Seismic Swarm S20190326.1: Eastern Turkey, March 2019

Eastern Turkey lies within the tectonically active Alpine-Himalayan orogenic belt, where the Arabian Plate converges northward with the Eurasian Plate at rates of approximately 15–20 mm per year. This interaction is accommodated primarily along the East Anatolian Fault Zone, a major left-lateral strike-slip system extending over 500 km, and associated secondary faults. The region experiences frequent seismic activity due to this transpressional regime, with historical records documenting destructive events such as the 1939 Erzincan earthquake (magnitude 7.8) and the 1976 Çaldıran earthquake (magnitude 7.3). Updated tectonic models from the past decade confirm ongoing strain accumulation along these structures, consistent with GPS measurements showing localized deformation.

Between 06:04 on 25 March 2019 and 09:12 on 29 March 2019, SeismoSight recorded Swarm S20190326.1 in Eastern Turkey. Over 99 hours and 7 minutes, the sequence comprised 52 earthquakes. Depths ranged predominantly between 3 km and 14 km, with the majority clustered at 5–7 km, indicating shallow crustal sources typical of the East Anatolian Fault system.

The sequence initiated with a magnitude 2.1 event at 7 km depth. Activity intensified within hours, featuring a magnitude 4.6 earthquake at 10 km depth on 25 March at 11:29:26, followed by numerous events of magnitude 2.0–2.5. Subsequent notable shocks included a magnitude 3.8 at 5 km on 25 March, a magnitude 3.4 at 5 km on 26 March, magnitudes 3.9 and 3.3 on 27 March at 5–7 km, and additional magnitude 3.3 and 3.9 events on 28 March at depths of 5–10 km. The swarm concluded with a magnitude 2.1 event at 7 km depth.

Magnitudes remained below 5.0 throughout, with 45 events between 2.0 and 2.9, six between 3.0 and 3.9, and one at 4.6. This distribution aligns with swarm characteristics, where energy release occurs through numerous moderate events rather than a single mainshock-aftershock pattern. Depths stayed consistently shallow, reflecting brittle failure in the upper crust.

Since 1 January 2000, only two prior swarms have been documented in the same Eastern Turkey sector: one in 2010 and one in 2015. The 2019 sequence represents the third such episode, suggesting episodic swarm behavior possibly linked to fluid migration or aseismic slip transients along local fault segments.

This swarm provides insight into background seismicity levels in a high-hazard zone. While individual events posed low risk, the pattern underscores the need for continuous monitoring to distinguish swarms from precursors to larger earthquakes in the East Anatolian Fault Zone.

References

SeismoSight internal swarm classification S20190326.1
USGS Earthquake Catalog (Eastern Turkey events, 2000–2023)
Emre et al., 2018, Active Fault Map of Turkey (updated 2023 revision)
Reilinger et al., 2022, GPS-derived plate motions in the Eastern Mediterranean