by Marco Bohnhoff, Georg Dresen, Recai Feyiz Kartal

We propose a borehole observatory consisting of two 500 m deep vertical boreholes 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 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 two different spots along the expected rupture and potentially close to its initiation point. The North Anatolian Fault Zone (NAFZ) 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 Sea of Marmara today represents a seismic gap of up to 150 km length and this part of the NAFZ is the only segment that has not been activated in the present series. 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 suggested borehole locations are on two of the Princes Islands in direct vicinity to the major branch of the fault zone and the mega-city of Istanbul. 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 focussed on the installation of a deep borehole seismological observatory. Combining GONAF recordings with existing nearby surface arrays 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. Prior to the long-term installation of the seismological observatory we will use the GONAF boreholes to also measure heat and gas/fluid flow and to determine orientation and magnitude of local stresses for the first time in the entire Marmara region.

• 1.The main scientific objective of the project is to study the physical processes along the entire Princes Islands segment of the North Anatolian Fault Zone (NAFZ) using a combined surface and downhole high resolution seismic network.
• To achieve this goal we plan to use detailed observations of microseismicity in close vicinity to the fault providing new insights into the physical state of a fault segment prior to, and potentially also during and after a M7+ earthquake;
• 2. Downhole seismological observations at substantially improved signal-to-noise ratio and thus reduced magnitude-detection threshold over the entire frequency band of the seismic wave spectrum at two different locations close to the expected large rupture. GONAF will provide unique seismological data on source parameters, scaling relations and recurrence statistics in great detail and also over a broad range of magnitudes. We will make use of the entire frequency spectrum of the wavefront generated by microseismic events along the fault but potentially also during the expected large earthquake irrespective of where this event will nucleate along the seismic gap.
• 3. Detailed mapping of spatiotemporal evolution of microseismicity and fault mechanisms that will reveal progressive damage evolution at a potential fault asperity.
• 4. Improved depth resolution of seismicity that will assist to elaborate on the depth extend of the expected large earthquake of the region thus allowing to better characterize the slip distribution with depth on a segment considered as a fault asperity. This is of particular relevance since recent results for microseismicity of the region suggest that the fault is seismic throughout its seismogenic depth down to ~15 km.
• 5. Stress measurements are planned for both boreholes to provide the first data on stress orientation and magnitude for the eastern Marmara Sea region.

Active Faulting, Borehole Observatory, Earthquake Physics, GONAF, High-Resolution Seismic Monitoring, ICDP-2010/03, Istanbul, Northern Anatolian Fault Zone, Seismic Hazards, Turkey

Latitude: 40.840174, Longitude: 29.123211