Dynamics of İzmit Earthquake Postseismic Deformation and Loading of the Düzce Earthquake Hypocenter
by Elizabeth Harding Hearn, Roland Bürgmann, and Robert E. Reilinger
We have developed dynamic finite-element models of İzmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip (via either velocity-strengthening frictional slip or linear viscous creep) or by distributed linear viscoelastic relaxation of the lower crust. We find that velocity-strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time-dependent Global Positioning System data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation. Our best frictional afterslip model fits the main features of postseismic slip inversions, in particular, high slip patches at (and below) the hypocenter and on the western Karadere segment, and limited afterslip west of the Hersek Delta (Bürgmann et al., 2002). The model requires a weakly velocity-strengthening fault, that is, either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate-dependence of friction (a-b) than is indicated by laboratory experiments.
Our best afterslip model suggests that the Coulomb stress at the Düzce hypocenter increased by 0.14 MPa (1.4 bars) during the İzmit earthquake (assuming right-lateral slip on a surface dipping 50° to the north), and by another 0.1 MPa during the 87 days between the İzmit and Düzce earthquakes. In the Marmara Sea region (within about 160 km of the İzmit earthquake rupture), this model indicates that the Coulomb stresses increased by 15%–25% of the coseismic amount during the first 300 days after the earthquake. Three hundred days after the earthquake, postseismic contributions to Coulomb stressing rate on the Maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate. Our estimates of postseismic Coulomb stress are highly model dependent: in the Marmara region, the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic Coulomb stresses than the frictional afterslip model.
Near-field stress and fault-zone rheology estimates are sensitive to the Earthʼs elastic structure. When a layered elastic structure is incorporated in our model, it yields a Coulomb stress of 0.24 MPa at the Düzce hypocenter, significantly more than the 0.14 MPa estimated from the uniform elastic model. Because of the higher near-field coseismic stresses, the layered elastic model requires a higher value of velocity-strengthening parameter (A-B) ([a-b] times effective normal stress σ′) to produce comparable postseismic slip. (A-B) is estimated at 0.4 and 0.2 MPa, respectively, for the layered and uniform elastic models. These results highlight the importance of understanding the Earthʼs elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses.