by Georg Dresen, Marco Bohnhoff, Mustafa Aktar, Haluk Eyidogan, Hisao Ito

The North Anatolian Fault Zone (NAFZ) is the most active plate-bounding strike-slip fault in Europe following an EW trend offshore through the Sea of Marmara within less than 20 km south of Istanbul. The fault has produced a series of devastating earthquakes during the 20th century starting in 1939 in eastern Anatolia and then propagating westwards. The most recent M>7 earthquakes occurred in 1999 near Izmit and Düzce. We propose the installation of a deep borehole observatory in a 1000 m deep borehole at the offshore part of the North Anatolian Fault Zone. The proposed GONAF observatory is located on Sivriada island (Princes Islands) representing the only possible onshore monitoring site close to the NAFZ and the city of Istanbul. GONAF is located at the transition between the western end of the 1999 Izmit rupture and an up to 150 km long seismic gap of the NAFZ below the Sea of Marmara. This segment may have accumulated a 4-5 m slip deficit since the last event in 1766 and is now considered to be close to failure. GONAF is focussed on the installation of a deep borehole geophysical observatory with the principal aim to monitor microearthquake activity prior to an expected large earthquake and close to the seismically active part of the fault zone at a depth between 4 to15 km. It will involve different types of downhole seismometers covering a wide frequency range and will be combined with an array of existing surface and planned shallow borehole seismometers to enhance the resolution and magnitude-detection threshold of seismological observations by several orders of magnitude. This allows studying the rupture process and interaction of small-magnitude events close to the source region of the expected Marmara earthquake with unprecedented detail. Heat and fluid flow close to the NAFZ will be monitored. The orientation and magnitude of local stresses and strains at the fault will be determined to assist estimating the in-situ strength at a major plate boundary fault. GONAF will give new insights into potential transients and scale dependence of physical processes responsible for the preparation of a large earthquake in a transform fault zone during the seismic cycle. GONAF constitutes an advanced and multi-disciplinary monitoring effort that will also provide important contributions towards enhanced ground shaking models. Moreover, the GONAF observatory will improve near-real time hazard assessment and earthquake early-warning for the mega-city of Istanbul with its >13 million inhabitants.

• The principal scientific objectives of GONAF are to study the physical processes that control deformation and earthquake generation at the North Anatolian Fault Zone which is one of the major plate bounding strike-slip faults on Earth. The GONAF site is located at the Marmara seismic gap of the NAFZ where a major earthquake is expected to occur based on our current knowledge of historical seismicity, earthquake recurrence times, stress modeling and fault structure. The planned observatory will also help improving the capability of seismic hazard assessment and earthquake early-warning for the nearby mega-city of Istanbul with its >13 million inhabitants. We focus on the installation of a deep borehole geophysical observatory with the aim to primarily monitor microseismic activity in direct vicinity of the fault, but also stress, heat and fluid flow. A deep borehole geophysical observatory in close proximity to the NAFZ near a branching point between strike-slip/normal and strike-slip/thrust fault segments of the NAFZ will allow addressing key questions such as:
• 1. How do earthquakes nucleate and how do earthquake ruptures propagate?,
• 2. How do earthquake source parameters scale with magnitude?,
• 3. How is deformation partitioned between aseismic creep and seismic slip at the termination of a major earthquake rupture?,
• 4. What is the orientation of the principal stresses close to the fault zone?,
• 5. Is the North Anatolian Fault Zone in North-Western Turkey a weak or a strong plate boundary?

Borehole Observatory, Europe, GONAF, ICDP-2009/04, Marmara Sea, NAFZ, Northern Anatolian Fault Zone, Seismic Monitoring, Turkey

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