remote sensing Article Secondary Fault Activity of the North Anatolian Fault near Avcilar, Southwest of Istanbul: Evidence from SAR Interferometry Observations Faqi Diao 1,*, Thomas R. Walter 2, Federico Minati 3, Rongjiang Wang 2, Mario Costantini 3, Semih Ergintav 4, Xiong Xiong 1 and Pau Prats-Iraola 5 1 State Key Laboratory of Geodesy and Earth’s Dynamics, Institute of Geodesy and Geophysics, Chinese Academy of Sciences, Wuhan 430077, China; [email protected] 2 GFZ-German Research Centre for Geosciences, Telegrafenberg, Potsdam 14473, Germany; [email protected] (T.R.W.); [email protected] (R.W.) 3 e-GEOS—An ASI/Telespazio Company, Via Tiburtina 965, Rome 00156, Italy; [email protected] (F.M.); [email protected] or [email protected] (M.C.) 4 Department of Geodesy, Kandilli Observatory and Earthquake Research Institute, Bo˘gaziciUniversity, Istanbul 34680, Turkey; [email protected] 5 DLR German Aerospace Center, Microwaves and Radar Institute, Weßling 82234, Germany; [email protected] * Correspondence: [email protected]; Tel.: +86-27-6888-1314 Academic Editors: Magaly Koch and Prasad S. Thenkabail Received: 12 July 2016; Accepted: 11 October 2016; Published: 18 October 2016 Abstract: Strike-slip faults may be traced along thousands of kilometers, e.g., the San Andreas Fault (USA) or the North Anatolian Fault (Turkey). A closer look at such continental-scale strike faults reveals localized complexities in fault geometry, associated with fault segmentation, secondary faults and a change of related hazards. The North Anatolian Fault displays such complexities nearby the mega city Istanbul, which is a place where earthquake risks are high, but secondary processes are not well understood. In this paper, long-term persistent scatterer interferometry (PSI) analysis of synthetic aperture radar (SAR) data time series was used to precisely identify the surface deformation pattern associated with the faulting complexity at the prominent bend of the North Anatolian Fault near Istanbul city. We elaborate the relevance of local faulting activity and estimate the fault status (slip rate and locking depth) for the first time using satellite SAR interferometry (InSAR) technology. The studied NW-SE-oriented fault on land is subject to strike-slip movement at a mean slip rate of ~5.0 mm/year and a shallow locking depth of <1.0 km and thought to be directly interacting with the main fault branch, with important implications for tectonic coupling. Our results provide the first geodetic evidence on the segmentation of a major crustal fault with a structural complexity and associated multi-hazards near the inhabited regions of Istanbul, with similarities also to other major strike-slip faults that display changes in fault traces and mechanisms. Keywords: fault activity; interferometric synthetic aperture radar (InSAR); persistent scatterer interferometry (PSI); Istanbul; seismic hazard assessment; North Anatolian fault 1. Introduction Crustal strike-slip faults are found to be highly segmented, with patches of high or low slip, and parasitic or associated faults that may sporadically become seismically active [1–3] and often localize at geometric bends of the main fault trace [4]. The North Anatolian Fault (NAF) is one of the most active strike-slip faults in the world and can be traced from eastern Turkey to the northern Aegean Sea with a scale of ~1200 km [5]. Near the Almacık block, the NAF separates into two branches, and the northern Remote Sens. 2016, 8, 846; doi:10.3390/rs8100846 www.mdpi.com/journal/remotesensing Remote Sens. 2016, 8, 846 2 of 17 Remote Sens. 2016, 8, 846 2 of 17 thebranch northern of the branch NAF(NNAF) of the NAF continues (NNAF) submarine continues into submarine the Marmara into the Sea, Marmara aligning Sea, with aligning the north with and thesouth north slopes and ofsouth the slopes Çınarcık of the Basin Çınarc [6] (Figureık Basin1). [6] The (Figure strike 1). of The the northernstrike of the branch northern then branch bends fromthen bendsthe northwest from the to northwest the west [to6]. the As wewest shall [6]. discussAs we inshall this discuss work, thein this localization work, the of thislocalization geometric of bendthis geometricis also the bend region is ofalso fault the complexity region of nearfault Küçükçekmececomplexity near Lake, Küçükçekmece associated with Lake, secondary associated faulting with secondaryprocesses thatfaulting can beprocesses observed that using can synthetic be observed aperture using radar synthetic (SAR) persistentaperture radar scatterer (SAR) interferometry persistent scatterer(PSI), a modern interferometry space geodetic (PSI), a technique.modern space geodetic technique. FigureFigure 1. 1. GeologicalGeological map map of of the the study study area. area. The The black black lines lines indicate indicate the the distribution distribution of active of active faults faults in thisin thisarea area [7], [7whereas], whereas the the dashed dashed gray gray lines lines show show the the trace trace of of the the secondary secondary faults faults southwest southwest of of IstanbulIstanbul [8]. [8]. The The red red line line and and red red star star show show the the coseismic coseismic rupture rupture and and epicenter epicenter of of the İIzmit/Düzce˙zmit/Düzce earthquakes,earthquakes, respectively. respectively. The The black black box box draws draws the the st studyudy area area of of this this paper. paper. The The dashed dashed orange orange boxes boxes showshow the the spatial spatial coverage coverage of of the the ascending ascending and and desce descendingnding track track in in this this area. area. NNAF: NNAF: Northern Northern branch branch ofof the the North North Anatolian Anatolian Fault; Fault; KL: KL: Küçükçekmece Küçükçekmece La Lake;ke; BL: BL: Büyükçekmece Büyükçekmece Lake. Lake. The The upper upper left left panel panel showsshows thethe shadedshaded relief relief map map of the of study the area, study in which area, the in isolate which red the areas isolate show activered landslidesareas show [9]. active landslides [9]. The 1999 Izmit/Düzce˙ earthquake disaster highlighted the high seismic hazard and revealed a The 1999 İzmit/Düzce earthquake disaster highlighted the high seismic hazard and revealed a westward propagation of earthquakes towards the Marmara Sea [10]. The tectonic evolution of the westward propagation of earthquakes towards the Marmara Sea [10]. The tectonic evolution of the NNAF beneath the Marmara Sea has been widely discussed in past decades; some authors suggest NNAF beneath the Marmara Sea has been widely discussed in past decades; some authors suggest that the NNAF deforms as a single shear zone [11], whereas others suggest that the crust beneath the that the NNAF deforms as a single shear zone [11], whereas others suggest that the crust beneath the Marmara Sea is separated by an E-W trending graben [12]. Besides, the evolution of the NNAF zone has Marmara Sea is separated by an E-W trending graben [12]. Besides, the evolution of the NNAF zone also been interpreted as a combination of en-echelon strike-slip faults and pull-apart basins [6,13,14] has also been interpreted as a combination of en-echelon strike-slip faults and pull-apart associated with numerous subparallel fault scarps of unknown activity [7,15]. These investigations basins [6,13,14] associated with numerous subparallel fault scarps of unknown activity [7,15]. These mostly focused on the fault activity beneath the Marmara Sea, but the possible activity of secondary investigations mostly focused on the fault activity beneath the Marmara Sea, but the possible activity faults north of the Marmara Sea remained to be studied. As the faults are shaping the land and directly of secondary faults north of the Marmara Sea remained to be studied. As the faults are shaping the affect the populated regions of western Istanbul, monitoring the fault activity in this region is of land and directly affect the populated regions of western Istanbul, monitoring the fault activity in great importance. this region is of great importance. Previous studies on seismic and bathymetric observations show evidence of the existence of Previous studies on seismic and bathymetric observations show evidence of the existence of secondary faults near the two lakes (Büyükçekmece Lake (BL) and Küçükçekmece Lake (KL); Figure1), secondary faults near the two lakes (Büyükçekmece Lake (BL) and Küçükçekmece Lake (KL); north of the Marmara Sea [8,16–19]. These secondary faults were found to be NW-SE oriented following Figure 1), north of the Marmara Sea [8,16–19]. These secondary faults were found to be NW-SE the geographic alignment of the lake, which also show general consistency with the inland drainage oriented following the geographic alignment of the lake, which also show general consistency with pattern and the scarps of the surrounding terrain [19]. Furthermore, the agreement between the strike the inland drainage pattern and the scarps of the surrounding terrain [19]. Furthermore, the and location of the secondary faults and off-shore active faults indicates that the secondary faults near agreement between the strike and location of the secondary faults and off-shore active faults indicates the lakes are not local fault systems, but related to the NW extension of the NNAF [16]. that the secondary faults near the lakes are not local fault systems, but related to the NW extension Near-field geodetic observations can provide an independent constraint on the present activity of of the NNAF [16]. these secondary faults. Recent GPS observations show local crust deformation (~5 mm/year) near Near-field geodetic observations can provide an independent constraint on the present activity KL [17,20], however, the inferred results may suffer from the spatial limitation of the survey-mode GPS of these secondary faults. Recent GPS observations show local crust deformation (~5 mm/year) near KL [17,20], however, the inferred results may suffer from the spatial limitation of the survey-mode GPS observation.