Multicycle Dynamics of the Aksay Bend Along the Altyn Tagh Fault in Northwest China: 1. A Simplified Double Bend

Abstract

I perform multicycle dynamic simulations of a restraining double bend that is simplified from the Aksay bend along the Altyn Tagh Fault in northwest China. The earthquake cycle includes a coseismic dynamic rupture phase and an interseismic stress loading and relaxation phase. The double bend fault system has two fault strands and each strand comprises a stem segment that strikes parallel to the maximum shear loading direction and a bend segment that deviates from the loading direction. I find that the restraining double bend is an effective barrier to dynamically propagating ruptures over multiple earthquake cycles, with a probability of rupture jumping from one strand to the other varying from 0% to 10%, depending on parameter values of the rate dependence of friction during the dynamic phase and the viscosity for stress relaxation during the interseismic phase. The critical condition for the primary rupture on the first strand to jump onto the second strand is that the preshear stress is close enough to the shear strength over a certain size of the fault patch on the second strand. These results on the simplified double bend provide important insights into rupture behavior of geometrically complex faults over multiple earthquake cycles in general and a reference to explore rupture behavior of the Aksay bend in particular.

Multicycle Dynamics of the Aksay Bend Along the Altyn Tagh Fault in Northwest China: 2. The Realistically Complex Fault Geometry

“…The slip‐ and rate‐weakening friction law has the following form (Duan, 2019), where f s , f d , f r , d 0 , v 0 , d , and v are the static, dynamic, and restrengthening coefficients of friction; the critical slip distance; the critical slip velocity; slip; and slip velocity on the fault, respectively. As shown for a typical path of the frictional coefficient as a function of slip and slip velocity in Duan (), the frictional coefficient in this law essentially drops from the static level to the dynamic level by following a slip‐weakening curve at the early stage of sliding and recovers to the restrengthening level by following a rate‐weakening curve at the late stage. The separate slip‐weakening law essentially has a form of equation .…”

“…We briefly discuss the method for multicycle dynamic modeling used in this study and refer readers to Duan () for details of the method. In this method, an earthquake cycle comprises an interseismic phase for fault stress loading and relaxation and a coseismic phase for dynamic rupture propagation (and seismic wave propagation).…”

“…Multiple earthquake cycles start with the first interseismic phase that loads the fault system from zero shear and ambient normal stresses until one fault point reaches failure (i.e., the shear stress reaches the shear strength, also known as the yield stress, which is the product of the frictional coefficient and the fault normal stress at the point). Then the fault shear and normal stresses at this moment are used as the initial fault stresses to simulate spontaneous rupture propagation of the first earthquake, which is initiated by a fixed rupture velocity within a small nucleation patch (Duan, ). After seismic waves propagate away from the fault, we terminate the dynamic simulation and the residual fault shear and normal stresses at that moment are used as the initial stresses for the second interseismic period.…”

“…We examine a set of models with variations in major parameters, including different types of friction law, different values of viscosity, and different critical velocity values in the slip‐ and rate‐weakening law. These major parameters could exert significant effects on multicycle dynamic rupture behavior as shown in a simplified double bend fault model (Duan, ), and they are relatively less constrained from observations than other model inputs such as fault geometry and loading rate. By varying these parameters, we can find common rupture behaviors that are robust, while differences among these models provide us with a range of possible rupture behaviors at the double bend.…”

“…Duan and Oglesby (, , ) applied the method to explore multicycle dynamics of idealized faults with a single bend, step over, or branch and found that heterogeneous fault stresses near these structural complexities and rupture histories play a critical role in rupture behavior on these faults. Duan () improved the method for idealized faults with more than one complexity in fault geometry.…”