Seismic Swarm PS20111023.1: Analysis of Events Near Van, Turkey

The seismic swarm designated PS20111023.1 occurred 15 km northeast of Van in eastern Turkey. It began at 10:41 on 23 October 2011 and concluded at 15:28 on 24 October 2011, spanning 28 hours and 46 minutes. During this period, ten earthquakes were recorded. The sequence initiated with a magnitude 7.1 event at 18 km depth, followed by nine additional shocks ranging from magnitude 4.6 to 5.9, with focal depths between 5 km and 10 km.

Event details unfolded as follows. At 10:48 on 23 October, a magnitude 5.6 earthquake struck at 9 km depth. Four minutes later, another magnitude 5.6 event occurred at 5 km. Subsequent shocks included a magnitude 4.6 at 10 km (11:00), magnitude 5.2 at 10 km (11:00), magnitude 5.0 at 10 km (11:10), magnitude 5.7 at 5 km (11:32), magnitude 5.1 at 5 km (18:10), and magnitude 5.9 at 5 km (20:45). The final event registered magnitude 5.0 at 10 km depth on 24 October at 15:28.

This swarm represents the sole seismic swarm recorded in the region since 1 January 2000. The preceding activity of comparable classification also dates to 2011. The magnitude 7.1 mainshock epicenter lay 27 km north-northeast of Van and approximately 29 km from the swarm centroid.

Eastern Turkey occupies a complex tectonic regime shaped by the collision of the Arabian and Eurasian plates along the Bitlis-Zagros suture zone. Convergence rates average 15–20 mm per year, producing north-south shortening accommodated primarily by thrust faulting and distributed deformation across the Eastern Anatolian Plateau. The Van basin sits near the junction of the Bitlis thrust belt and the North Anatolian Fault system’s eastern extensions, where crustal thickening and strike-slip components coexist. Historical records document recurrent moderate-to-large earthquakes in the area, consistent with this active collisional setting.

The 2011 sequence aligns with reverse-fault mechanisms typical of the regional stress field. Depths of the recorded events, predominantly shallow to mid-crustal, reflect brittle failure within the upper 20 km of thickened continental crust. No subsequent swarms have been identified in the catalog through the present, underscoring the relative infrequency of clustered activity compared with isolated mainshock-aftershock sequences in the same locale.

References
USGS Earthquake Catalog (event data for 23–24 October 2011)
Tectonic framework descriptions from the Geological Society of London Special Publications on Eastern Anatolia