Using the example of the 1992 M 7.3 Landers earthquake, we show that in the aftermath of a large earthquake the depth extent of aftershocks shows an immediate deepening from pre-earthquake levels, followed by a time-dependent postseismic shallowing. We use these seismic data to constrain the change in the depth of the seismic-aseismic transition with time throughout the earthquake cycle. Most studies of the seismic-aseismic transition have focussed on the effects of temperature and/or lithology on the transition either from brittle faulting to viscous flow or from unstable to stable sliding. A strain-rate dependent transient deepening of the brittle-ductile transition following a major earthquake is predicted by geological and laboratory observations. By analyzing the time-dependent depth distributions of aftershocks, we identify and quantify the temporal evolution of this transition. In the example of the Landers earthquake, its depth changes by as much as 3 km over the course of 4 years.