a deep Geophysical Observatory at the North Anatolian Fault
The key elements of GONAF are:
1. Implementation Phase: It is planned to drill eight up to 500 m deep vertical boreholes all around the eastern Sea of Marmara at close proximity to the surface trace of the main branch of the Marmara segment of the NAFZ below the Sea of Marmara. The primary purpose of the boreholes will be to install a deep geophysical borehole observatory at the NAFZ focusing on downhole seismic monitoring. After conducting open-hole geophysical logs it is planned to case and cement the hole down to the final depth. Coring is planned for selected depth sections and core material will be used to study rock physical and strength properties, to investigate lithology and microstructure and to perform stress relaxation testing. The holes will then be equipped with chains of borehole seismic instrumentation consisting of short-period, broadband and strong motion sensors to cover a broad part of the seismic frequency spectrum. It is also planned to install borehole strain and tilt meters as well as pressure and temperature sensors.
2. Observatory Phase: Once the GONAF observatory is up and running we intent to continuously monitor microearthquakes along the Princes Islands segment of the NAFZ with unprecedented resolution. GONAF recordings will improve the present magnitude-detection threshold and the precision of hypocenter determination by at least one order of magnitude. The downhole seismic recordings will be complemented by the surface-based PIRES seismic array on the Princes Islands that has been in operation since 2006 . The entire effort will significantly improve seismic monitoring conditions at a site where a major earthquake is expected to occur in the near future. GONAF will also allow a systematic determination and analysis of the relation between site-response and amplification of seismic waves with depth for the greater Istanbul area. The later information is highly relevant to prepare regional infrastructure in the broader Istanbul metropolitan area for the expected next major earthquakes at the Marmara segment of the NAFZ.
Figure: Satellite image of the eastern Sea of Marmara region with the Istanbul metropolitan region, the major fault segments of the NAFZ . The traces of the Marmara (red, last activated in 1766) and Izmit (green, last reactivated in 1999) fault segments are drawn simplified.