A seismic swarm designated S20210722.1 occurred 15 km south-southwest of Polichnítos on the island of Lesvos, Greece. The sequence began at 17:58 on 21 July 2021 and concluded at 07:19 on 24 July 2021, spanning 61 hours and 20 minutes. During this interval, 70 earthquakes were recorded.
The swarm featured events with magnitudes between 2.0 and 4.3 and focal depths ranging from 2 km to 15 km. Notable shocks included a magnitude 3.9 event at 23:03 on 21 July at 14 km depth, a magnitude 4.3 event at 00:48 on 22 July at 14 km depth, and a magnitude 4.1 event at 01:20 on 22 July at 10 km depth. Multiple magnitude 3.5–3.7 events clustered between 21 and 23 July, with the majority of activity concentrated in the first 36 hours. Depths remained predominantly in the upper crust, consistent with shallow tectonic release.
Lesvos lies within the Aegean extensional province, where back-arc spreading associated with the Hellenic subduction zone drives active normal faulting. The island sits near the western termination of the North Anatolian Fault system, which accommodates right-lateral strike-slip motion and transfers strain into the Aegean. This tectonic setting produces frequent moderate-magnitude earthquakes and occasional swarm-like sequences along north-northeast-trending normal faults.
Seismic swarms have been documented in the immediate vicinity since 2000, with three recorded episodes: one in 2008, one in 2017, and the 2021 sequence analyzed here. These events reflect episodic fluid migration or aseismic slip along pre-existing fault networks rather than mainshock-aftershock sequences driven by a single large rupture.
The 2021 swarm did not produce reported damage or injuries, consistent with the modest magnitudes involved. Continued monitoring by national and international networks remains essential for tracking strain accumulation in this high-hazard segment of the Aegean.
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