a deep Geophysical Observatory at the North Anatolian Fault
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GONAF aims at installing a borehole-based seismometer network at the offshore part of the NAFZ along the Princes Islands segment that includes the transition between the 1999 Izmit rupture and the current seismic gap (GONAF - Geophysical Observatory at the North Anatolian Fault). 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 North Anatolian Fault Zone (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 several different spots along the potential rupture.

Figure: Bathymetric map of the Sea of Marmara (modified after Armijo et al., 2005). The > 100 km long Marmara segment is located between the 1912 Ganos and 1999 Izmit ruptures. It has not been activated since 1766 and considered to host a M > 7 earthquake in the near future. The here proposed deep borehole observatory GONAF is located on the two outermost Prince Islands, Sivriada and Büyükada, in direct vicinity to the fault and close to the megacity of Istanbul with > 13 million inhabitants.

The North Anatolian Fault Zone (NAFZ) is the most active plate-bounding strike-slip fault in Europe that follows an EW trend offshore through the Sea of Marmara within less than 20 km south of Istanbul. The fault has produced a series of large and devastating earthquakes during the 20th century starting in 1939 in eastern Anatolia and then systematically propagating westwards. The most recent M > 7 earthquakes 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 and may have accumulated a slip deficit of up to 4-5 m since the last event in 1766. Recent estimates indicate a 35-70% probability for the occurrence of a M > 7 earthquake close to the population center of Istanbul by 2034. Owing to post-seismic stress redistribution after the 1999 Izmit earthquake the eastern part of the seismic gap along the Princes Islands segment is likely subjected to enhanced stresses.
The location of the observatory 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. GONAF is focused on the installation of a deep borehole seismological observatory. Combining GONAF recordings with the existing PIRES network and regional permanent stations will allow to substantially improve 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. GONAF will provide new insight into physical processes acting prior and potentially also during and after a large (M > 7) earthquake at a major transform fault zone during the seismic cycle. Moreover, GONAF is expected to address fundamental questions related to rupture dynamics, temporal changes of material properties and to refine and calibrate ground shaking models and near-real time hazard assessment for the mega-city of Istanbul with its > 13 million inhabitants.