Seismic Swarm S20230512.2 in Central Turkey: Analysis and Context

Central Turkey occupies a tectonically active zone at the junction of the Anatolian, Arabian, and Eurasian plates. The region experiences frequent seismicity driven by the westward extrusion of the Anatolian plate along the North Anatolian Fault and East Anatolian Fault systems. These major strike-slip structures accommodate relative motion between the converging plates, producing both large-magnitude events and smaller clustered activity. Historical records document destructive earthquakes throughout the Holocene, with paleoseismic studies indicating recurrence intervals of several hundred years for segments of the East Anatolian Fault.

Swarm S20230512.2 began at 00:54 on 12 May 2023 and concluded at 06:25 on 29 May 2023, spanning 413 hours and 30 minutes. During this interval, 228 earthquakes were recorded within Central Turkey. Analysis of the first 100 events reveals predominantly low-to-moderate magnitudes, with the majority falling between 2.0 and 3.5. Depths clustered tightly around 5 km, although a smaller subset occurred between 1 km and 16 km. The sequence opened with a 3.4 event at 5 km depth, followed rapidly by additional events of similar magnitude. Peak activity included a 4.5 event at 2 km depth on 12 May and a 4.9 event at 5 km depth on 21 May. Most events remained below magnitude 3.0, consistent with swarm behavior rather than a classic foreshock-mainshock-aftershock pattern.

Temporal distribution showed highest rates during the first 48 hours, with magnitudes generally decreasing after the initial larger shocks. Shallow focal depths suggest activity within the brittle upper crust, typical of the faulted carbonate and ophiolitic assemblages that underlie much of Central Turkey. No events exceeded magnitude 5.0 within the examined subset, and the swarm terminated without escalation to a larger mainshock.

Since 1 January 2000, only two seismic swarms have been identified in the region according to SeismoSight internal classification. The first occurred in 2023. This limited historical swarm frequency underscores the unusual nature of the May 2023 sequence relative to the background rate of isolated tectonic events.

The May 2023 swarm occurred several months after the devastating February 2023 doublet on the East Anatolian Fault, which produced widespread stress redistribution across Central Turkey. Such post-seismic stress changes can trigger swarm activity on secondary structures, although direct causal links require detailed modeling beyond the scope of this summary.

References
SeismoSight internal swarm catalogue (S20230512.2 parameters and event list).
USGS Earthquake Catalog (regional tectonics and historical seismicity).
Emre et al. (2018), Active fault map of Turkey, General Directorate of Mineral Research and Exploration.
Reilinger et al. (2006), GPS constraints on continental deformation in the Africa-Arabia-Eurasia plate collision zone, Journal of Geophysical Research.