Very early identification of a bimodal frictional behavior during the post-seismic phase of the 2015 <i>M</i>_w 8.3 Illapel, Chile, earthquake

It is well-established that the post-seismic slip results from the combined contribution of seismic and aseismic processes. However, the partitioning between these two modes of deformation remains unclear due to the difficulty of inferring detailed and robust descriptions of how both evolve in space and time. This is particularly true just after a mainshock when both processes are expected to be the strongest. Using state-of-the-art sub-daily processing of GNSS data, along with dense catalogs of aftershocks obtained from template-matching techniques, we unravel the spatiotemporal evolution of post-seismic slip and aftershocks over the first 12 h following the 2015 Mw 8.3 Illapel, Chile, earthquake. We show that the very early post-seismic activity occurs over two regions with distinct behaviors. To the north, post-seismic slip appears to be purely aseismic and precedes the occurrence of late aftershocks. To the south, aftershocks are the primary cause of the post-seismic slip. We suggest that this difference in behavior could be inferred only a few hours after the mainshock. We finish by showing that this information can potentially be obtained very rapidly after a large earthquake, which could prove to be useful in forecasting the long-term spatial pattern of aftershocks.