Complex postseismic vertical motion following megathrust earthquakes explained by simple mechanisms

Abstract

Postseismic deformation following large megathrust earthquakes is widely observed with geodetic measurements and coastal geological investigations and contains important information on subduction zone rheology and megathrust slip behaviour. Short-term (a few years) horizontal deformation exhibits a consistent pattern common to almost all large megathrust earthquakes. For example, the coastal area moves consistently in the seaward direction, but the trench area moves in the landward direction. The horizontal deformation is readily explained by the effects of viscoelastic relaxation (VER) of earthquake-induced stress and afterslip. However, the vertical deformation exhibits greater complexity. For example, the sense of coastal deformation varies not only between different earthquakes but also along strike for the same earthquake. In this work, by separately modelling the VER and afterslip processes of synthetic and real subduction earthquakes, we show that the vertical motion can be explained in a simple manner in the same conceptual framework as for the horizontal motion, although the vertical motion is more sensitive to details of the rheological structure and afterslip. VER results in a long-wavelength, tri-segment deformation pattern consisting of near-trench uplift, midway subsidence, and near-arc uplift. The near-trench uplift and midway subsidence follow coseismic uplift and subsidence, respectively, and are both controlled by the viscosity of the sub-slab oceanic mantle. The near-arc uplift results from viscoelastic relaxation in the presence of a cold and elastic forearc mantle wedge corner (the cold nose) and is controlled by the viscosity of the hot part of the mantle wedge beneath the arc and back arc (Luo & Wang, 2021). In contrast to VER, each afterslip patch results in a local bi-modal pattern of uplift and subsidence dominated by elastic deformation, with variable wavelengths depending on the location and size of the afterslip. The complexity in postseismic vertical motion arises mainly from the heterogeneity and site-specific nature of afterslip (Luo & Wang, 2022). If observations are made near the rupture area, the observed vertical postseismic motion can be very complex, because the effects of heterogeneous afterslip around or within the rupture zone can obscure or conceal the pattern of near-trench uplift and midway subsidence due to VER. Farther away from the rupture area, the observed postseismic deformation mainly reflects the contribution of VER, and near-arc uplift appears to be ubiquitous. Separating the common VER process and the site-specific afterslip effect helps to constrain mantle rheology and illuminate fault slip behaviour. Our work also has strong implications for deciphering paleoseismic estimates of coastal motion associated with ancient earthquakes to understand coseismic vs. postseismic contribution.