To estimate seismic hazard in complex, multiple fault systems, it is necessary to understand how stresses within the system are transferred. Accurate estimates of loading rate due to far-field tectonic motion and postseismic transients following large earthquakes are required. Using plausible lower crustal rheologies constrained by observations following the 1906 San Francisco earthquake, I show that postseismic relaxation may play a significant role in reloading the coseismic fault, providing 60--80% of the stress released during great earthquakes. Further, postseismic stressing rates along neighboring faults are highly dependent on the geometry and rheology of the lower crust/upper mantle. This implies that tectonic loading rate estimates are also highly dependent on lower crustal/upper mantle structure. Thus, improvement of seismic hazard estimates within complex, multiple fault systems is dependent on a better understanding of structure and rheology at depth.