Hydraulic control of the foreshocks and mainshock of the 2017 Valparaiso earthquake in central Chile

Slow-slip events (SSEs) are a well-known mode of aseismic deformation in subduction zones. Seismological and geological studies further suggest that SSEs enhance fault-zone permeability, enabling fluid migration from overpressured oceanic crust into the plate interface. However, it remains unclear whether the resulting pore-pressure changes dominate stress transfer and promote the commonly observed increase in seismicity during SSEs and, although less commonly, the occurrence of larger megathrust earthquakes. Here, we investigate the impact of an SSE that occurred three days before the 2017 Mw 6.9 Valparaíso earthquake in central Chile. We use a forward 4D hydromechanical (poroelastic) model and compare the resulting spatial stress changes with a high-resolution seismicity catalog of the foreshock sequence.

We simulate the SSE by prescribing a geodetically inferred slip distribution on the fault interface and assume an overpressured oceanic crust, together with transient permeability enhancement due to SSE-induced local fracturing of the plate interface. We compute stress transfer driven by these pore-pressure changes along the plate interface and compare the results with widely used models that consider elastic stress changes only. Our results show that fluid migration into the plate-interface zone generates stress changes of ~1–10 MPa, overwhelmingly dominated by pore-pressure variations. The largest stress (and pore-pressure) changes spatially correlate with zones of increased seismicity, repeating earthquakes, and the mainshock. In contrast, the elastic-only scenario produces stress changes that are two to three orders of magnitude smaller and shows a much weaker spatial correspondence with the observed seismicity.

Our modeling results indicate that transient permeability enhancement during SSEs enables fluid redistribution that fundamentally controls stress transfer along the plate interface. We conclude that pore-pressure changes exert first-order control on earthquake precursors in subduction zones, offering a physical explanation for foreshock clustering and the triggering of large earthquakes during SSEs. These findings highlight the importance of incorporating fluid–rock interactions in models of seismic hazard and earthquake nucleation.