Weak ductile shear zone beneath a major strike‐slip fault: Inferences from earthquake cycle model constrained by geodetic observations of the western North Anatolian Fault Zone

GPS data before and after the 1999 İzmit/Düzce earthquakes on the North Anatolian Fault Zone (Turkey) reveal a preseismic strain localization within about 25 km of the fault and a rapid postseismic transient. Using 3-D finite element calculations of the earthquake cycle in an idealized model of the crust, comprising elastic above Maxwell viscoelastic layers, we show that spatially varying viscosity in the crust can explain these observations. Depth-dependent viscosity without lateral variations can reproduce some of the observations but cannot explain the proximity to the fault of maximum postseismic velocities. A localized weak zone beneath the faulted elastic lid satisfactorily explains the observations if the weak zone extends down to midcrustal depths, and the ratio of relaxation time to earthquake repeat time ranges from ~0.005 to ~0.01 (for weak-zone widths of ~24 and 40 km, respectively) in the weakened domain and greater than ~1.0 elsewhere, corresponding to viscosities of ~1018 ± 0.3 Pa s and greater than ~1020 Pa s. Models with sharp weak-zone boundaries fit the data better than those with a smooth viscosity increase away from the fault, implying that the weak zone may be bounded by a relatively abrupt change in material properties. Such a change might result from lithological contrast, grain size reduction, fabric development, or water content, in addition to any effects from shear heating. Our models also imply that viscosities inferred from postseismic studies primarily reflect the rheology of the weak zone and should not be used to infer the mechanical properties of normal crust.