Marmara Sea region in northwestern Turkey with the North Anatolian Fault Zone (NAFZ) separating Eurasia from Anatolian.
The offshore Marmara fault where a major earthquake is overdue is indicated by the red line. The black lines to either side are the two last major ruptures of the region, the 1912 Ganos and the 1999 Izmit earthquakes. The Marmara section has not produced a large earthquake since 1766 but is known to rupture every ~250 years based on historical records. The yellow stars mark repeating earthquakes found in a recently published study indicating fault creep (green rectangle) while the fault portion offshore of Istanbul (blue rectangle) is locked.
Bottom right: Location map of the Anatolian region with the main tectonic features framing the North Anatolian Fault. The black box marks the enlarged area. Bold black arrows indicate the direction of the plate motion with respect to stable Eurasia (modified from Bohnhoff et al., 2017b).
SOURCE GONAF[1]
[1] GONAF (Geophysical borehole Observatory at the North Anatolian Fault) is a joint research venture between Helmholtz Centre Potsdam German Research Centre for Geosciences Potsdam (GFZ) and the Turkish Disaster and Emergency Presidency (AFAD) in Ankara. It is co-funded by GFZ, AFAD, the Turkish Ministry of Development, the International Scientific Drilling Program (ICDP), and the German Helmholtz Association (HGF). The aim of this project is to monitor earthquake activity at low magnitude-detection threshold in the Istanbul / eastern Sea of Marmara region where a major (M>7) earthquake is pending.
GONAF is currently comprised of seven 300 m deep vertical seismic profiling stations and four collocated 100 m deep borehole strainmeters. Five of the stations are located on the land surrounding the Princes Islands segment below the eastern Sea of Marmara and two are on the near-fault Princes Islands south of Istanbul. The 300 m boreholes have 1, 2, and 15 Hz 3-C seismometers near their bottoms. Above this are vertical, 1 Hz, seismometers at ∼ 210, 140, and 70 m depths.