ALBERT

Description: Western Turkey provides spectacular examples of the two end-member models of deformation of the continental lithosphere, with strain localization along the North Anatolian fault and distributed north–south extension along the Aegean coast. To provide constraints on the mechanisms of continental deformation, we present a new high-resolution image of lithospheric structure along a ~650 km transect crossing Western Anatolia at 28°E longitude from the Black Sea to the Mediterranean. More than 2600 receiver functions are computed from records of teleseismic earthquakes at 40 broadband seismic stations with an average spacing of 15 km. Lateral variations of crustal thickness and Vp / Vs ratio are inferred from both H - k and common conversion point stacks. We observe long-wavelength variations of Moho depth from ~31 km in the Thrace basin to ~25 km beneath the Marmara Sea, ~32 km beneath the Izmir–Ankara suture, ~25 km beneath the Menderes Massif and ~20 km on the coast of the Mediterranean. No mid- or lower-crustal interface is visible in the migrated depth section and seismic discontinuities are confined to the shallow crust. The small-amplitude and long-wavelength lateral variations of the Moho topography suggest that viscous flow in a hot lower crust has smoothed out the lateral variations of crustal thickness induced by Cenozoic continent–continent collision. The crust–mantle boundary is flat beneath the central and southern Menderes Massif. The rougher Moho topography and more heterogeneous crust of the Marmara region likely result from transtension in the North Anatolian Fault Zone superimposed onto Aegean extension. The Moho of the subducted African lithosphere is observed dipping northward between ~40 and ~60 km depth at the southern end of the profile. The abrupt termination of the subducted slab only 50 km to the north of the Mediterranean coast confirms the slab tear inferred from previous tomographic studies.

Description: The North Anatolia Fault Zone (NAFZ) is a transform zone 1600 km in length representing the plate boundary between the westward moving Anatolian Plate and stable Eurasia. Almost the entire fault zone has failed during the last century except for the Sea of Marmara section, which is located in direct vicinity to the city of Istanbul. In this study, we investigate the crustal anisotropy along the eastern Marmara section of the NAFZ based on shear-wave splitting. We measure seismic anisotropy parameters, namely, the fast polarization direction (PD) and time delay (TD), by analyzing local seismicity recorded at selected seismographs operated throughout the eastern Sea of Marmara region. Our shear-wave splitting (SWS) observations indicate a predominant northwest–southeast-oriented PD, which is sub-parallel to both the orientation of the regional S Hmax in northwest Turkey and the local NAFZ strike along the Princes’ Islands segment. Toward the south, at the Armutlu Peninsula, we find a different PD pattern reflecting local fault strikes, S Hmax as well as strain asymmetry between different crustal blocks across the fault zone. Applying strict quality criteria enables us to identify robust, preferred fast PDs, which suggests that initially observed PD heterogeneities are sometimes caused by second-order effects in the data rather than by varying PDs. Comparing TD and hypocentral depth distribution, we find the depth extent of the anisotropy is confined to the uppermost 10-km depth of crust. We combine our SWS results with those from previous studies conducted along the San Andreas fault (SAF) and NAFZ in order to investigate the relation of angular deviations of the PDs from regional S Hmax and local fault strikes with fault-zone distance. We find that fast PDs are mainly controlled by the local fault structure in close proximity to a fault zone (5 and 10 km) while they are controlled by crustal stress at off-fault locations (5 and 10 km).

Description: The North Anatolia fault zone (NAFZ) is a transform zone 1600 km in length representing the plate boundary between the westward moving Anatolian Plate and stable Eurasia. Almost the entire fault zone has failed during the last century except for the Sea of Marmara section, which is located in direct vicinity to the city of Istanbul. In this study, we investigate the crustal anisotropy along the eastern Marmara section of the NAFZ based on shear-wave splitting. We measure seismic anisotropy parameters, namely, the fast polarization direction (PD) and time delay (TD), by analyzing local seismicity recorded at selected seismographs operated throughout the eastern Sea of Marmara region. Our shear-wave splitting (SWS) observations indicate a predominant northwest-southeast-oriented PD, which is sub-parallel to both the orientation of the regional S (sub Hmax) in northwest Turkey and the local NAFZ strike along the Princes' Islands segment. Toward the south, at the Armutlu Peninsula, we find a different PD pattern reflecting local fault strikes, S (sub Hmax) as well as strain asymmetry between different crustal blocks across the fault zone. Applying strict quality criteria enables us to identify robust, preferred fast PDs, which suggests that initially observed PD heterogeneities are sometimes caused by second-order effects in the data rather than by varying PDs. Comparing TD and hypocentral depth distribution, we find the depth extent of the anisotropy is confined to the uppermost 10-km depth of crust. We combine our SWS results with those from previous studies conducted along the San Andreas Fault (SAF) and NAFZ in order to investigate the relation of angular deviations of the PDs from regional S (sub Hmax) and local fault strikes with fault-zone distance. We find that fast PDs are mainly controlled by the local fault structure in close proximity to a fault zone (5 and 10 km) while they are controlled by crustal stress at off-fault locations (5 and 10 km).

Description: We investigate earthquakes with similar waveforms in order to characterize spatiotemporal microseismicity clusters within the North Anatolian fault zone (NAFZ) in northwest Turkey along the transition between the 1999 Izmit rupture zone and the Marmara Sea seismic gap. Earthquakes within distinct activity clusters are relocated with cross-correlation derived relative travel times using the double-difference method. The spatiotemporal distribution of microearthquakes within individual clusters is resolved with relative location accuracy comparable to or better than the source size. High-precision relative hypocenters define the geometry of individual fault patches, permitting a better understanding of fault kinematics and their role in local-scale seismotectonics along the region of interest. Temporal seismic sequences observed in the eastern Sea of Marmara region suggest progressive failure of mostly nonoverlapping areas on adjacent fault patches and systematic migration of microearthquakes within clusters during the progressive failure of neighboring fault patches. The temporal distributions of magnitudes as well as the number of events follow swarmlike behavior rather than a mainshock/aftershock pattern.

Description: Over the last century the North Anatolian Fault Zone in Turkey has produced a remarkable sequence of large earthquakes. These events have now left an earthquake gap south of Istanbul and beneath the Marmara Sea, a gap that has not been filled for 250 years. Here we investigate the nature of the eastern end of this gap using microearthquakes recorded by seismographs primarily on the Princes Islands offshore Istanbul. This segment lies at the western terminus of the 1999 Mw7.4 Izmit earthquake. Starting from there, we identify a 30-km-long fault patch that is entirely aseismic down to a depth of 10 km. Our evidence indicates that this patch is locked and is therefore a potential nucleation point for another Marmara segment earthquake—a potential that has significant natural hazards implications for the roughly 13 million Istanbul residents immediately to its north.

Description: The Marmara segment of the North Anatolian Fault Zone (NAFZ) in NW Turkey currently represents a ‚seismic gap’ that has not been activated since 1766 and that has the potential to host a M〉7 earthquake in the near future close to the Mega-City Istanbul with its 14 million inhabitants. To characterize the Marmara fault segment a local seismic network was installed offshore Istanbul on the Princes Islands in the direct vicinity to the fault (PIRES campaign) as part of the GFZ Plate Boundary Initiative. Recordings of the local microseismicity now allow for characterization of the fault below the Sea of Marmara for the first time. Distinct seismically active spots indicate that strain is released only partially along the seismic gap. Implementing recordings from seismic stations throughout the area results in stable fault plane solutions at refined hypocentral precision and an unprecedented perception of the seismotectonic setting. Current expansion of the PIRES network will provide for an increase in the data base at improved hypocentral resolution and also enable the performance of comparative studies e.g. with the San Andreas Fault in California.

Description: The North Anatolian Fault Zone (NAFZ) below the Sea of Marmara represents a 'seismic gap' where a major earthquake is expected to occur in the near future. The Marmara segment of the NAFZ is located between the 1912 Ganos and 1999 Izmit ruptures and is the only segment that has not ruptured since 286 - Seismologie - Naturgefahren und Geophysikund Geophysik 1766. To monitor the microseismic activity at the main fault branch offshore of Istanbul below the Cinarcik Basin a permanent seismic network (PIRES) was installed in 2006 on the Princes Islands, at a few kilometers distance to the main fault branch. PIRES recordings are combined with data from local permanent stations of the Turkish network and the ARNET seismic network on the Armutlu peninsula in order to get the best available azimuthal control for the target area. We obtain a well-resolved hypocenter catalog of microseismicity allowing us to discriminate seismically active from inactive fault patches along the eastern part of the Marmara seismic gap. The results show that the seismicity generally tends to cluster slightly off the main fault, probably along splay faults, with a well-defined internal spatiotemporal migration. This probably means that the main fault is locked and the slip is transferred to secondary structures which might be due to the major NAFZ branch approaching a late stage of the seismic cycle. Furthermore, PIRES recordings are also used for characterization and preselection of borehole locations in the frame of the ICDP-GONAF project aiming at installing a downhole geophysical observatory throughout the eastern Marmara region.