Seismic Swarm S20111023.2: Eastern Turkey Earthquake Sequence Analysis

Eastern Turkey occupies a tectonically complex zone within 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 deformation along major structures including the East Anatolian Fault and Bitlis-Zagros suture zone. The area around Van Province features active thrust and strike-slip faults that accommodate regional shortening, producing frequent shallow crustal seismicity.

The M 7.1 mainshock on 23 October 2011, located 27 km NNE of Van and only 7 km from the swarm center, highlighted the region’s seismic hazard. That event caused widespread damage and triggered aftershock sequences. Historical records indicate persistent activity since at least the instrumental era, with multiple damaging earthquakes linked to the same fault systems.

Swarm S20111023.2 began at 11:10 on 23 October 2011 and concluded at 05:00 on 7 November 2011. Over 353 hours and 50 minutes, 739 earthquakes were recorded. This marked the first swarm in the region since systematic monitoring began in 2000; only one additional swarm has occurred since that date.

Analysis of the initial 100 events reveals predominantly shallow foci. Depths ranged from 2 km to 20 km, with the majority occurring between 2 km and 10 km. Magnitudes spanned 2.2 to 4.8, indicating a swarm dominated by small-to-moderate events rather than a classical mainshock-aftershock sequence. Early activity included several events of M 4.0–4.8 within the first hours, followed by a sustained rate of M 3.0+ shocks through 25 October. The distribution shows clustering at depths of 5–7 km during peak activity, consistent with slip on shallow crustal structures.

The swarm’s temporal evolution featured an initial energetic phase on 23 October, with multiple M 4+ events, transitioning to lower-magnitude but frequent seismicity over subsequent days. No events exceeded M 4.8 in the examined subset, underscoring the swarm character distinct from the preceding M 7.1 mainshock.

Such swarms provide insight into fluid migration or aseismic slip processes that can modulate seismic release in compressional regimes. Continued monitoring remains essential given the proximity to populated centers and the documented recurrence of significant earthquakes in the Van region.

References
SeismoSight internal swarm catalog S20111023.2
USGS Earthquake Catalog (historical regional events)