Three M>8 earthquakes have occurred in Mongolia during a 52-year period (1905–1957). Since these earthquakes were well separated in space (400 km), the coseismic stress change is far too low (0.001 bar) to explain mutual earthquake triggering. By contrast, postseismic relaxation gradually causes a significant stress change (0.1–0.9 bar) over large distances. Using a spring-slider model to simulate earthquake interaction, we find that viscoelastic stress transfer may be responsible for the earthquake time clustering observed in active tectonic areas. Therefore, revealing postseismic strain by satellite geodesy and modeling earthquake clusters could improve our understanding of earthquake occurrence, especially in zones where large earthquakes have already struck in past decades.