Seismic Swarm S20130421.1 in Central Turkey: Characteristics and Tectonic Context

Seismic swarm S20130421.1 occurred in central Turkey between 23:23 on 20 April 2013 and 01:45 on 24 April 2013. Over 74 hours and 22 minutes, the sequence produced 64 earthquakes. Magnitudes ranged from 1.7 to 3.6, with the largest event recorded at 21:06 on 21 April. Focal depths remained shallow, predominantly between 2 km and 16 km, consistent with upper-crustal brittle failure.

The swarm exhibited typical features of clustered microseismicity: an initial moderate event (M 3.4) followed by numerous smaller aftershocks and several additional events above M 3.0 distributed throughout the first 48 hours. Activity declined steadily after 22 April, with only isolated events above M 2.5 recorded on 23 and 24 April. Such temporal patterns indicate fluid-driven or stress-transfer mechanisms rather than a single mainshock-aftershock sequence.

Central Turkey lies within the Anatolian continental block, bounded by the North Anatolian Fault to the north and the East Anatolian Fault Zone (EAFZ) to the southeast. The EAFZ accommodates left-lateral strike-slip motion between the Arabian and Anatolian plates at rates of approximately 6–10 mm per year. The 2013 swarm centroid lies roughly 13 km from the surface rupture of the 6 February 2023 M 7.8 Pazarcık earthquake, which nucleated on a segment of the EAFZ and produced extensive surface faulting across Kahramanmaraş province.

Historical records maintained since 1 January 2000 document three swarms in the broader region, the earliest occurring in 2012. These episodic clusters highlight persistent low-level strain accumulation along secondary faults subsidiary to the main EAFZ strands. The 2023 mainshock released accumulated elastic strain over centuries, demonstrating that even modest swarms can occur within zones capable of generating large-magnitude events.

Seismic monitoring by national and international networks continues to refine fault segmentation models for the EAFZ. Updated hazard assessments incorporate both the 2013 swarm parameters and the 2023 rupture geometry to better constrain recurrence intervals and ground-motion predictions for central and southeastern Anatolia.

References

USGS Earthquake Catalog (2023 Kahramanmaraş sequence)
SeismoSight internal swarm classification database