Seismic Swarm S20111030.1: Eastern Turkey Analysis

Eastern Turkey lies within the complex tectonic framework of the Alpine-Himalayan orogenic belt, where the Arabian Plate converges northward with the Eurasian Plate at rates of approximately 2–3 cm per year. This collision drives active thrust faulting and crustal shortening, particularly along the Bitlis-Zagros suture zone. The region around Van experiences frequent seismicity due to these compressional stresses, with shallow crustal earthquakes predominating.

The M7.1 earthquake of 23 October 2011, located 27 km north-northeast of Van, occurred on a blind thrust fault and produced extensive surface deformation. Swarm S20111030.1 initiated six days later, centered approximately 13 km from the mainshock epicenter. This temporal proximity suggests the swarm represents triggered aftershock activity within the same fault system.

The swarm spanned 50 hours and 49 minutes from 19:20 on 29 October to 22:09 on 31 October 2011, registering 46 events. Magnitudes ranged from 2.0 to 3.8, with the largest event (M3.8) recorded at 04:29 on 30 October at 5 km depth. Depths clustered between 2 km and 20 km, indicating activity confined to the upper crust. Multiple events reached M3.0 or above, including M3.7 at 15:22 on 31 October and M3.6 at 12:03 on 30 October, both at shallow depths of 5 km and 2 km respectively.

Event distribution showed temporal clustering, with heightened activity during daytime hours on 30 October. Depths remained consistently shallow, consistent with aftershock sequences in compressional regimes where stress redistribution occurs along pre-existing faults. No events exceeded M4.0, distinguishing this swarm from mainshock-aftershock sequences with larger triggered shocks.

Since 2000, six seismic swarms have been documented in the region, with this event marking the first recorded instance. Such swarms highlight episodic strain release in an area already stressed by the recent M7.1 mainshock.

Geological Context and Historical Seismicity

The Van basin sits at the eastern margin of the Anatolian Plateau, influenced by both Arabian-Eurasian convergence and westward escape of the Anatolian Plate along the North and East Anatolian faults. Historical records document destructive earthquakes in 1941 and 1976, underscoring long-term seismic hazard. Post-2011 monitoring has improved resolution of microseismicity, revealing persistent low-magnitude clusters along thrust structures.

Insights from Swarm Characteristics

Analysis of the 46 events reveals a typical post-mainshock swarm pattern: rapid onset, moderate magnitudes, and shallow focal depths. The predominance of events between 2–12 km depth aligns with the rupture zone of the preceding M7.1 earthquake. This sequence likely facilitated continued stress adjustment without producing significant new damage.

References

• SeismoSight internal swarm classification S20111030.1
• USGS Earthquake Catalog (event data for 23 Oct 2011 Van earthquake)
• Geological Survey of Turkey regional tectonic reports