Stress loading following a Mw7.8 megathrust earthquake in the northern Ecuadorian forearc

Whether driven by fluids or aseismic slip, seismic swarms release stress. In the Ecuadorian subduction zone, the 2016 Pedernales Mw7.8 megathrust earthquake caused aftershocks associated with afterslip. It also triggered distant seismic swarms several months after the mainshock. 82 stations were temporarily deployed following the megathrust earthquake. Stress and fluid-pressure changes following the Pedernales earthquake could have triggered a seismic swarm, approximately 60 km north of the 2016 epicenter 8 months after the mainshock. This upper plate swarm consists of three bursts, each larger in terms of number of events and magnitude. The observed migration of seismicity is consistent with pore-fluid diffusion. The third burst of seismicity initiated with the largest event (Mw5.4) of the swarm. Modeled stress change supports nearby triggering of seismicity and enhanced fluid diffusivity. We interpret a pressure back front following this event preventing the occurrence of seismicity near its hypocenter. In northern Ecuador, several Mw5.9 + megathrust events between 2015 and 2025 have loaded the region where a Mw7.7 megathrust earthquake occurred in 1958. Although large megathrust ruptures may result from stress release due to accumulated slip deficit on the plate interface, loading caused by intermediate-sized earthquakes may also contribute to the triggering of larger events.