Geodetic Evidence of Time‐Dependent Viscoelastic Interseismic Deformation Driven by Megathrust Locking in the Southwest Japan Subduction Zone

Abstract

The time‐variable features of interseismic deformation in subduction zones are poorly understood and commonly ignored. Here, by incorporating century‐long leveling data and contemporary global navigation satellite systems (GNSS) velocities, we investigate the temporal evolution of interseismic deformation in southwest Japan using two‐dimensional viscoelastic earthquake‐cycle models. We find that a steady‐state continental mantle viscosity of 1019 Pa·s is required to explain these two geodetic data sets. Our preferred model predicts significant variations in the surface velocity field throughout the entire interseismic period due to viscous mantle flow driven by ongoing megathrust locking, indicating that the multiyear GNSS‐derived velocity field represents a snapshot of time‐varying interseismic deformation. The locking depth and locking time exert significant influences on the horizontal and vertical deformation patterns. Our findings highlight the importance of the interseismically relaxing mantle and the necessity of considering such an effect in determining the current locking state along the Nankai subduction zone.

Crustal Shortening and Rheological Behavior Across the Longmen Shan Fault, Eastern Margin of the Tibetan Plateau

“…The lateral extension of the eastern part of the lower crust ( D ) is set to be a free parameter in the modeling (Figure S2 in Supporting Information ). We utilize the Maxwell rheology body to model the long‐term viscoelastic relaxation of the lower crust and upper mantle (Li et al., 2020). Initial tests indicate that the surface deformation is insensitive to the viscosity of the western lower crust ($${\eta }_{lc}^{w}$$) beneath eastern Tibet and the viscosities of lower crust and upper mantle beneath the Sichuan Basin, which are fixed at 10 18 , 10 22 , and 10 23 Pa·s, respectively (Diao et al., 2018; Huang et al., 2014; Shi & Cao, 2008).…”

“…The elastic model assumes a steady‐state deformation pattern during interseismic periods, and is widely used owing to the simple formulation and reasonable explanation of the observations. The viscoelastic model, which incorporates the viscoelastic relaxation effects driven by stress changes during earthquake cycles, can predict time‐dependent interseismic deformation (e.g., Diao et al., 2019; Li et al., 2020; Savage & Prescott, 1978; Wang et al., 2012).…”

“…(Hao et al, 2014). White vectors represent horizontal GPS velocities with 95% confidence interval relative to the Eurasian plate (Zheng et al, 2017) during earthquake cycles, we can explore the long-term viscosity with interseismic geodetic observation (Diao et al, 2019(Diao et al, , 2021Li et al, 2020;Wang et al, 2012).…”

“…Previous investigations suggested low crustal shortening rates of 0-3 mm/a (e.g., Chen et al, 2000;Shen et al, 2005;Thatcher, 2007;Zheng et al, 2017), but other competing results proposed shortening rates of 5-6 mm/a (Thompson et al, 2015;Wang & Shen, 2020). These studies used elastic models and neglected the viscoelastic relaxation effects induced by stress accumulation and release during earthquake cycles, which may bias the estimation of kinematic parameter, such as fault slip rate and locking depth (Diao et al, 2019(Diao et al, , 2021Li et al, 2020). Besides, only horizontal deformation rates were used by these previous studies.…”

“…During the interseismic period of earthquake cycles (∼10 4 yrs), the stress accumulated by fault locking is partially relaxed by slow viscous flow of the ductile lower crust and upper mantle, leading to observable surface deformation (Savage & Prescott, 1978; Wang et al., 2012). Based on the newly developed models that include viscoelastic effects during earthquake cycles, we can explore the long‐term viscosity with interseismic geodetic observation (Diao et al., 2019, 2021; Li et al., 2020; Wang et al., 2012).…”