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Megathrust earthquakes impose changes of differential stress and pore pressure in the lithosphere‐asthenosphere system that are transiently relaxed during the postseismic period primarily due to afterslip, viscoelastic and poroelastic processes. Especially during the early postseismic phase, however, the relative contribution of these processes to the observed surface deformation is unclear. To investigate this, we use geodetic data collected in the first 48 days following the 2010 Maule earthquake and a poro‐viscoelastic forward model combined with an afterslip inversion. This model approach fits the geodetic data 14% better than a pure elastic model. Particularly near the region of maximum coseismic slip, the predicted surface poroelastic uplift pattern explains well the observations. If poroelasticity is neglected, the spatial afterslip distribution is locally altered by up to ±40%. Moreover, we find that shallow crustal aftershocks mostly occur in regions of increased postseismic pore‐pressure changes, indicating that both processes might be mechanically coupled.

<p>After large earthquakes at subduction zones, the plate interface continues moving due to mostly frictional afterslip processes. Below depths of 60 km, little frictional afterslip is to be expected on the plate interface due to low shear strength, lack of apparent geodetic interseismic locking, and low seismic moment release from aftershocks. However, inversion models that consider an elastic crust above a mantle with viscoelastic rheology result in a significant portion of afterslip at depths > 60 km. In this study, we present a forward 3D geomechanical-numerical model with power-law rheology that simulates dislocation creep processes for the crust and upper mantle in combination with an afterslip inversion. The linear rheology case is also considered for comparison. We estimate the cumulative viscoelastic relaxation and the afterslip distribution for the first six years following the 2010 M<sub>w</sub> 8.8 Maule earthquake in Chile. The cumulative afterslip distribution is obtained from the inversion of the residual surface displacements between continuous GPS (cGPS) observations and predicted displacements from viscoelastic forward modelling. We investigate three simulations: two with the same dislocation creep parameters in the slab and upper mantle but different ones in the continental crust, and another with elastic properties in the crust and slab and a linear viscoelastic upper mantle. Our preferred simulation is the one with power-law rheology in the crust and upper mantle with a weak continental crust since the corresponding afterslip distribution shows the best overall fit to the cGPS displacements (cumulative and time series) as well as having a good correlation with aftershock activity. In this simulation, most of the viscoelastic relaxation occurs in the continental lower crust beneath the volcanic arc due to dislocation creep processes. The resulting afterslip pattern from the inversion is reduced at depths > 60 km, which correlates well with the spatial distribution of cumulative seismic moment release from aftershocks. We conclude that by allowing for non-linear stress relaxation in the continental lower crust due to dislocation creep processes, the resulting afterslip distribution is in better agreement with the physical constraints from the shear strength of the plate interface at depth, the predicted locking degree, and the aftershock activity.</p>

In the aftermath of large earthquakes, the Earth surface displays time-dependent deformation patterns on different spatiotemporal scales that may last several of years or decades due to the relaxation of coseismically imposed stress and pore pressure changes in the lithosphere-asthenosphere system (e.g., Hergert & Heidbach, 2006;Hughes et al., 2010; K. Wang et al., 2012, and references therein). These relaxation processes are aseismic postseismic slip on the fault interface (afterslip), poroelastic processes in the upper crust, and viscoelastic relaxation in the lower crust and upper mantle (e.g.,

Upper-plate aftershocks following megathrust earthquakes are particularly dangerous as they may occur close to the highly populated shore. Aftershock numbers decay with time, imposing a time-dependent seismic hazard. While coseismic stress transfer cannot explain this time-dependency, transient postseismic deformation due to afterslip, viscoelastic relaxation, and pore-pressure diffusion are potential candidates. We investigate which of these three processes is the key driver of the upper-plate aftershocks following the 2014 Mw=8.1 Iquique, northern Chile, earthquake. We first use a 4D (space and time) model to reproduce the postseismic deformation observed in geodetic data. We then analyze the spatiotemporal stress changes produced by individual postseismic processes and compare them to the distribution of upper-plate aftershocks. Our results reveal that stresses produced by coseismically-induced pore-pressure diffusion best correlate in space and time with increased upper-plate aftershock activity. Moreover, an increase in pore-pressure diffusion reduces the three principal stresses likewise. Hence, all faults, regardless of their orientation, are brought closer to failure. This may explain the diversity of faulting styles of upper-plate aftershocks. Our findings provide new insights into the link between pore-pressure diffusion and upper-plate deformation in subduction zones with implications for time-dependent seismic hazard assessment.

<p>Large earthquakes impose differential stresses in the crust and upper mantle that are transiently relaxed during the postseismic phase mostly due to afterslip on the fault interface, viscoelastic relaxation in the lower crust and upper mantle, and poroelastic rebound in the upper crust. During the last years, the wealth of geophysical and geodetic observations, as well as great effort in forward and inverse modelling have allowed a better comprehension of the role of these mechanisms during the postseismic period. However, it is still an open question to what extent postseismic processes contribute to the surface deformation signal, especially during the early postseismic period. In this study, we use GNSS and InSAR observations collected in the first 48 days following the 2010 Maule earthquake in Chile along with a model approach that integrates afterslip, poroelasticity, and temperature-controlled power-law (non-linear viscosity) rheology. The afterslip distribution is obtained from a geodetic data inversion after removing the poro-viscoelastic component by forward modelling to the geodetic data. We find that our model approach explains the geodetic cumulative signal 14% better than a pure elastic model inverting for afterslip. This improvement is mainly produced by the better fit to the geodetic signal at the volcanic and back-arc regions due to the inclusion of non-linear viscoelastic processes, which can explain > 60% of the observed surface displacements in these regions. We also show that poroelastic processes play a significant role locally, specifically near the region where the coseismic slip was largest. Here, poroelastic processes explain most of the cumulative observed GNSS uplift signal and produce surface landward patterns that affect the horizontal GNSS component by up to 15% in the opposite direction. If poroelastic processes are ignored, our results reveal that the resulting afterslip amplitude is both amplified and suppressed by up to 40% in regions of ~50 x 50 km<sup>2</sup>. Our findings have implications for the calculation of the postseismic slip budget, and therefore the seismic hazard assessment of future earthquakes.</p>

<p>Aftershocks are a time-dependent (exponential decay) phenomenon in the aftermath of large earthquakes. In subduction zones, those occurring in the upper plate are of special concern given their potential seismic hazard, as they may produce substantial surface shaking close to highly populated cities. Therefore, the understanding of the mechanisms that drive upper-plate aftershocks is of utmost importance to improving seismic hazard assessment. Transfer of static coseismic stresses has been commonly proposed to explain this; however, they fail to explain their exponential decay over time. This time-dependency is observed in postseismic geodetic measurements, suggesting that the processes that control the postseismic surface deformation also govern or at least are involved in the generation of upper-plate aftershocks. Here, the postseismic surface deformation is dominated by aseismic slip along the fault interface (afterslip), non-linear viscoelastic relaxation in the lower crust and upper mantle, and pore-pressure diffusion in the crust. Despite great research efforts, however, the key driver remains elusive.</p> <p>In this study, we investigate which postseismic mechanism mainly controls the occurrence of aftershocks in the upper plate in subduction zones using the 2014 Mw=8.1 Iquique earthquake, northern Chile, as a study case. We employ a 4D numerical forward model to simulate the transient poroelastic and non-linear viscoelastic relaxation, whose contributions are subtracted from the cumulative Global Navigation Satellite System (GNSS) measurements to then invert for afterslip. Using realistic rock material properties, we first show that this approach explains the surface displacements during the first nine months of postseismic deformation recorded by continuous GNSS. For the same period, we then compute the spatiotemporal Coulomb Failure Stress changes (ΔCFS) that result from individual postseismic processes and compare them with the upper-plate aftershocks using a high-resolution seismicity catalog and focal mechanisms. We show for the first time that the ΔCFS produced by pore-pressure diffusion induced by the mainshock are unambiguously better correlated in space and time with the increase in upper-plate aftershocks than those from afterslip or non-linear viscous relaxation. In addition, pore-pressure diffusion lowers the effective normal stress of the stress tensor more effectively, while its resulting ΔCFS are relatively independent of the fault orientation. The latter would also explain the diversity of faulting styles in the upper plate exhibited by focal mechanisms following the 2014 Iquique earthquake and other subduction zone earthquakes. Our findings provide new insights into the link between pore-pressure diffusion and upper-plate deformation in subduction zones with implications for time-dependent seismic hazard.</p>