The NAFZ is a right-lateral strike-slip fault. However, the transtensional setting of the broader Marmara region also produces surprisingly large normal faulting events, e.g., a Ms 6.3 earthquake along the Çınarcık segment below the eastern Sea of Marmara in 1963 (Ayhan et al., 1981). There is an on-going debate on whether the pending Marmara event will be pure strike-slip or might include a normal component with consequent tsunami potential (Yalciner et al., 2002; LePichon et al., 2001; Armijo et al., 1999, 2005). Accordingly, for all the risk factors we have cited, it is a pressing matter to resolve the dominant kinematic setting along the Marmara section. This is especially true for the Princes Islands segment just offshore from Istanbul.
Two equally important objectives are to identify potentially creeping and locked fault patches (asperities) - the latter as potential nucleation points for a large rupture with subsequent directivity effects and ground shaking. The installation of seismometer arrays in boreholes allows not only relatively noise-free waveform recordings, but also studies of near-surface ground motion amplification effects. Fault creep observed on borehole strainmeters provides strong constraints on the accumulation of unreleased strain, both in terms of the amount of fault slip and its areal distribution. The fact that the entire Marmara section of the NAFZ is under water makes high-resolution seismic and geodetic monitoring of its seismicity and potential creep a challenging task. Our first step in this regard was to install the surface-based PIRES seismic network on the Princes Islands in 2006. The PIRES network has lowered the magnitude of completeness for local microseismicity by more than 1 order of magnitude to ∼1.4 (Raub et al., 2017).
Based on a 4-year PIRES seismicity catalog, Bohnhoff et al. (2013) were able to identify a ≥ 30 km long aseismic fault patch along the Princes Islands segment extending down to a depth of 10 km. It was concluded that this segment is currently locked and thus represents a potential nucleation point for the next Marmara event. This view is also supported by Global Positioning Satellite (GPS) data (Ergintav et al., 2014). Beyond the PIRES network, better monitoring of the eastern Sea of Marmara and the Princes Islands segment would be difficult to achieve with surface instruments due to the area's ubiquitous anthropogenic noise. Moreover, near-fault, subareal locations only exist on the Princes Islands. Given these boundary conditions, the GONAF concept of borehole-based geophysical instrumentation surrounding the eastern Sea of Marmara region was conceived and developed in the context of an International Continental Drilling Program (ICDP) project. Overall, the goal of the GONAF project is a substantially better characterization of the seismic gap first identified by the PIRES and other associated networks. In the long run, outside of any pending significant earthquake, this objective will allow scientific correlations with the findings of other strike-slip faults, such as the San Andreas fault in California. In addition to the study of brittle failures in the form of seismic events, GONAF is also addressing the role of aseismic deformation processes through strain monitoring. These processes included fault creep and slow earthquakes, which are now recognized as playing equally fundamental roles in the seismomechanics of the earth's crust.