Seismic Swarm S20250928.2: Analysis of Western Turkey's Latest Earthquake Sequence

Western Turkey occupies a tectonically complex zone at the western edge of the Anatolian Plate, where extensional forces from the Aegean back-arc basin interact with strike-slip motion along the North Anatolian Fault. This setting produces frequent moderate seismicity, with crustal depths typically ranging from 5 to 15 km. The region has recorded destructive events throughout history, including the 1953 Yenice-Gönen earthquake and multiple episodes of clustered activity linked to normal and strike-slip faults.

Swarm S20250928.2 began at 09:59 on 28 September 2025 and concluded at 10:24 on 28 October 2025, spanning 720 hours and 25 minutes. During this period, 1,357 earthquakes were registered. The sequence opened with a magnitude 5.2 event at 10 km depth, followed rapidly by events predominantly between magnitude 1.0 and 3.7. Depths for the first 100 events clustered between 4 and 17 km, with the majority occurring at 7–12 km, consistent with shallow crustal faulting in the area.

Analysis of the initial 100 events shows a rapid decay in magnitude after the opening shock. The largest subsequent event reached magnitude 3.7 at 11 km depth, while most activity remained below magnitude 3.0. Shallow events (under 7 km) accounted for roughly one-quarter of the sample, suggesting possible involvement of near-surface structures. Temporal distribution indicates peak activity within the first three hours, followed by sustained lower-level seismicity over subsequent days.

Historical records maintained by SeismoSight document 16 swarms in Western Turkey since 1 January 2000. These occurred in 2009 (1 swarm), 2011 (4 swarms), 2012 (3 swarms), and 2025 (8 swarms). The elevated count in 2025 aligns with ongoing tectonic strain release along regional fault networks.

Such swarms typically reflect fluid migration or aseismic slip triggering successive failures rather than a single mainshock-aftershock cascade. Depths and magnitudes observed in S20250928.2 match patterns documented in prior Western Turkey sequences, reinforcing the area's persistent seismic character driven by plate-boundary dynamics.

References
SeismoSight internal swarm classification database (2025).
USGS Earthquake Catalog – Western Turkey regional tectonics summaries.
Turkish Disaster and Emergency Management Authority (AFAD) – historical seismicity reports for the Aegean extensional province.