THE QUESTION OF LONG-TERM FAULT STRENGTH ACROSS DEFORMING ZONES IN TURKEY

In this study we make an inverse approach to the problem of fault strength distribution in major deforming zones in Turkey. We use a simple frictional model defined for a thin-sheet lithosphere. Although we are aware that the width of the deforming zone around even geometrically simple strike-slip faults increases with depth and that the physics of the phenomenon is not exactly represented by a thin-sheet approach, we believe that this simple approach can produce first-order quantitative clues on the problem . We treat the body forces for separate cases where we use seismologically determined crustal thicknesses, a large-scale standard crustal model and another simple model where Airy isostasy is almost satisfied. We also use different values of long-term pore pressure to see the overall effect of the variation of this parameter. Using a variational approach we solve the linear force balance equations where the forcing is simply the horizontal gradients in gravitational potential energy per unit area (GPE). This is similar to calculating the Green’s function for the Stokes equations for a viscous, non-accelerating thin-sheet continuum. We then exploit the linearity of the problem to solve for the stress boundary conditions to reach a style match between the stress and the strain rates (calculated using a GPS dataset) within the domain. In this preliminary work we assumed Airy isostasy and assumed no lateral density variations within the crust. The seismogenic thickness in Anatolia rarely exceeds 18 km and especially in western Turkey most of the deformation is known to be occuring seismically. Still, the brittle-ductile transition depth might change place to place. Rather than spatially varying the stress integration depths, we assumed uniform stress integration depth but experimented with various depth values. One interesting outcome of our calculations is that, for all reasonable stress integration depth, the fault friction coefficients in the Marmara zone are larger than the rest of NAF to the east. As the stress integration depth increases, the zone that characterizes the "large friction" Marmara zone extends slightly to the east. Assuming a brittle-ductile transition depth of around 15-17 km, we see that the faults in western Turkey have friction coefficients of around 0.1-0.2 (under long-term hydrostatic pore pressure conditions) which is similar to most of the findings in San Andreas fault zone whereas in the eastern part of NAF the values are around 0.1 or less. Coincidentally, eastern portion of NAF is also characterized by less seismicity during the instrumental epoch but this might be due to lack of high density observational coverage there. When we look at the fault strength, rather than solely the friction coefficient, we see a stronger variation in the rest of the NAF, strong Sinop zone being confined from the west and from the east by less strong zones.