Numerical study of splay faults in subduction zones: The effects of bimaterial interface and free surface

Splay faults are a characteristic branching fault system in some subduction zones. We model a megathrust earthquake rupture with a branching fault, inhomogeneous media, and a free surface, using a spontaneously propagating mode II crack on a bimaterial interface. For this purpose, we develop an explicit finite element code to solve the elastodynamic equations and a slip‐weakening friction law on the fault plane. In a homogeneous prestress condition, the rupture on the branching fault is enhanced when the upper medium is more compliant. Even when the upper medium is less compliant, a fast rupture propagating at a velocity close to the generalized Rayleigh velocity produces the strong dynamic stress around the rupture front and activates the branching fault. When prestress linearly increases with depth from zero at the free surface, only a slight change of normal stress that is due to seismic waves causes an initiation of rupture on the branching fault at the free surface, and this rupture propagates downward on the branching fault. The arrival of this descending rupture at the branching point greatly changes the stress field in the surrounding area. It affects the slip distribution on the main fault and even terminates the main rupture propagation if it occurs before the arrival of the main rupture on the branching point.