The Marmara section of the North Anatolian Fault Zone (NAFZ) runs under water and is located less than 20 km from the 15-million-person population center of Istanbul in its eastern portion. Based on historical seismicity data, recurrence times forecast an impending magnitude M>7 earthquake for this region. The permanent GONAF [1](Geophysical Observatory at the North Anatolian Fault) has been installed around this section to help capture the seismic and strain activity preceding, during, and after such an anticipated event. Mavi Boncuk |
The Sea of Marmara Region with the offshore Marmara Section of the North Anatolian Fault Zone (NAFZ) delineated in red. In its continuation lie the ruptures of the 1912 Ganos and 1999 Izmit earthquakes (delineated in black). Location of topographic features are indicated with capital letters: Western High (WH), Central Basin (CB), Central High (CH), Çınarcık Basin (CCB), Armutlu Peninsula (AP) all after (Bécel et al. 2009), Gulf of Gemlik (GG), Erdek Tombolo (ET), Ganos Mountains (GM) all after (Şengör et al. 2014). Permanent seismic stations are marked by different symbols according to their associated network. The bathymetry, submarine faults and onshore faults are after Le Pichon et al. (2001), Armijo et al. (2005) and the Turkey General Directorate of Mineral Research and Exploration (pers. comm.), respectively. The extent of larger cities (Istanbul, Izmit, Bursa) is indicated by pink areas. The inset in the lower right shows the area of study in the broader tectonic regime where the movement of the Anatolian Plate is given with respect to stable Eurasia. 6812 epicenters of selected seismicity are shown. Events within blue areas are likely to be quarry blasts (after Wollin et al. 2018)
SEE ALSO: Megacity Istanbul Project Reports
Municipality Disaster Management Center (AKOM), Istanbul, Turkey
22 October, 2008
[1] 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. Marmara Sea region in northwestern Turkey with the North Anatolian Fault Zone (NAFZ) separating Eurasia from Anatolian.
The offshore Marmara fault where a major earthquake is overdue is indicated by the red line. The black lines to either side are the two last major ruptures of the region, the 1912 Ganos and the 1999 Izmit earthquakes. The Marmara section has not produced a large earthquake since 1766 but is known to rupture every ~250 years based on historical records. The yellow stars mark repeating earthquakes found in a recently published study indicating fault creep (green rectangle) while the fault portion offshore of Istanbul (blue rectangle) is locked.
Bottom right: Location map of the Anatolian region with the main tectonic features framing the North Anatolian Fault. The black box marks the enlarged area. Bold black arrows indicate the direction of the plate motion with respect to stable Eurasia (modified from Bohnhoff et al., 2017b).
The strainmeter boreholes are located within a few meters of the seismometer boreholes and contain horizontal strain tensor sensors and 2 Hz 3-C seismometers at their bottoms. This selection of instruments and depths was made so as to ensure high-precision and broad-frequency earthquake monitoring and vertical profiling, all under low-noise conditions.
GONAF is the first ICDP-driven project with a primary focus on long-term monitoring of fault-zone dynamics. It has already contributed to earthquake hazard studies in the Istanbul area in several ways. Combining GONAF recordings with existing regional seismic stations now allows monitoring of the NAFZ offshore of Istanbul down to magnitudes M < 0. GONAF also improves the resolution of earthquake hypocenters and source parameters, better defining local fault branches, their seismicity, and earthquake potential. Using its vertical distribution of sensors, it has directly measured depth-dependent seismic side-effects for ground shaking studies. GONAF is starting to address fundamental questions related to earthquake nucleation, rupture dynamics, temporal changes in material properties and strain.
SEE ALSO: A unified earthquake catalogue for the Sea of Marmara Region, Turkey, based on automatized phase picking and travel-time inversion: Seismotectonic implications Author links open overlay panelChristopherWollinaMarcoBohnhoffabPatriciaMartínez-GarzónaLudgerKüperkochcChristinaRauba Show more https://doi.org/10.1016/j.tecto.2018.05.020
Highlights
• Compilation of a new 10-year hypocenter catalogue for the Marmara region based on an optimized virtual seismic network
• Employment of a novel automated waveform picking procedure as well as an iterative travel time inversion
• Identification of seismically active and inactive fault patches along the offshore Marmara segment of the North Anatolian Fault Zone where a M7+ earthquake is overdue
• Observation of coherent fault segments and local faulting regimes from high-precision relocated hypocenters and focal mechanisms
Abstract
The Marmara section of the North Anatolian Fault Zone (NAFZ) is late in its seismic cycle and can be expected to produce a magnitude M up to 7.4 earthquake during the next decades in direct vicinity to the 15-million population center Istanbul. This setting translates the seismic hazard into very high risk and makes a thorough understanding of the current seismotectonic setting of this NAFZ section a pressing task. The absence of near-fault stations along the most part of the offshore Marmara section limits the reliability of existing seismicity catalogues for this region. For the first time we combine the different regional permanent networks thereby optimizing azimuthal coverage and present a refined hypocenter catalogue for the Sea of Marmara on this basis. Compared to the original locations, adoption of a refined automated technique to determine precise onset times for the different body waves and an iterative travel-time inversion scheme, lead to substantial improvement of 6812 absolute earthquake locations, particularly in the epicentral distribution. The automated processing is explained in detail. Further optimization is achieved through relative relocation of 4407 earthquakes. Our catalogue covers more than a decade (2006–2016) with a regional moment-magnitude of completeness of Mc = 2.1. The epicentral distribution delineates the Marmara Section, i.e. the northern NAFZ branch, as the seismically most active fault strand. We identify several aseismic fault patches that are interpreted to represent locked parts of the fault. Seismic activity in the past decade predominantly occurs off the main fault on the edges of the aseismic patches, supporting previous studies that the Marmara section of the NAFZ contains both locked and creeping fault portions. Single-event focal mechanisms (2.7 ≤ Mw ≤ 4.5) indicate that currently both strike-slip and normal faulting occur, confirming the transtensional setting of the region.