Postseismic displacements following great subduction earthquakes show significant long-wavelength and time-dependent patterns caused primarily by transient viscoelastic relaxation processes occurring broadly at depth. However, the Earth’s viscosity structure and time-dependent variations are still poorly understood, especially in the years immediately following a great earthquake. Here we investigate the spatiotemporal variation of mantle viscosity proximal and distal to the southern Andes using 8 years of continuous high-resolution GPS observations following the 2010 Mw 8.8 Maule earthquake in south Central Chile. We remove the potential influences of relocking and afterslip on far-field GPS displacements and estimate viscosities that can explain the 3-D displacements. The optimal viscosity structure exhibits a low-viscosity (~1018Pa s) mantle beneath the Andean volcanic arc and high-viscosity (>1022Pa s) cratonic mantle, indicating a dependence of transient viscosity on temperature. Comparisons of the viscosity distributions at different times show that mantle viscosities increase with time throughout the study region. Viscosity increase is generally fastest in the mantle wedge beneath the Andes and slows down with increasing distance from the source region of the Maule earthquake. Such temporal viscosity evolution may indicate a stress dependence of the viscosity proximal to the rupture zone, while regions east of the Andes act as a relatively rigid body (i.e., cratonic mantle) with much higher viscosity. Our results thus suggest that both temperature structure and stress state contribute to spatiotemporal variations of the mantle viscosity. Heterogeneous spatiotemporal variations of viscosity seem to control the expansion and duration of the postseismic deformation and therefore the postseismic stress evolution.
Shaoyang Li 1,2; Jonathan Bedford 1; Marcos Moreno 1,3; William D. Barnhart 2; Matthias Rosenau 1; and Onno Oncken 1
1 Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Potsdam, Germany
2 Department of Earth and Environmental Sciences, University of Iowa, Iowa City, IA, USA
3 Department of Geophysics, University of Concepcion, Concepcion, Chile