Fluid migration before and after a large megathrust earthquake

To better understand the distribution of seismicity and slip behavior in the Ecuadorian forearc, several networks of temporary stations were installed between 2011 and 2022. These temporary networks complement the densification of the permanent network between 2010 and 2018 by the Instituto Geofisico – Escuela Politecnica Nacional (IG EPN). These deployments provide a time series of recorded seismicity to study time-dependent physical properties in the region of the 2016 Mw 7.8 Pedernales megathrust earthquake. We generate a unified consistent catalog and relocate the seismicity through a joint tomography inversion of both earthquake hypocenters and the velocity structures. Our results reveal a transient signal in the distribution of high Vp/Vs. Before the megathrust earthquake, high Vp/Vs is concentrated south of the 2016 Pedernales rupture area. Fluids, located below the plate interface at 25 km prior to the earthquake, migrated to the upper 10 km of the overriding plate in the post-seismic phase. In a later period between 2020 and 2022, high Vp/Vs concentrates north of the rupture, coinciding with a subducting seamount. Focal mechanisms document a perturbation of the stress tensor in the first three weeks following the 2016 Pedernales earthquake. This is the smallest subduction earthquake following which a stress tensor perturbation was reported. A decrease in the stress shape ratio following the stress perturbations reflects a delayed increase in fluid pressure. Along with afterslip and aftershocks triggered by a large megathrust earthquake, stress perturbation may be a mechanism leading to fluid displacement in the upper plate.