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.
The strainmeter boreholes are located within a few meters of the seismometer boreholes and contain horizontal strain tensor sensors and 2 Hz 3-C seismometers at their bottoms. This selection of instruments and depths was made so as to ensure high-precision and broad-frequency earthquake monitoring and vertical profiling, all under low-noise conditions.
GONAF is the first ICDP-driven project with a primary focus on long-term monitoring of fault-zone dynamics. It has already contributed to earthquake hazard studies in the Istanbul area in several ways. Combining GONAF recordings with existing regional seismic stations now allows monitoring of the NAFZ offshore of Istanbul down to magnitudes M < 0. GONAF also improves the resolution of earthquake hypocenters and source parameters, better defining local fault branches, their seismicity, and earthquake potential. Using its vertical distribution of sensors, it has directly measured depth-dependent seismic side-effects for ground shaking studies. GONAF is starting to address fundamental questions related to earthquake nucleation, rupture dynamics, temporal changes in material properties and strain.