Abstract
In this study we derive the stress tensor and its local variations throughout the Marmara region, Turkey. Based on a recently compiled 10-year earthquake catalogue, we directly invert first-motion polarity data and quantify confidence intervals for the principal stress orientations. We find a combined strike-slip and normal faulting stress field for the Marmara region generally reflecting the overall transtensional setting. However, the results clearly show moderate local variations of the stress field. The largest (sigma _1) and intermediate (sigma _2) principal stresses show an average regional trend of N125{^circ }E and locally varying plunges. The least principal stress (sigma _3) is well resolved in its confidence interval and consistent throughout the region with an average trend of sim N35{^circ }E and a subhorizontal plunge. The eastern Sea of Marmara shows local stress field orientations with pronounced strike-slip (northern part) and normal faulting (southern part) components. Along the central Marmara region, normal faulting tends to dominate, while a well resolved strike-slip stress regime is found in the western Sea of Marmara region. Regarding the faulting mechanism of an earthquake with magnitude up to 7.4 which is expected in this area in direct vicinity of the Istanbul metropolitan region, our results imply that neither strike-slip nor normal faulting kinematics can be excluded.
Highlights
The vast majority of earthquakes are caused by failure of critically stressed faults within the seismogenic layer of the earth’s crust and along active tectonic plate boundaries (e.g. Wallace 1951; Bott 1959; McKenzie 1969; Vavrycuk 2015; Hardebeck and Okada 2018)
The Sea of Marmara region in northwestern Turkey is located along the plate-bounding North Anatolian Fault Zone (NAFZ) at the transition zone between the pure right-lateral part of the NAFZ to the east and the north–south extensional Aegean region to its southwest
Except for the westernmost areas of the region, the orientation of the r1-axis is consistent in its trend of about N125E but generally less well constrained in its plunge, the 95% confidence interval of its marginal probability density functions (PDFs) forming a great circle in the plane spanned with the r2-axis
Summary
The vast majority of earthquakes are caused by failure of critically stressed faults within the seismogenic layer of the earth’s crust and along active tectonic plate boundaries (e.g. Wallace 1951; Bott 1959; McKenzie 1969; Vavrycuk 2015; Hardebeck and Okada 2018). The Sea of Marmara region in northwestern Turkey is located along the plate-bounding North Anatolian Fault Zone (NAFZ) at the transition zone between the pure right-lateral part of the NAFZ to the east and the north–south extensional Aegean region to its southwest This setting results in a first-order transtensional tectonic regime (McClusky et al 2000; Le Pichon et al 2015; Bohnhoff et al 2016b) and in the opening of the Sea of Marmara as a pull-apart structure The submarine Marmara segment of the NAFZ currently represents a seismic gap capable of generating a major (M [ 7) earthquake in the decades (Parsons 2004; Bohnhoff et al 2013; Murru et al 2016) This translates into significant hazard and risk for the neighboring Istanbul metropolitan area with its 15? We discuss which type of faulting can be expected for the pending Marmara earthquake and its potential consequences for seismic risk scenarios
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