The evolution of the seismic‐aseismic transition during the earthquake cycle: Constraints from the time‐dependent depth distribution of aftershocks

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.