Pore-pressure diffusion controls upper-plate aftershocks following the 2014 Mw 8.2 Iquique earthquake (northern Chile)

Upper-plate aftershocks following megathrust earthquakes can pose severe time-dependent hazard because they often occur close to densely populated regions, increasing the risk to structures already weakened by the mainshock. Although aftershock rates commonly follow Omori–Utsu temporal decay, the physical mechanisms controlling their non-linear time dependency and the diversity of faulting styles in the upper plate remain unclear. Because coseismic static stress transfer cannot explain this time dependency, transient postseismic processes — afterslip, viscoelastic relaxation, and fluid-driven pore-pressure diffusion — are potential candidates.

Here, we combine comprehensive seismological and geodetic observations with a 4D (space–time) hydro-mechanical numerical model to identify the dominant postseismic stress-change process controlling upper-plate aftershocks of the 2014 M_w 8.2 Iquique megathrust earthquake in northern Chile. We reproduce GNSS-observed postseismic deformation during the first nine months and separate the contributions from afterslip, viscoelastic relaxation, and poroelastic deformation in both horizontal and vertical components. In particular, poroelastic deformation contributes substantially to the near-field vertical signal. We then compute spatiotemporal Coulomb Failure Stress (CFS) changes for each process and compare them to the distribution of upper-plate aftershocks.

Our results show that CFS changes driven by coseismically induced pore-pressure changes best explain the observed aftershock pattern in both space and time. Furthermore, increasing pore pressure reduces effective normal stress largely independent of fault orientation, promoting failure across a broad range of faulting styles, consistent with observed focal-mechanism diversity. This implies that time-independent elastic ΔCFS calculations on optimally oriented faults may be insufficient to assess the response of upper-plate faults to megathrust earthquakes, and that transient, pore-pressure stress changes must be considered. Overall, our results link postseismic deformation, stress transfer, and pore-fluids in the upper plate, and provide a basis for physics-based, time-dependent aftershock forecasting constrained by forearc hydraulic properties.