Resolving depth-dependent subduction zone viscosity and afterslip from postseismic displacements following the 2011 Tohoku-oki, Japan earthquake

Abstract

We developed a 3-D, viscoelastic finite element model of the M9 2011 Tohoku-oki, Japan earthquake capable of predicting postseismic displacements due to viscoelastic relaxation and afterslip. We consider seismically inferred slab geometries associated with the Pacific and Philippine Sea Plate and a wide range of candidate viscoelastic rheologies. For each case, we invert for afterslip based on residual surface displacements (observed GPS minus that predicted due to viscoelastic relaxation) to develop combined viscoelastic relaxation and afterslip models.We are able to find a mechanical model that fully explains all observed geodetic on-land and seafloor horizontal and vertical postseismic displacements. We find that postseismic displacements are in about equal parts due to viscoelastic relaxation and afterslip, but their patterns are spatially distinct. Accurately predicting both horizontal and vertical on-land postseismic displacements requires a mantle wedge viscosity structure that is depth dependent, reflecting the manner in which temperature, pressure, and water content influence viscosity. No lateral heterogeneities within the mantle wedge viscosity structure beneath northern Honshu are required. Westward-directed postseismic seafloor displacements may be due flow via lowtemperature, plastic creep within the lower half of a Pacific lithosphere weakened by plate

Modeling Seismic Cycles of Great Megathrust Earthquakes Across the Scales With Focus at Postseismic Phase

“…viscoelastic relaxation in the mantle has controlled postseismic deformation at least since one month after the Great Tohoku earthquake and that the afterslip downdip of the main rupture zone might have been substantially overestimated (Freed et al, 2017;Sun et al, 2014;Sun & Wang, 2015;Hu et al, 2016). However, we infer that even at the time range from one hour to a month after the great earthquake, viscoelastic deformation may be very active and therefore the deforming domain may be much larger than previously thought and imaged with the conventional linear models.…”

Monitoring of the spatio-temporal change in the interplate coupling at northeastern Japan subduction zone based on the spatial gradients of surface velocity field

“…After the 2011 Tohoku-oki earthquake, the displacement rate field in the Tohoku region is strongly influenced by the visco-elastic response (e.g. Watanabe et al 2014;Sun et al 2014;Freed et al 2017), and it is difficult to deduce the information with respect to the interplate coupling based on the spatio-temporal changes in the displacement rate gradients.…”

Stress change before and after the 2011 M9 Tohoku-oki earthquake

“…We are mainly concerned with the overall process and defer a more detailed attempt to match the stress field evolution to later visco-elastic modeling work which captures the GPS geodetic time-series fully, rather than considering cumulative post-seismic displacements as was done by Freed et al (2017).…”

“…Figures 7d-i show time-dependent stress from the visco-elastic plus afterslip approach of the modified Freed et al (2017) model, evaluated for the first seven years after the M9. These stresses would be perturbations to the background stress and are expected to lead to a differential effect, comparable to our stress anomaly inferences of Figures 7a-c if the crustal background stress is of comparable amplitude, which we discuss further below.…”

“…This correction does not affect any homogeneous medium slip inversions (since geodetic displacements, rather than stress-based quantities were used as data), and even in the presence of strong heterogeneity, inversion results for slip should be affected by 10% (Hashima et al, 2016). The visco-elastic model of Freed et al (2017) was thus recomputed with both shear modulus and viscosities scaled up by 1.5 to have the same Maxwell decay time as in the original model.…”