Coseismic slip propagation on the Tohoku plate boundary fault facilitated by slip‐dependent weakening during slow fault slip

Slow slip phenomena, including earthquake afterslip and discrete slow slip events (SSEs), may have a major effect on megathrust earthquakes in subduction zones; however, the nature of this effect is still incompletely understood. Here we report that in friction experiments using samples from the plate boundary fault zone in the 2011 Tohoku‐Oki earthquake, an increase in sliding velocity can induce a change from steady state frictional strength or slip‐strengthening friction to slip‐weakening frictional behavior. Specifically, significant slip weakening is caused by velocity steps with initial slip rates consistent with afterslip observed after the largest Tohoku earthquake foreshock on 9 March 2011. This suggests that the foreshock afterslip, which is slightly faster than discrete SSEs, is favorable for fault weakening that may have contributed to the large coseismic slip during the main shock on the shallow plate boundary fault during the Tohoku‐Oki earthquake. Key Points At slow slip rates, velocity steps induce changes in friction slip dependence An increase in sliding velocity from more than 1 μm/s induces a change from slip strengthening to slip weakening Slip velocities slightly faster than SSE velocities are a favorable initial condition for earthquake slip