Historic and modern catalogs of seismicity in California suggest a migration of earthquakes (M≥5.6) along the San Jacinto Fault; these events appear to travel down the fault with a migration speed of 1.7 km/year (Sanders [1993>). This migration is explained by postseismic strain diffusion due to viscoelastic relaxation from the great Fort Tejon earthquake in 1857. We model this postseismic effect and find that significant stress diffuses down the San Jacinto fault for distances in excess of 200 km and the corresponding migration may be a result of Coulomb triggering from this stress perturbation. The level of postseismic stress that seems to be the trigger level for most of the events is of order 1 bar. Since the temporal evolution of the postseismic strain field is mainly dependent on the inelastic properties of the lower crust and uppermost mantle, the observed migration enables a viscosity estimate of ~4¿1018 Pas for this region of California. ¿ 2001 American Geophysical Union