Neotectonics and seismicity of eastern simav graben, Kütahya – Turkey

Kaplan, Mustafa
The Simav graben is an about 1-9 km wide, 50 km long and approximately WNW-trending active depression. It is located along the western section of a seismogenic belt with a depth of 10-15 km. It is the 10-30 km wide and approximately 500 km long active fault system, namely the Akşehir-Simav fault system. The Simav graben was developed on the northern Menderes Massif under the control of active faults comprising the Simav section of the Akşehir-Simav fault system. The Simav and Akdere grabens include two different fills separated by an intervening angular unconformity: (a) Early Miocene-Middle Miocene deformed graben fill, and (b) Quaternary modern and undeformed graben fill. Geological structures and stratigraphic relationships reveal that there are four tectonic periods differentiated in the study area: (a) N-S extension along the Simav detachment fault, (b) E-W extension and formation of Demirci, Akdere and Selendi grabens, (c) NW-SE compression period characterized by strike-slip faulting, deformation of older graben fills, separation and displacement of the Akdere and Demirci grabens and “non-deposition” in the region, (d) NNE-SSW extension controlled neotectonic period that is characterized by normal faulting and development of both the modern Simav and Akdere grabens. The Simav County and a number of settlements in the size of town and villages are located on and very close to the graben margin-bounding normal faults. Most of them are active and have a capacity of creating destructive earthquakes. This was proved once more by the occurrence of two recent earthquakes. These are the 17 February 2009 Naşa (Simav) (Mw = 5.3) and the 19 May 2011 Söğüt (Simav) (Mw =5.9) earthquakes. The source of the Söğüt earthquake is the Nadarçamı or Seyirkaya fault comprising the Simav fault zone. These faults are normal faults dipping towards NNE. NNE extension and normal faulting mechanism is consistent with the focal mechanism solutions, epicenter location and focus depth and GPS velocity analysis. Simav and a number of settlements located around the active faults are under the threat of earthquake hazard. For this reason, deterministic seismic hazard maps of the study area were prepared to define the earthquake hazard in the Simav region by using the available geological and seismological data as well as a reasonable attenuation relationship for the region. PGA value for Simav city center soil site condition is 0.398 g.