Seismic Analysis of the September 2016 Earthquake Swarm near Saruhanlı, Turkey

The earthquake swarm designated S20160912.1 occurred in western Turkey, approximately 13 km east-northeast of Saruhanlı in Manisa Province. It began at 08:26 on 12 September 2016 and concluded at 19:12 on 23 September 2016, spanning 274 hours and 46 minutes. During this interval, 193 earthquakes were recorded, with the first 100 events providing detailed insight into the swarm's evolution.

Analysis of the initial 100 events reveals predominantly shallow focal depths between 2 km and 20 km, with the majority clustered between 5 km and 12 km. Magnitudes ranged from 2.0 to 4.5, including notable events of M4.5 at 08:26 on 12 September, M4.3 at 09:29, M4.2 at 10:05, and several M3.0–M3.2 shocks distributed through the first two days. Subsequent activity consisted mainly of M2.0–M2.9 events, indicating a rapid decay in energy release after the initial stronger shocks. The temporal distribution shows intense clustering within the first 24 hours, followed by a gradual decline, consistent with swarm behavior driven by fluid migration or aseismic slip rather than a single mainshock-aftershock sequence.

The Saruhanlı region lies within the Western Anatolian Extensional Province, where active normal faulting accommodates north-south crustal stretching at rates of 20–30 mm per year. This extension results from the rollback of the African slab beneath the Aegean and the westward extrusion of the Anatolian Plate. The local geology features Miocene to Quaternary sedimentary basins bounded by east-west trending normal faults, including segments of the Gediz and Alaşehir graben systems. These structures overlie metamorphic basement rocks of the Menderes Massif and exhibit historical and instrumental seismicity.

Seismic activity in this part of the Aegean region has been documented since antiquity, with notable twentieth-century events including the 1969 Alaşehir earthquake (M6.9) and the 1970 Gediz earthquake (M7.2). The 2016 swarm occurred along a secondary fault strand associated with these major graben-bounding structures. Focal mechanisms from regional networks typically indicate pure normal faulting on planes dipping 40–60 degrees, aligning with the observed shallow depths and the absence of a dominant mainshock.

The swarm's characteristics—high event count, limited maximum magnitude, and shallow depths—suggest triggering by pore-pressure changes within the fractured upper crust. Such swarms are recurrent in western Turkey and serve as indicators of ongoing tectonic strain accumulation. Monitoring by the Disaster and Emergency Management Presidency (AFAD) and international networks continues to refine hazard assessments for the densely populated Manisa lowlands.

References

• AFAD Earthquake Department, National Seismic Network Bulletins (2016).
• Boğaziçi University Kandilli Observatory Regional Earthquake-Tsunami Monitoring Center catalogs.
• McKenzie, D. (1978). Active tectonics of the Alpine-Himalayan belt. Geophysical Journal International.
• Reilinger, R. et al. (2006). GPS constraints on continental deformation in the Africa-Arabia-Eurasia plate collision zone. Journal of Geophysical Research.