Seismic Swarm S20200122.4 in Western Turkey: Analysis and Regional Context

Western Turkey occupies a tectonically active zone shaped by the interaction of the African, Arabian, and Eurasian plates. The region experiences extensional tectonics linked to the Aegean back-arc system and strike-slip motion along the North Anatolian Fault Zone. These forces produce frequent shallow crustal earthquakes, with historical events including the destructive 1999 Izmit earthquake of magnitude 7.6. Seismicity clusters often occur along secondary faults branching from major plate boundaries.

Seismic swarms represent episodic bursts of activity without a single dominant mainshock. In Western Turkey, such swarms have been documented since instrumental recording began. Since 2000, three swarms have been identified in the broader region: one in 2001 and two in 2016. These episodes typically involve hundreds of events concentrated over days to weeks, with magnitudes rarely exceeding 6.0 and focal depths under 20 km.

Swarm S20200122.4 began at 19:22 on 22 January 2020 and concluded at 04:30 on 16 February 2020, spanning 585 hours and 8 minutes. During this interval, 673 earthquakes were registered across Western Turkey. The sequence opened with a magnitude 5.6 event at 5 km depth, followed rapidly by aftershocks. Magnitudes ranged from 2.5 to 5.6, with the majority falling between 2.5 and 3.9. Depths remained predominantly shallow, between 1 km and 19 km, consistent with upper-crustal faulting in the extensional regime.

Examination of the first 100 events reveals a rapid onset and sustained high rate of occurrence. The initial 24 hours accounted for the strongest shaking, including events of magnitude 4.5, 4.3, 4.1, 4.2, 4.4, and 4.0. Depths for these larger shocks clustered between 5 km and 14 km. Subsequent events showed a gradual decline in magnitude while maintaining similar depth distribution. By the end of the first 100 events, activity had transitioned to lower-energy releases, with most magnitudes between 2.5 and 3.6. This pattern indicates fluid-driven or stress-transfer mechanisms typical of swarm behavior rather than classic mainshock-aftershock sequences.

The swarm's temporal evolution aligns with known characteristics of Western Turkish seismicity, where clusters often migrate along fault segments over periods of weeks. No surface rupture was associated with the largest events, reflecting the modest energy release. Cumulative moment from the sequence remained well below thresholds that produce significant damage in populated areas.

Ongoing monitoring by regional networks continues to refine understanding of swarm recurrence. Future episodes may provide additional data on the interplay between regional extension and local fault structures.

References

SeismoSight internal classification records for Swarm S20200122.4.
United States Geological Survey Earthquake Catalog (regional Western Turkey events).
European-Mediterranean Seismological Centre historical bulletins.