Seismic Swarm S20111119.2 in Eastern Turkey

Eastern Turkey occupies a tectonically complex zone within the Alpine-Himalayan orogenic belt, where the Arabian Plate converges with the Eurasian Plate at rates of approximately 2–3 cm per year. This convergence drives active deformation along the East Anatolian Fault Zone and associated thrust and strike-slip structures, producing frequent shallow crustal earthquakes. The region’s geology features young volcanic rocks, folded sedimentary sequences, and elevated topography resulting from ongoing continental collision that began in the Miocene.

Seismic swarms represent clusters of earthquakes occurring in rapid succession without a single dominant mainshock, often linked to fluid migration or aseismic slip along fault networks. Swarm S20111119.2, recorded in eastern Turkey, began at 16:08 on 18 November 2011 and concluded at 21:19 on 21 November 2011, spanning 77 hours and 11 minutes. During this interval, 70 earthquakes were registered, with magnitudes ranging from 1.9 to 3.9 and focal depths predominantly between 2 and 10 km, indicating shallow crustal sources.

The sequence initiated with events of magnitude 2.3–3.1 on 18 November, concentrated at depths of 2–7 km. Activity persisted through 19 November with a steady rate of events mostly between 2.1 and 3.2, including several at 7 km depth. On 20 November the swarm reached its peak intensity, featuring the largest event of magnitude 3.9 at 2 km depth along with multiple shocks of 3.1–3.3. The final recorded event, magnitude 2.6 at 7 km, occurred on 21 November.

This swarm constitutes the earliest documented swarm in the regional catalogue since 1 January 2000. Five such swarms have been identified in eastern Turkey through the present, underscoring episodic clustered seismicity superimposed on the area’s background tectonic loading.

The shallow depths and moderate magnitudes observed are consistent with the region’s active fault systems, where small-scale stress perturbations can trigger dense sequences of events. Continued monitoring of similar swarms contributes to refined understanding of fault mechanics and seismic hazard assessment in this high-strain continental collision zone.

References

SeismoSight internal swarm classification records.
Regional tectonic summaries from the United States Geological Survey Earthquake Hazards Program.
Plate-boundary studies published by the Incorporated Research Institutions for Seismology.