Deep Ductile Shear Zone Facilitates Near‐Orthogonal Strike‐Slip Faulting in a Thin Brittle Lithosphere

Abstract

AbstractSome active fault systems comprise near‐orthogonal conjugate strike‐slip faults, as highlighted by the 2019 Ridgecrest and the 2012 Indian Ocean earthquake sequences. In conventional Mohr‐Coulomb failure theory, orthogonal faulting requires a zero frictional coefficient (pressure‐insensitive), which is unlikely in the brittle lithosphere. The simulations developed here show that near‐orthogonal faults can form in the brittle layer by inheriting the geometry of orthogonal shear zones nucleated in the deep ductile (pressure‐insensitive) layer. In particular, if the brittle layer is sufficiently thinner than the ductile fault root, near‐orthogonal faulting is preserved at the surface. The preservation is further facilitated by a depth‐dependent strength in the brittle layer. Conversely, faults nucleated within the brittle layer are unlikely to form at orthogonal angles. Our model thus offers a possible explanation for orthogonal strike‐slip faulting and reveals the significant interactions between the structure of faults in the brittle upper lithosphere and their deep ductile roots.