Estimates of Seismogenic Strength for Deforming Fault Zones in Turkey

We apply an inverse approach to estimate the frictional strength of the seismogenic layer within the major deforming zones of Turkey. We define a fault friction model that utilizes results from thin-sheet methodologies. Our representation of the physics of the phenomenon is oversimplified when using our thin-sheet approximation because the width of the deforming zone around even geometrically simple faults increases with depth. However, this approach does produce quantitative insight on the question of frictional strength for faults of the upper crust. Using a variational approach we solve the linear force balance equations where the forcing is defined by 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. Our thin-sheet approximation neglects stresses due to flexure as well as small shear stresses at the base of the crustal layer. We then exploit the linearity of the problem to solve for the stress field boundary conditions that arise from far-field plate interactions. Finally we solve the force-balance equations for the horizontal deviatoric stress field associated with gradients of GPE and the stress field boundary conditions. We seek a match in the styles and directions of the deviatoric stress field associated with the seismogenic layer to strain field indicators. These deformation indicators are defined by a kinematic strain rate and velocity field model calculated using GPS data. We assume that the relationship between deviatoric stress directions and kinematic strain rate directions is isotropic and the directions and style of the principal axes of kinematically defined strain rates are the appropriate strain field indicators. We treat the body forces for separate cases where we use (1) seismologically determined crustal thicknesses, (2) a large-scale standard crustal model, and (3) a simple model where Airy isostasy is almost satisfied. In preliminary work we assume Airy isostasy with no lateral density variations within the crust. The seismogenic thickness in Anatolia rarely exceeds 18 km especially in western Turkey and in general the brittle-ductile transition depth (BDTD) is spatially variable. Rather than spatially varying our stress integration depths, we instead define our depths of integration from the surface of variable topography to a uniform depth (assumed BDTD) below sea level. We test various integration depths to be used as a proxy for the thickness of the seismogenic layer. We estimate that depth integrated deviatoric stress magnitudes range between 0.05-0.75 TN/m for a seismogenic layer defined to uniform base 20 km below sea level. This implies seismogenic strength magnitudes of 0.05-1.5 TN/m and indicates that at hydrostatic pore pressure, coefficients of friction on active faults are very low. One outcome of our calculations is that the fault friction coefficients in the Marmara zone are greater than the rest of North Anatolia Fault (NAF) to the east. A seismogenic thickness of 15-17 km defines fault friction coefficients of around 0.1-0.2 for western Turkey and friction coefficients of ≦ 0.1 for the eastern part of NAF.