AbstractWe study numerically the effects of fault roughness on the nucleation process during earthquake sequences. The faults are governed by a rate and state friction law. The roughness introduces local barriers that complicate the nucleation process and result in asymmetric expansion of the rupture, nonmonotonic increase in the slip rates on the fault, and the generation of multiple slip pulses. These complexities are reflected as irregular fluctuations in the moment rate. There is a large difference between first slip events in the sequences and later events. In the first events, for roughness amplitude br ≤ 0.002, there is a large increase in the nucleation length with increasing br. For larger values of br, slip is mostly aseismic. For the later events there is a trade‐off between the effects of the finite fault length and the fault roughness. For br ≤ 0.002, the finite length is a more dominant factor and the nucleation length barely changes with br. For larger values of br, the roughness plays a larger role and the nucleation length increases significantly with br. Using an energy balance approach, where the roughness is accounted for in the fault stiffness, we derive an approximate solution for the nucleation length on rough faults. The solution agrees well with the main trends observed in the simulations for the later events and provides an estimate of the frictional and roughness properties under which faults experience a transition between seismic and aseismic slip.