Seismic Swarm S20250420.1 in Western Turkey: Characteristics and Regional Context

A seismic swarm designated S20250420.1 occurred in Western Turkey from 10:42 on 19 April 2025 to 15:43 on 23 April 2025. Over 101 hours, the event sequence registered 107 earthquakes. Analysis of the first 100 events reveals a cluster of low-to-moderate magnitude activity concentrated at shallow depths, consistent with the region's ongoing tectonic deformation.

The swarm initiated with events of magnitude 2.4 and 2.2 at depths of 6 km and 11 km, respectively. Magnitudes ranged from 0.7 to 4.0, with the largest shock (M4.0) recorded at 17:24 on 21 April at 9 km depth. Depths predominantly fell between 5 km and 15 km, indicating activity within the upper crust. Multiple events clustered around 7–12 km, reflecting typical rupture zones in the local fault network. Time distribution showed peak activity on 21 April, with 28 events in the initial dataset occurring that day.

Western Turkey occupies a tectonically active zone at the western termination of the North Anatolian Fault and within the Aegean extensional province. This setting arises from the northward motion of the Arabian Plate and the rollback of the Hellenic subduction zone, producing widespread normal and strike-slip faulting. The crust experiences rapid extension rates of 20–30 mm per year, accommodated by numerous active faults that generate both mainshock-aftershock sequences and swarm-type activity.

Historical records document eight swarms in the region since 2000, occurring in 2009 (one swarm), 2011 (four swarms), and 2012 (three swarms). These episodes share similar characteristics with S20250420.1, featuring dense clusters of small-magnitude events over days to weeks without a dominant mainshock. Such swarms are common in areas of fluid migration or aseismic slip along faults, though specific triggering mechanisms for individual sequences require detailed modeling.

The shallow focal depths observed in the current swarm align with the brittle-ductile transition in Western Turkey’s continental crust, typically 10–15 km. This depth range facilitates frequent microseismicity that contributes to long-term strain release. No damage or felt reports beyond instrumental detection were associated with the sequence, underscoring its low-energy nature.

Continued monitoring remains essential given the region’s seismic potential. Integration of swarm statistics with regional fault maps supports improved probabilistic assessments of future activity.

References
SeismoSight internal swarm classification database.
USGS Earthquake Hazards Program regional tectonic summaries.
Peer-reviewed literature on Aegean extensional tectonics (post-2020 updates).