Geological faults comprise large-scale segmentation and small-scale roughness that govern earthquake processes and associated seismic radiation. Recent large earthquakes demonstrate that standard techniques for seismic hazard assessment for such faults (fault systems) are insufficient. This problem can be overcome by numerical simulations for multi-scale geometrical complex faults to investigate their rupture dynamics and seismic radiation. In this review, I discuss recent work to understand effects of large-scale segmentation and small-scale roughness on rupture evolution and near-source shaking. Using numerical simulations, we find that rupture incoherence due to fault roughness leads to high-frequency spectral decay consistent with observations. Waveform characteristics and comparisons with empirical ground-motion relations show that rough-fault rupture simulations generate realistic synthetic seismogram that can be used for engineering applications. We also show that for segmented faults, the spatial distribution of the regional stress is of critical importance as it determines the initial stress on the fault system. Similarly, the rupture nucleation point has significant impact on the resulting rupture process and earthquake size. Consequently, seismic hazard assessment for such fault systems must include more earthquake physics to capture the possible near-source shaking levels of future earthquakes.