Earthquake Swarm S20110206.1: Seismic Activity in Western Turkey, February 2011

Western Turkey lies within one of the most seismically active regions on Earth, shaped by complex interactions among the Eurasian, African, and Arabian tectonic plates. The area forms part of the Aegean extensional province, where crustal stretching produces frequent normal and strike-slip faulting. Major structures such as the North Anatolian Fault and the Western Anatolian Graben System accommodate this deformation, resulting in elevated background seismicity and occasional earthquake swarms. Historical records document destructive events throughout the 20th century, including the 1999 İzmit earthquake (Mw 7.6) along the North Anatolian Fault and multiple moderate shocks in the 1960s and 1970s near the Aegean coast.

Swarm S20110206.1 began at 07:29 on 5 February 2011 and concluded at 17:36 on 8 February 2011, spanning 82 hours and 6 minutes. During this interval, 62 earthquakes were recorded across western Turkey. Magnitudes ranged predominantly between 2.1 and 2.9, with a single event reaching 4.2. Focal depths clustered between 2 km and 15 km, although isolated events occurred at 0 km and 30 km. The sequence exhibited a typical swarm pattern: an initial rise in activity on 5 February, sustained elevated rates through 6 and 7 February, and a gradual decline concluding on 8 February. No single mainshock dominated; instead, energy released through numerous comparable events.

The largest shock (magnitude 4.2) occurred at 02:08 on 6 February at a reported depth of 0 km. Preceding and following events maintained lower magnitudes and slightly greater depths, consistent with fluid-driven or aseismic-slip triggering mechanisms often observed in extensional regimes. Depth distribution indicates that most ruptures nucleated within the upper crust, where brittle failure is favored. The absence of aftershock decay typical of mainshock-aftershock sequences further supports classification as a swarm rather than a triggered cascade.

This episode represents the second swarm documented in the instrumental catalog since 1 January 2000, following an earlier swarm in 2010. Such recurrent swarm behavior underscores the region’s capacity for clustered, moderate-magnitude seismicity without immediate progression to larger events. Monitoring by regional networks continues to provide essential data for understanding stress transfer and potential hazard implications along the western Anatolian fault systems.

References

• United States Geological Survey Earthquake Catalog
• European-Mediterranean Seismological Centre bulletins
• Boğaziçi University Kandilli Observatory seismic reports