Seismic Analysis of the 2011 Simav Earthquake Swarm in Western Turkey

The Simav region in Kütahya Province, western Turkey, lies within the Aegean extensional tectonic province. This area experiences active normal faulting driven by north-south crustal stretching between the Anatolian and African plates. The Simav Graben and associated fault systems, including segments of the Simav Fault Zone, accommodate this extension, resulting in frequent shallow seismicity at depths typically between 2 and 15 km. Historical records document destructive earthquakes in the broader Aegean region, with modern monitoring confirming ongoing activity linked to these structures.

Swarm S20110519.1 was recorded 12 km east-northeast of Simav. It began at 19:59 on 19 May 2011 and concluded at 11:13 on 1 July 2011, spanning 1023 hours and 14 minutes. During this period, 2795 earthquakes were detected. This event represents the second swarm in the region since 2000, following a single swarm in 2009.

The sequence opened with a magnitude 5.8 event at 7 km depth on 19 May at 20:15:22, marking the largest shock. Subsequent early activity included a 3.8 foreshock and numerous aftershocks. Analysis of the first 100 events reveals a rapid succession of tremors, predominantly with magnitudes between 2.0 and 4.0. Depths clustered between 2 and 10 km, consistent with the shallow crustal faulting characteristic of the Simav area. The initial 24 hours featured several events above magnitude 4.0, such as 4.6, 4.3, and 4.4 shocks, followed by a dense cluster of smaller events. This pattern indicates swarm-like behavior driven by fluid migration or stress triggering along local faults rather than a classic mainshock-aftershock decay.

Geological context supports the observed shallow focus and high event rate. The Simav basin is bounded by active normal faults that have produced similar swarms in the Aegean extensional domain. Updated regional studies confirm that such sequences reflect ongoing tectonic extension at rates of several millimeters per year.

This swarm underscores the persistent seismic hazard in western Turkey. Continued monitoring remains essential given the region's history of both isolated events and clustered activity.

References

SeismoSight internal swarm classification data S20110519.1.
USGS Earthquake Catalog for regional tectonics of western Turkey.
Active tectonics of the Aegean region, Geological Society of London Special Publications.