Seismic Swarm S20111023.3: Analysis of Eastern Turkey's October-November 2011 Activity

Eastern Turkey occupies a tectonically active zone within the Alpine-Himalayan belt, where the Arabian Plate converges northward with the Eurasian Plate at rates of approximately 15-20 mm per year. This interaction produces a network of strike-slip and thrust faults, including the East Anatolian Fault Zone and the Bitlis-Zagros suture. The region's geology features extensive volcanic fields, ophiolitic complexes, and sedimentary basins shaped by ongoing compression since the Miocene. Historical records document frequent moderate-to-large earthquakes, with notable events such as the 1976 Çaldıran quake and the destructive 2011 Van mainshock underscoring persistent seismic hazard.

Seismic swarm S20111023.3 began at 11:27 on 23 October 2011 and concluded at 03:29 on 5 November 2011, spanning 304 hours and 2 minutes. During this interval, 415 earthquakes were recorded across eastern Turkey. The swarm represents the sole such episode documented since 1 January 2000, highlighting its rarity within the instrumental record.

Examination of the first 100 events reveals predominantly shallow foci, with depths ranging from 2 km to 23 km and clustering between 2 km and 10 km. Magnitudes varied from 2.4 to 4.8, with several events exceeding 4.0 on the initial day. Early activity included a 4.7 event at 10 km depth shortly after onset, followed by repeated sequences of 3.5-4.5 shocks at depths under 10 km. Later events in the sequence maintained similar shallow characteristics, with isolated deeper occurrences near 19-22 km. This pattern indicates fluid-driven or stress-transfer mechanisms typical of swarm behavior rather than a single mainshock-aftershock cascade.

The temporal distribution shows intense clustering within the first 24 hours, transitioning to sustained lower-rate activity through early November. Such swarms in convergent settings often correlate with minor fault reactivation amid regional compression. Given the single historical precedent since 2000, this episode provides valuable data on episodic seismicity in the area.

Further monitoring and integration with regional fault models remain essential for assessing future potential. The event underscores eastern Turkey's dynamic crustal environment shaped by long-term plate interactions.

References

SeismoSight internal swarm classification records.
Regional tectonic summaries from peer-reviewed geological literature on the Arabian-Eurasian collision zone.