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.
The location of the borehole seismometer network is unique representing the only possible long-term monitoring sites along the NAFZ segment below the Sea of Marmara and the city of Istanbul in an onshore location. Combining the high-resolution network recordings with existing nearby surface arrays and regional permanent stations will allow to substantially improve seismic monitoring conditions along the entire Princes Islands segment by lowering the magnitude-detection threshold by at least one order of magnitude thus allowing to study the spatial and temporal evolution of microseismic activity prior to the expected Marmara earthquake with unprecedented detail. The GONAF project is expected to provide new insights into physical processes acting prior and potentially also during and after a large (M>7) earthquake and to refine and calibrate ground shaking models and near-real time hazard assessment for the mega-city of Istanbul.