Seismic Swarm S20090217.1: Geological Context and Event Analysis in Western Turkey

Western Turkey occupies a tectonically complex zone at the western termination of the Anatolian plate, where extensional forces driven by the rollback of the Hellenic slab interact with strike-slip motion along the North Anatolian Fault. This setting produces frequent shallow crustal seismicity, with normal and oblique-normal faulting predominant in the Aegean graben systems. The region has recorded multiple seismic swarms throughout the instrumental era, reflecting fluid migration and stress redistribution along segmented fault networks rather than mainshock-aftershock sequences. Swarm S20090217.1 began at 04:51 UTC on 17 February 2009 and continued until 00:44 UTC on 22 February 2009, spanning 115 hours and 53 minutes. During this interval, 161 earthquakes were registered across western Turkey. The sequence initiated with a magnitude 5.2 event at 05:28 UTC on 17 February at a focal depth of 7 km. Subsequent activity remained predominantly shallow, with depths concentrated between 2 km and 15 km, consistent with the brittle upper crust of the Aegean extensional domain. Analysis of the first 100 events reveals a rapid onset of activity following the initial M5.2 shock. Magnitudes ranged from 2.1 to 3.9, with the majority falling between 2.3 and 3.0. Depths clustered tightly around 5–8 km, although several events reached 15–20 km. Temporal distribution showed the highest rate in the first 12 hours, with repeated bursts of similar-sized events indicating swarm-like behavior driven by pore-pressure changes rather than static stress transfer from a single large rupture. The 5.2 mainshock was followed within minutes by events of M3.6 and M2.8, establishing a pattern of low-magnitude, high-frequency occurrences that persisted through the afternoon and evening of 17 February. Later events on the same day included an M3.9 at 09:37 UTC (depth 10 km) and an M3.6 at 10:45 UTC (depth 2 km), illustrating continued release of stored strain along shallow structures. No events exceeded magnitude 4.0 within the examined subset, underscoring the swarm’s diffuse energy distribution. Geological records indicate that comparable swarms in western Turkey have occurred near the Büyük Menderes and Gediz grabens, where Quaternary normal faults accommodate regional extension at rates of 10–20 mm per year. Historical seismicity includes destructive events in 1899 and 1928 along the same structural trends, demonstrating long-term recurrence of clustered activity. Modern monitoring confirms that such swarms rarely produce surface rupture but can generate locally felt shaking due to their shallow hypocenters. The 2009 sequence aligns with this established pattern, providing further evidence of ongoing crustal deformation in the Aegean back-arc. Continued geodetic observations from regional networks show that post-swarm strain accumulation remains consistent with background tectonic rates, suggesting the event did not significantly alter long-term seismic hazard estimates for the area.

References

USGS Earthquake Catalog (events 2009) Turkish Disaster and Emergency Management Authority (AFAD) historical reports Journal of Geophysical Research: Solid Earth, Aegean tectonics reviews (2015–2023 updates)