Seismic Swarm S20170528.1: A Detailed Overview of Western Turkey Activity

Western Turkey occupies a tectonically complex zone at the convergence of the African, Eurasian, and Arabian plates. The region features active normal and strike-slip faulting linked to the westward escape of the Anatolian plate along the North Anatolian Fault and extension within the Aegean back-arc. This setting produces frequent shallow seismicity, with historical records documenting destructive events such as the 1999 Izmit earthquake and numerous earlier shocks that have shaped local building practices and monitoring networks.

Between 15:53 on 27 May 2017 and 20:55 on 2 June 2017, the SeismoSight system registered Swarm S20170528.1, comprising 199 earthquakes over 149 hours and 2 minutes. The sequence began with a magnitude 5.1 event at 10 km depth. Subsequent activity included multiple events above magnitude 4.0, notably two magnitude 4.0 shocks within two minutes on 28 May, a magnitude 4.6 event, a magnitude 4.8 event, and a magnitude 4.9 shock at only 3 km depth. Depths throughout the swarm remained predominantly shallow, between 1 km and 12 km, consistent with the extensional regime of western Turkey.

Analysis of the first 100 events shows a rapid aftershock sequence following the initial magnitude 5.1 shock. Magnitudes clustered between 2.0 and 3.5, with 12 events reaching or exceeding magnitude 4.0. The largest events occurred within the first 24 hours, after which activity declined while maintaining a steady rate of smaller shocks. Depths averaged around 6 km, indicating a shallow crustal source volume.

Since 1 January 2000, three swarms have been identified in the same area according to internal classification criteria. Earlier episodes occurred in 2016 (two swarms) and 2017 (one swarm prior to S20170528.1). These recurrent swarms illustrate the persistent seismic productivity of the region without a single dominant mainshock-aftershock pattern.

The 2017 swarm did not produce reported damage or casualties, reflecting both the moderate magnitudes involved and established seismic building standards in Turkey. Continued monitoring by national networks remains essential given the region's high strain rates and history of larger events.

References
SeismoSight internal swarm catalog (S20170528.1 parameters and event list).
Regional tectonic summaries from peer-reviewed literature on Aegean-Anatolian plate interactions.