Seismic swarm S20170616.1 occurred near the coast of western Turkey, beginning at 00:12 on 16 June 2017 and concluding at 20:16 on 17 June 2017. Over 44 hours and three minutes, the sequence produced 47 earthquakes. This event represents one of only two documented swarms in the region since 2000, alongside a single swarm in 2011, underscoring the infrequent nature of such clustered activity in this segment of the Aegean tectonic domain.
Western Turkey lies within an active extensional tectonic setting driven by the rollback of the Hellenic subduction zone and the westward extrusion of the Anatolian plate. The crust experiences north-south stretching accommodated primarily by normal faults that trend east-west. This regime produces shallow seismicity, with most events occurring at depths between 5 and 15 km. The coastal zone features a combination of onshore and offshore fault systems that have generated both moderate swarms and larger mainshock-aftershock sequences throughout the Holocene.
The 2017 swarm displayed a typical swarm signature: a gradual increase in event rate, multiple events of comparable magnitude without a single dominant mainshock, and a relatively rapid decay. Magnitudes ranged from 2.0 to 5.2, with the largest shock (magnitude 5.2 at 7 km depth) occurring late in the sequence on 17 June at 19:50. Earlier peaks included a magnitude 4.2 event at 10 km depth on 16 June and another magnitude 4.2 at 12 km depth on 17 June. Depths throughout remained shallow, consistent with the regional normal-faulting environment where brittle failure occurs above the ductile transition zone.
Temporal distribution showed two distinct phases of elevated activity. The first phase on 16 June featured numerous events between 2.0 and 3.4 in magnitude, concentrated between 00:12 and 23:25. A second, more energetic phase developed on 17 June after 19:33, culminating in the magnitude 5.2 shock and followed by several events between 2.9 and 3.8. Such bimodal patterns are characteristic of fluid-driven or stress-transfer processes within fault networks rather than classic aftershock decay.
Regional geological records indicate that western Turkey has experienced repeated seismic episodes linked to the same extensional faults. Instrumental and historical catalogs document destructive earthquakes in the Aegean coastal belt dating back centuries, reflecting long-term strain accumulation and release. The low frequency of swarms—only two recorded since 2000—suggests that most strain release occurs through isolated mainshock sequences rather than prolonged swarm activity.
The 2017 swarm provides a clear illustration of how moderate-magnitude clusters can occur without producing a single dominant rupture. Continued monitoring of similar sequences remains essential for refining seismic hazard assessments along Turkey’s western margin, where population centers lie close to active normal faults.
References
SeismoSight internal swarm catalog (S20170616.1 parameters and historical statistics).
USGS Earthquake Catalog (regional tectonics and depth distribution).
NOAA National Centers for Environmental Information (historical Aegean seismicity).