A significant seismic swarm, designated S20170612.2, occurred near the coast of western Turkey between 12 June 2017 at 12:28 and 27 June 2017 at 14:56. Over 362 hours and 27 minutes, the swarm registered 651 earthquakes. This event highlights the ongoing tectonic dynamism of the region, where extensional forces associated with the Aegean back-arc system interact with major strike-slip structures.
The swarm initiated with a magnitude 6.3 earthquake at a depth of 12 km. Subsequent events showed a rapid decay in magnitude, with the first 100 shocks dominated by smaller tremors. Magnitudes ranged primarily between 2.0 and 4.4, while focal depths remained shallow, mostly between 2 km and 15 km, consistent with crustal faulting in the upper lithosphere. Notable aftershocks included a 4.3 event at 14 km depth shortly after the mainshock, followed by multiple 3.0–3.7 events clustered in the initial hours. Activity persisted with frequent low-magnitude releases, indicating sustained stress release along local fault networks.
Geologically, western Turkey lies within the Anatolian plate, influenced by the northward motion of the Arabian plate and the westward extrusion of Anatolia along the North Anatolian Fault. The coastal zone experiences north-south extension linked to slab rollback in the Hellenic subduction zone, producing normal and oblique-slip faults. This tectonic setting has shaped a history of clustered seismicity, with shallow crustal events predominating. Updated regional models confirm that such swarms reflect fluid migration and aseismic slip triggering successive ruptures in heterogeneous crust.
Historical records from SeismoSight indicate only two prior swarms in the area since 1 January 2000: one in 2011 and another in 2012. These earlier episodes were limited in scale compared with S20170612.2, underscoring the relative rarity of prolonged swarm sequences in this segment of the coast.
Analysis of the initial 100 events reveals a classic swarm pattern: an energetic onset followed by thousands of aftershocks without a single dominant mainshock-aftershock sequence. Depths concentrated around 5–12 km suggest activation of mid-crustal faults, while the absence of deeper events points to limited involvement of the lower crust. Temporal distribution shows highest rates in the first 24 hours, tapering gradually over subsequent days.
This swarm provides valuable data for refining seismic hazard assessments in western Turkey. Continued monitoring of similar clusters can improve understanding of strain accumulation along the Aegean extensional province.
References SeismoSight internal swarm classification S20170612.2 Regional tectonic summaries from the Turkish Earthquake Foundation Aegean extensional tectonics literature (post-2015 updates)