Fault interaction is believed to influence seismicity and crustal deformation, but the mechanics of fault interaction over various time scales remain poorly understood. We present here a numerical investigation of fault coupling and interaction over multiple time scales, using the San Andreas fault and the San Jacinto fault in southern California as an example. The San Andreas fault is the Pacific–North American plate boundary, but in southern California, a significant portion of the relative plate motion is accommodated by the subparallel San Jacinto fault. We developed a three-dimensional viscoelastoplastic finite-element model to study the ways in which these two faults may have interacted (1) during and following individual earthquakes, (2) over multiple seismic cycles, and (3) during long-term steady-state fault slip. Our results show that the cluster of nine moderate-sized earthquakes (M 6–7) on the San Jacinto fault since 1899 may have lowered the Coulomb stress on the southern San Andreas fault, delaying the “Big One,” an earthquake of magnitude 7.8 or greater that may result from rupture of much of the southern San Andreas fault. In addition to the static Coulomb stress changes associated with individual earthquakes, variations of seismicity over seismic cycles on one fault can influence the loading rate on the other fault. When the San Jacinto fault experiences clusters of earthquakes such as those in the past century, the loading rate on the San Andreas fault can be lowered by as much as ∼80%. Over longer time scales, these two faults share the slip needed to accommodate the relative plate motion. Hence, an increase in slip rate on one of these two faults causes complementary decrease on the other, which is consistent with geological observations.