Abstract Slow‐slip events (SSE) are a key mode of aseismic deformation and can enhance fault permeability through fracturing, enabling fluid migration from the overpressured oceanic crust to the plate interface. Whether the resulting poroelastic stress changes promote seismicity and larger megathrust events, however, remains unclear. We investigate this process for the Mw = 6.9 Valparaíso earthquake in Chile using a 4D poroelastic model and a high‐resolution seismicity catalog. Model scenarios with an overpressured oceanic crust and SSE‐induced permeability enhancement produce stress changes of 1–10 MPa—dominated by pore‐pressure changes, which are up to two orders of magnitude higher than those from elastic models (<0.04 MPa). Repeating earthquakes, foreshocks, and the mainshock all locate in regions of increased pore pressure. Our results quantitatively link fluid migration and pressure changes to the foreshock sequence and the mainshock, indicating that transient hydraulic processes at the plate interface have the potential to trigger foreshock seismicity.

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