What repeating earthquakes can tell us about postseismic slip and fluid circulation in the Ecuadorian subduction zone

The Ecuador-Colombia subduction zone is a complex and spatially heterogeneous region that hosts both shallow aseismic slip and large megathrust earthquakes, and where both  inter-seismic and post-seismic seismicity have been linked to aseismic slip. Repeating earthquakes, which are the result of repeated loading and failure of single asperities on a fault, are a valuable tool in studying aseismic slip as well as in monitoring the evolution of fault properties over time. In this study, we search for repeating earthquakes within one year of aftershocks following the April 16th, 2016 Mw 7.8 Pedernales earthquake, and we analyze their relationship to afterslip and the evolution of their source properties.  We calculate waveform cross-correlation coefficients (CC) on 4762 catalog events, and use a threshold CC of 0.95 to sort events into preliminary families, which are then completed using template-matching and relocated using HypoDD. In total, 376 earthquakes were classified into 62 families of 4 to 15 earthquakes. Additionally, the magnitudes, corner frequencies and stress drops of 136 repeaters were determined using spectral ratios. We find an increase in the recurrence time of repeating events with time after the mainshock, highlighting a possible timeframe for the afterslip’s deceleration. However, repeating earthquakes appear to concentrate around the areas of largest afterslip release, where afterslip gradient is the highest. This suggests that while most repeating aftershocks are linked to afterslip release, the afterslip gradient may play a bigger role in determining their location than previously thought. We also find that repeaters in the region near the trench are unusual, in that their stress drops are anomalously low and systematically decrease over the postseismic period, hinting at a potential increase in pore fluid pressure in this region over time.