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Abstract:
The Germencik geothermal field with 240 °C fluid temperature is one of the most important energy production areas located at the westernmost end of the Büyük Menderes Graben (western Türkiye). The northern boundary faults of the graben have a dip angle larger than 50° and have listric characteristics. In this brief study, the earthquake activity between 2007 – 2024 in the Germencik geothermal field is improved with the double-difference inversion method to image the deep fault surfaces down to 20 km of the crust. Considering the steeply southward dipping boundary faults in the Büyük Menderes Graben, listric normal faults in the upper crust are demonstrated with earthquake hypocenters for the first time. The deep image of the listric faults indicates that these are the main carriers for the upwelling hot geothermal fluid (~240 °C), which are schematically shown in the conceptual geothermal models of Germencik. On the other hand, a significant microearthquake activity was observed in 2009, when the energy production of the Germencik was starting. The slope of the Gutenberg-Richter relation of the activity (b-value = 1.55) is higher than the tectonically normal value of 1.0 and may be related to the first geothermal operations. However, the seismological data of the national network after 2011 do not show a long-time evident relation between moderate/large earthquake occurrence and geothermal operation activity in Germencik.
Tan, O., 2025. Imaging fault geometries in the upper crust using high-resolution earthquake hypocenters in the Germencik geothermal field, western Türkiye. Turkish Journal of Earth Sciences, 34, 68-77, https://doi.org/10.55730/1300-0985.1946. [ PDF ]
Figure 1. Geological setting (modified from Şimsek and Gülgör, 1988) and seismicity of Germencik. Cyan circles are the earthquakes between 2007 and 2024 located in this study. The earthquakes between 1960 and 2006 are shown with grey circles (from the TURHEC database by Tan, 2021). Solid line are normal faults from the MTA Active Fault Map (Emre et al., 2013). Büyük Menderes Detachment Fault is denoted with thick red line. Co: Conglomerate, Sa: Sandstone, Sc: Schist, Si: Siltstone. Geothermal fields (G.F.) are also shown. Inset map: The selected seismological stations (green triangles) around Germencik (red dot).
Figure 3. Statistical characteristics of the Germencik earthquakes. a) Seismic activity histogram through the years. b) Gutenberg-Richter relations for three different periods. c) Horizontal (epicenter) and vertical (depth) uncertainties of the absolute event locations.
Figure 4. Improved earthquake epicenters in Germencik after the DD inversion (a). The depth section of A-A' profile with 8 km wide, centered at Mursallı village, is given in (b). Solid grey line denotes the listric normal fault surface with a clear hypocentral image. Dashed lines (LF) denote deep listric normal faults. F: Mapped normal fault traces on the surface. BMGDF: Büyük Menderes Graben Detachment Fault. ÖGF: Ömerbeyli geothermal field.
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