes, with the region also influenced by strike-slip faulting linked to the westward extrusion of the Anatolian plate. Shallow crustal depths predominate, typically under 30 km, reflecting the complex interplay of subduction-related thrusting and extensional tectonics in the overriding plate. Historical seismicity includes destructive events such as the 1881 Chios earthquake and the 2017 Kos earthquake (Mw 6.6), both of which produced significant ground shaking across nearby islands. Updated regional monitoring confirms ongoing activity consistent with long-term strain accumulation along the arc. Swarm S20201030.2 began at 11:51 UTC on 30 October 2020 and concluded at 00:36 UTC on 24 November 2020, encompassing 588 hours and 45 minutes during which 881 earthquakes were recorded. The sequence initiated with a magnitude 7.0 mainshock at 21 km depth. Subsequent analysis of the first 100 events reveals a classic aftershock decay pattern: magnitudes ranged from 2.5 to 5.3, with the majority falling between 2.5 and 4.0. Depths clustered between 1 km and 33 km, though most aftershocks occurred at 5–10 km, indicating activation of shallow crustal structures. Following the mainshock, the initial 30 minutes featured multiple events above magnitude 4.0, including a 4.9 at 13:00 UTC and a 4.6 pair within the first seven minutes. Activity remained elevated for several hours, with events such as the 5.3 at 15:14 UTC marking a notable secondary peak. Depths showed modest variation, trending slightly shallower after the first hour, consistent with fluid migration or stress redistribution in the upper crust. Overall, the first 100 events demonstrate rapid energy release concentrated in the hours immediately after the mainshock, with decreasing frequency and magnitude thereafter. This temporal distribution aligns with Omori-law aftershock behavior observed in subduction-zone settings. No events exceeded the mainshock magnitude, and the sequence remained confined to the Dodecanese region without migration beyond typical aftershock distances. The swarm underscores the persistent seismic hazard of the Hellenic Arc and the value of dense local networks for real-time tracking of post-mainshock sequences.

References

• Hellenic Arc tectonics: McKenzie (1972), Le Pichon & Angelier (1979), updated GPS studies (2020–2023).
• Regional earthquake catalog: National Observatory of Athens, USGS Earthquake Catalog (2020 event parameters).