Modeling fully dynamic earthquake cycles on a bent fault governed by rate- and state-dependent friction using EQsimu

Abstract:

We develop a finite element (FE) dynamic earthquake simulator, EQsimu, to model fully dynamic earthquake cycles on geometrically complex faults. The fault is governed by rate- and state-dependent friction (RSF). EQsimu integrates an existing explicit FE code EQdyna for coseismic dynamic ruptures and a newly developed implicit FE code EQquasi for quasi-static phases of an earthquake cycle, including nucleation, post-seismic and interseismic processes. EQdyna and EQquasi are coupled through on-fault physical quantities of shear and normal stresses, and slip-rates and state variables in RSF. EQsimu can handle time steps ranging from hundredths of a second to a fraction of a year based on a variable time-stepping scheme and resolve the cohesive zone at rupture fronts of dynamic ruptures with small enough element sizes.

We apply EQsimu to a 3-D strike-slip fault with a bend, assuming elastic off-fault behavior. Complex earthquake event patterns spontaneously emerge in the simulation, and the fault demonstrates two phases in its evolution. In the first phase, there are three types of dynamic ruptures: ruptures breaking the whole fault from left to right, ruptures being halted by the bend, and ruptures breaking the whole fault from right to left. As the fault bend experiences more ruptures, the zone of stress heterogeneity near the bend widens and the earthquake sequence enters the second phase showing only repeated ruptures that break the whole fault from left to right. The two-phase behaviors of this bent fault system suggest that a 10° bend may conditionally stop dynamic ruptures at the early stage of the fault system evolution and will eventually not be able to stop ruptures as the fault system evolves.

We work further toward fully dynamic earthquake cycles on geometrically complex faults with off-fault plasticity. This task brings a challenging question that seems unique to geometrically complex faults governed by RSF. What should stress states in the volume surrounding a fault and initial conditions in RSF be? If the fault system is governed by RSF and slips at steady state, then stress states, initial slip rates and frictional parameters in RSF must be heterogeneous. We explore several possibilities of these heterogeneities and their effects on dynamic rupture behavior of the fault system.