GONAF - A deep Geophysical Observatory at the North Anatolian Fault
The GONAF project involves the installation of a high-resolution borehole seismic observatory at the North Anatolian Fault Zone (NAFZ) consisting of several shallow (up to 500m deep) vertical boreholes in onshore locations around the eastern Sea of Marmara/NW Turkey. The principal scientific objective is to study physical processes acting before, during and after the expected M>7 earthquake along the Princes Islands segment of the NAFZ by monitoring microseismic activity at significantly reduced magnitude detection threshold and improved hypocentral resolution. It is also intended to study wave propagation characteristics of a large earthquake using downhole seismic recordings at different spots along the potential rupture.
The most recent M>7 earthquakes at the NAFZ occurred in 1999 near Izmit and Düzce and temporarily produced accelerated seismic activity along the NAFZ south of the greater Istanbul area below the Sea of Marmara now representing a seismic gap of up to 150 km length. This part of the NAFZ is the only segment that has not been activated in the present series of major earthquakes and may have accumulated a slip deficit of up to 4-5 m since the last event in 1766.
Figure 1: The North Anatolian Fault Zone (NAFZ) in NW Turkey. The white rectangle indicates the location of the GONAF borehole observatory that focuses on the eastern part of the seismic gap below the Sea of Marmara. Red lines mark fault branches of the NAFZ, yellow bold lines represent the last major ruptures of the region in 1912 and 1999, respectively. Stars indicate hypocenters of major (M>6.8) earthquakes during the last 2000 years (after Ambraseys, 2002: Parsons, 2004). The white arrow indicates the GPS-derived horizontal motion of the Anatolian plate with respect to stable Eurasia (Figure after Bohnhoff et al., 2011).