| Popis: |
Earthquakes on intra-continental faults pose substantial seismic hazard to populated areas. The interaction of faults is an important mechanism of earthquake triggering and can be investigated by the calculation of Coulomb stress changes. Using three-dimensional finite-element models, co- and postseismic stress changes and the effect of viscoelastic relaxation on dip-slip faults are investigated. The models include elastic and viscoelastic layers, gravity, ongoing regional deformation as well as source and receiver fault zones. A parameter study with a systematic fault geometry, which is independent of a specific earthquake, shows that high coseismic stress increase occurs in along-strike prolongation of the source fault and in small areas parallel to the source fault. The coseismic slip and coefficient of friction influence the magnitude of stress changes, while the fault dip also influences the distribution. The stress changes can be explained by the spatial distribution of the coseismic strain. Differences in normal and thrust fault models are mainly caused by the different fault dips. The postseismic stress changes – caused by viscoelastic relaxation and interseismic stress increase – modify the coseismic stress changes that stress-triggering zones can change to stress-shadow zones and vice versa. Stress changes induced by viscoelastic relaxation can outweigh the interseismic stress increase so that negative stress changes can persist for decades. The lower the viscosity of the lower crust or lithospheric mantle, the more pronounced is the effect of viscoelastic relaxation in the first years. Layers with low viscosity define the area of highest postseismic velocities and hence determine relaxation and stress changes. The application of the model to the active Wasatch fault system in the eastern Basin and Range Province (Utah) is the first study in which an entire series of earthquakes on a natural fault system is simulated in a finite-element model using realistic fault geometries and palaeo-seismological data to investigate the co- and postseismic Coulomb stress changes for palaeo-earthquakes and the future evolution. The coseismic stress changes extend over all modelled fault segments. The postseismic stress changes and velocities show that the postseismic relaxation dominates the first years after the earthquake, while in the hundredth year the stress increase by the regional stress field dominates. The analysis of the stress changes since the last event per fault segment shows that the Brigham City segment (~780 bar) and Salt Lake City segment (~510 bar) have accumulated the most stress since the last earthquake. Modelled hypothetical present-day earthquakes suggest that present-day ruptures on the Brigham City segment or Salt Lake City segment could experience M ~7.1 or M ~7.0 earthquakes, respectively, which pose high seismic hazard for the metropolitan areas. |
