Seismic Swarm S20151129.2 in Central Turkey

Central Turkey occupies a tectonically complex zone at the western margin of the Anatolian Plate, where northward motion of the Arabian Plate and westward extrusion of Anatolia produce distributed strike-slip and normal faulting. The region’s seismic character reflects the interaction of the North Anatolian Fault system to the north and the East Anatolian Fault to the southeast, with additional deformation accommodated along secondary structures within the Anatolian plateau. Historical records document recurrent moderate-to-large earthquakes, including the 1939 Erzincan event and the 1944 Gerede earthquake, underscoring the area’s long-term seismic productivity.

Swarm S20151129.2 began at 00:28 on 29 November 2015 and concluded at 13:17 on 3 December 2015, spanning 108 hours and 49 minutes. During this interval, 60 earthquakes were recorded. The sequence lacked a single dominant mainshock and instead exhibited a gradual rise and decay of activity, consistent with swarm-type behavior. The largest event reached magnitude 4.9 at a depth of 10 km within the first minute of the swarm; subsequent activity included multiple events of magnitude 3.4–3.8 at depths between 2 km and 17 km. The majority of hypocenters clustered between 2 km and 12 km depth, indicating shallow crustal deformation.

Temporal analysis shows peak rates during the first 12 hours, followed by a steady decline. Notable late-stage events on 1 December included magnitudes 3.2 and 3.7, both at 5 km depth, illustrating continued energy release without escalation. Depths remained predominantly shallow, suggesting fluid-assisted or thermally influenced faulting typical of swarm sequences in this part of the Anatolian Plate.

Historical context within the SeismoSight catalogue indicates only one prior swarm in central Turkey since 1 January 2000, occurring in 2010. This low frequency highlights the episodic nature of swarm activity relative to the more frequent isolated earthquakes driven by regional plate-boundary faults.

The 2015 swarm provides insight into localized stress release along secondary structures that may not always produce large single events. Continued monitoring of similar sequences can improve understanding of how distributed deformation contributes to the overall seismic hazard of central Turkey.

References

SeismoSight internal catalogue
USGS Earthquake Catalog (historical regional events)