Seismic Swarm S20120619.1: Analysis of Western Turkey Earthquake Sequence

Western Turkey occupies a tectonically complex zone at the convergence of the African, Eurasian, and Arabian plates. The region experiences ongoing extension linked to rollback of the Hellenic slab and westward extrusion of Anatolia along the North Anatolian Fault. These processes produce a combination of normal and strike-slip faulting that accounts for elevated seismicity levels compared with many other continental interiors.

The area has recorded destructive earthquakes throughout recorded history. Notable modern events include the 1999 Izmit and Düzce earthquakes along the North Anatolian Fault, which together caused more than 18,000 fatalities. Instrumental records since 1900 show that western Turkey averages several magnitude-5 or larger events per decade, with many occurring at shallow depths between 5 and 20 km.

Between 18 and 20 June 2012, a swarm comprising 34 events was recorded in western Turkey. Activity began at 16:28 UTC on 18 June and concluded at 17:33 UTC on 20 June, spanning 49 hours and 5 minutes. The sequence included one event of magnitude 4.6 at 9 km depth, with the majority of shocks falling between magnitude 1.9 and 3.3 and depths ranging from 2 km to 19 km. The largest event occurred early on 19 June at 01:46 UTC, followed by numerous smaller aftershocks clustered within the first 24 hours.

The temporal pattern showed an initial peak within hours of the magnitude-4.6 shock, with 22 events registered on 19 June. Activity declined steadily on 20 June, ending with a final magnitude-2.4 event. Depths remained predominantly shallow, consistent with the extensional fault systems that dominate the region.

Since 1 January 2000, seven swarms have been documented in western Turkey. Earlier swarms occurred in 2009 (one swarm), 2011 (four swarms), and 2012 (two swarms, including the present sequence). Such episodic clustering is typical of the area’s fluid-influenced fault zones, where pore-pressure changes can trigger distributed microseismicity without producing a single large mainshock.

This swarm illustrates the background seismic hazard that persists in western Turkey. Although individual events remained below damaging thresholds, the occurrence underscores the need for continued monitoring of the North Anatolian Fault system and associated extensional structures.

References

United States Geological Survey Earthquake Hazards Program, Western Turkey Seismicity.
European-Mediterranean Seismological Centre, Regional Earthquake Catalogue.
Turkish Earthquake Foundation, Active Fault Map of Western Anatolia.