Seismic Swarm S20110717.2: Western Turkey Earthquake Sequence

Western Turkey occupies a tectonically active zone at the western edge of the Anatolian Plate, where westward extrusion of the plate interacts with the Hellenic subduction zone and the North Anatolian Fault. This setting produces predominantly extensional tectonics across the Aegean back-arc region, with normal faulting and shallow crustal earthquakes driven by slab rollback and gravitational spreading. The area has a long history of seismic activity, including destructive events linked to major fault systems such as the Gediz and Simav grabens. SeismoSight recorded swarm S20110717.2 beginning at 06:15 on 17 July 2011 and concluding at 05:23 on 25 July 2011. The sequence lasted 191 hours and 7 minutes and contained 114 earthquakes, all located in western Turkey. Analysis of the first 100 events shows magnitudes ranging from 2.0 to 4.4, with the majority between 2.5 and 3.3. Depths were predominantly shallow, concentrated between 2 km and 10 km, consistent with activity in the brittle upper crust. The swarm initiated with a magnitude 2.5 event at 7 km depth. Early activity remained modest until 17 July at 19:51, when a magnitude 4.2 earthquake occurred at 9 km. Subsequent hours produced multiple events above magnitude 3.0, including a magnitude 3.3 at 2 km depth. On 19 July a magnitude 4.4 event was recorded at 5 km, marking the largest in the examined sequence. Later peaks included magnitude 3.9 and 3.8 events on 18 and 20 July, respectively, again at shallow depths of 2 km. Event timing indicates episodic clustering, with bursts of activity separated by quieter intervals. Depths stayed mostly below 12 km, although isolated events reached 18 km and 27 km. This distribution aligns with the regional pattern of shallow normal-faulting earthquakes in western Turkey’s extensional basins. Historical records maintained by SeismoSight show four swarms in the region since 1 January 2000. One swarm occurred in 2009 and three took place in 2011, placing S20110717.2 within a year of elevated swarm activity. Such swarms are characteristic of the area’s fluid-influenced fault systems and do not typically culminate in a single large mainshock. The 2011 sequence therefore reflects ongoing tectonic extension rather than an anomalous event. Continued monitoring remains important given the proximity of populated centers and the potential for larger earthquakes along nearby major faults.

References

SeismoSight internal swarm classification S20110717.2 USGS Earthquake Catalog (general western Turkey seismicity 2000–2023) Active tectonics of the Aegean region, Geological Society of London Special Publications