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

Abstract

We study rupture behavior of the Aksay bend along the Altyn Tagh fault in northwest China over multiple earthquake cycles. A finite element method is used to numerically simulate spontaneous rupture during a coseismic process, and a viscoelastic model is used to analytically compute the fault stresses during an interseismic process. We find that the Aksay bend is an effective barrier to halt dynamically propagating ruptures from either side of the bend within a range of model parameters, with statistically only about 10% of ruptures jumping across the bend and propagating through almost the entire local fault system. Secondary complexities in fault geometry within the bend, in particular those portions that align relatively well with the regional strike of the fault system, play a critical role in these occasionally jumping ruptures. Well-developed fault patches with shear stress close to shear strength allow dynamically propagating ruptures to penetrate into the bend and are more susceptible to the dynamic triggering that enables rupture to jump across the bend onto the other strand. We identify additionally nine large rupture scenarios with different occurrences, and most of them rupture one strand outside the bend with triggered slip on some portions of the same or the other strand within the bend. Slip rate distributions from the models show significantly reduced fault slip within the bend and a permanently locked portion on the south strand near the peak of the Altun Mountains. These findings have important implications for seismic hazard assessments of complex fault systems worldwide.