Slow rupture and weakly pressure-sensitive strength enables compressional branching:

18 The 2012 M8.6 off-Sumatra earthquake, the largest strike-slip and intraplate earthquake recorded 19 to date, followed an exceptionally tortuous rupture path. It featured two episodes of branching 20 into fault segments that were experiencing increased compressive dynamic stresses, hence 21 increased frictional strength. Meng et al. (2012) attributed this unexpected compressional 22 branching to slow rupture and weak pressure-sensitivity of the fault strength. Here, by 23 conducting 2D dynamic rupture simulations, we confirm that the compressional branching can 24 only occur under slow rupture speed and low apparent friction coefficient. Poroelastic effects can 25 also contribute by buffering the dynamic clamping. We suggest that serpentinized minerals may 26 provide low friction, fluids and dynamic weakening down to 25 km depth, and ductile shear 27 heating instability may provide a weakening mechanism from 40 to 60 km depth. The absence of 28 known weakening mechanism in the intermediate depth range suggests that rupture may 29 penetrate over velocity-strengthening regions. 30