The 1999 İzmit, Turkey, Earthquake: Nonplanar Fault Structure, Dynamic Rupture Process, and Strong Ground Motion

We simulated dynamic rupture propagation along various nonplanar fault models of the 1999 İzmit, Turkey, earthquake using a boundary integral equation method. These models were inferred from geological and geodetic observations. Based on these results, we modeled seismic-wave propagation around the fault system using a finite difference method. We focused on the effect of different fault geometries on the rupture process and seismic-wave propagation. Numerical simulation results imply a rapid and continuous rupture propagation from the İzmit– Sapanca Lake segment to the Sapanca–Akyazi segment. The rupture under Sapanca Lake appears to have propagated not on a disconnected fault segment but along a smooth fault structure with a bend of only a few degrees. The observational complexity of the surface breaks, however, can be best simulated by a highly segmented fault model. This infers that fault geometric characters observed in the field reflect near-surface structure and that seismological and geodetic features are controlled by global fault structure at depth. Then we investigated the effect of frictional parameters and the initial stress field. In order to explain near-field seismograms at station SKR, located a distance of a few kilometers from the fault, we had to force the rupture to propagate at shallow depth close to the station. In order to obtain this, we had to introduce a finite cohesive force in the friction law that allows stress accumulation and release in the shallow crust. The external stress field had to be large enough for the rupture to propagate at very rapid speed. Our simulation results show that it is important to include detailed fault geometry in the numerical simulation, and to constrain frictional parameters and the initial stress field, for understanding of the full dynamic process of an earthquake.