Seismic Swarm S20170422.1: Western Turkey Earthquake Sequence of April 2017
A seismic swarm designated S20170422.1 occurred in Western Turkey between 14:12 UTC on 21 April 2017 and 13:58 UTC on 27 April 2017. The sequence lasted 143 hours and 46 minutes, during which 119 earthquakes were recorded. The first 100 events provide a clear picture of swarm behavior characterized by a rapid onset, absence of a dominant mainshock, and a steady release of energy through numerous small to moderate tremors.
The initiating event reached magnitude 4.9 at a depth of 10 km. Subsequent activity remained predominantly shallow, with depths ranging from 1 km to 28 km and most events occurring between 4 km and 11 km. Magnitudes clustered between 2.0 and 3.3, with only a few exceeding 3.0. Notable events included a magnitude 3.6 earthquake at 10 km depth on 22 April and magnitude 3.3 events on 22 and 23 April. The temporal distribution showed the highest frequency within the first 48 hours, followed by a gradual decline while maintaining a consistent low-magnitude character.
Western Turkey lies within the Aegean extensional province, a tectonically active region shaped by the interaction of the African, Eurasian, and Arabian plates. Extension is accommodated primarily by normal faulting along north-south trending structures linked to slab rollback along the Hellenic Arc. The broader Anatolian plate moves westward at rates of approximately 2 cm per year, driven by the Arabia-Eurasia collision to the east. This setting produces frequent earthquake swarms, often associated with fluid migration or aseismic slip on faults rather than classic mainshock-aftershock sequences.
Historical records document repeated seismic activity in the region. Notable events include the 1953 Yenice-Gönen earthquake (magnitude 7.2) and the 1899 Menderes Valley sequence, both illustrating the capacity for damaging normal-fault ruptures. Instrumental monitoring since the late twentieth century has confirmed elevated background seismicity, with swarms commonly occurring near the intersections of major grabens such as the Gediz and Büyük Menderes.
Analysis of the first 100 events reveals a classic swarm pattern: clustered occurrence without a single dominant rupture, shallow focal depths consistent with upper-crustal extension, and a rapid decay in event rate after the initial peak. The largest events remained below magnitude 4.0 after the opening shock, supporting the interpretation of distributed strain release rather than progressive failure on a single fault segment.
The 2017 swarm adds to the growing catalog of monitored sequences that help refine seismic hazard models for Western Turkey. Continued high-resolution recording of such episodes improves understanding of how transient deformation and fluid processes interact within the actively extending Aegean crust.