FaultLab: Geodynamics
Geodynamic modelling study in FaultLab project aims to establish the crustal viscosity structure beneath North Anatolian Fault Zone (NAFZ). Viscosity structure of crust and/or upper mantle has generally been estimated from geodetic observations of post-seismic or inter-seismic deformation. However, the viscosity structure obtained from either post- or inter-seismic deformation does not necessarily explain the other deformation. It is therefore important to estimate a viscosity structure that explains the surface deformation at any stage of the earthquake cycle. The GPS velocities before and after the 1999 İzmit and Düzce earthquakes in the western NAFZ provide excellent opportunity to estimate such a viscosity structure.
In this study, using a 3D finite element model, the linear Maxwell visco-elastic response to a repeated strike-slip faulting event is examined under the condition of a constant far-field tectonic loading. We describe the model behaviour for three basic crustal viscosity structures beneath an upper elastic layer: uniform viscosity (UNV), depth-dependent viscosity (DDV) and localised weak zone (LWZ), in which the ratio Τ/Δt of the Maxwell relaxation time Τ to the earthquake cycle period Δt is principal determinant of the surface velocity variation during the cycle.
Our numerical experiments have found that the observed GPS velocity profiles require such a localised weak zone (LWZ) beneath the western North Anatolian Fault Zone that the thickness and width of LWZ are ~ 12 km and ~ 20 - 40 km, respectively, and the weakened and the non-weakened domains should have the ratio Τ/Δt of ~ 0.01 and ~ ≥ 1.0, which scales the viscosity value each in the domain to be ~ 1018 Pa s and ~ ≥ 1020 Pa s, respectively.