Insights into the mechanisms enabling compressional branching of the 2012 Mw 8.6 Off-Sumatra earthquake: earthquake cycle and rupture simulation in orthogonal conjugate strike-slip fault systems.

The 2012 Mw 8.6 off-Sumatra earthquake is the largest strike-slip and intraplate earthquake recorded to date with an exceptionally tortuous rupture path. It featured two episodes of branching first into fault segments that were experiencing increased compressive dynamic stresses, hence increased frictional strength. While the branches in the positive Coulomb stress lobes rupture with a delay. Meng et al. (2012) attributed this unexpected compressional branching to slow rupture and weak pressure-sensitivity of the fault strength. Here, by conducting 3D earthquake cycle and dynamic rupture simulations on a T-shape strike-slip fault system (as shown in the figure), we investigate the effect of different frictional coefficients, heterogeneous pre-stress, and slightly oblique fault systems on the rupture pattern. We confirm that compressional branching can occur with a low frictional coefficient of 0.2 and a low pre-stress on the dilatational branch due to previous earthquakes. In these cases, earthquakes nucleate on fault 1. Then rupture first propagates to fault 2, leaving fault 3 unbroken or broken with a delay. We suggest that serpentinized minerals or ductile shearing may provide the low apparent frictional coefficient. We also reproduce several earthquake sequences reminiscent of the 2019 Ridgecrest earthquake sequence. In these sequences, the first earthquake ruptures mainly fault 1. The first earthquake then triggers an aseismic slip at the position of the maximum Coulomb stress change on fault 3, which finally develops into an earthquake 1 to ~ 100 days after the first earthquake. Poroelastic effects are another alternative mechanism for the compressional branching. The slip on fault 1 may change the pore pressure instantaneously, which buffers the dynamic clamping on fault 2 and the unclamping on fault 3. With time, the pore fluid diffusion will allow the pore pressure disturbance to decrease. This mechanism may explain the delayed rupture of the two dilatational branches in the 2012 off-Sumatra earthquake and the time gap between the Mw 6.4 and the Mw 7.1 Ridgecrest earthquakes.