QUANTIFYING THE EFFECTS OF ABSOLUTE FRICTION AND SYMMETRY ON THRUST FAULT RUPTURE

For faults where geometrical symmetry is broken, the free surface boundary condition cannot be ignored. This situation allows the fault to experience changes in the normal stress which changes the shear stress. The free surface boundary condition creates a case of seismic positive feedback as the advancing rupture becomes more asymmetric. Once a fault’s symmetry is broken, slip is affected by absolute friction. In this study, we use numerical models to quantify the contributions of absolute friction on the temporal evolution of thrust fault rupture.

Our approach uses dynamic finite element modeling to simulate rupture propagation and slip during a thrust earthquake. We model a 2-dimensional 40 km-long, 15º dipping thrust fault within a homogenous solid using the finite element code FaultMod developed by Michael Barall. For each condition of absolute friction, three models are conducted: (1) where the upper fault tip terminates at the free surface; (2) where the upper fault tip is buried 9 km; and (3) where the upper fault tip is buried 120 km. For the latter model, seismic radiation is unable to reflect off the free surface to the fault before slip ceases. For the other two models, seismic radiation from the free surface can interact with the fault as rupture propagates, thereby causing changes in the normal stress and influencing slip.

Preliminary results for a purely symmetric fault in a whole-space both confirm and demonstrate that different frictional and stress parameterizations produce identical final slip results in a symmetric configuration. For models that approach the free surface, where the free surface boundary condition is apparent, we find that the evolution of fault rupture and slip do indeed depend on the absolute level of friction. We believe our findings will provide valuable insight in investigating real-world thrust earthquakes, specifically demonstrating the relationship between friction and geometrical symmetry to the temporal evolution of slip and stress during an earthquake.