Sudden subduction channel and mantle wedge weakening leads to the vertical deformation pattern changes before and after great subduction zone earthquakes

Abstract:

Modern geodetic measurements reveal that the viscoelastic deformation is prevalent after great subduction zone earthquakes requiring more comprehensive models to explain the transient well-recorded postseismic deformation. Specifically, these models necessitate the testing of various rheological settings different from validated interseismic models for certain geological heterogeneities, e.g. subduction channel. This discrepancy between the models favored for inter- and post-seismic deformation hints at the possibility of a time-depended rheological variation during the phase change of earthquake cycle. Here, we use a set of two-dimensional viscoelastic finite element models to investigate the time evolutions of the effective viscosities in the relaxing bodies and their effects on vertical deformation for the continuous GPS records before and after the 2010 Mw 8.8 Maule and the 2014 Mw 8.2 Iquique earthquakes, of South-Central and Northern Chile. Effective viscosities are defined as the best-fitting Maxwell viscosities for the pre-determined time windows. Geological structures of the models are constrained independently by geophysical and geological data. Our results reveal a sudden decrease in effective viscosities of the subduction channel and mantle wedge immediately following the earthquake and the slow recovery of these effective viscosities during the postseismic phase. In comparison to the models without weak subduction channel and mantle wedge, the weakening of these bodies adds a subsidence-uplift deformation waveform close to the trench and shifts an uplift zone into the measurable inland area, i.e., ~100 to 200 km away from the trench. The variations of the viscosity in these bodies may reflect a negative dependence of the viscosity on the strain rate of material, which is suddenly elevated by coseismic-introduced stress perturbation. Therefore, we suggest this geophysical process leads to the first order waveform change of vertical deformation away from the trench before and after a great earthquake. Our proposed vertical deformation mechanism may be common and applicable to broader subduction zone systems due to similar time evolutions of vertical deformation have been observed after other subduction zone earthquakes, such the 2004 Sumatra and 2011 Tohoku earthquakes.