Replacing the elastic model with a simple viscoelastic model for more reliable estimation of geodetic locking depths of strike-slip faults

Inferring fault locking depth from interseismic geodetic observations is important to assessing seismic hazard. Interseismic deformation changes with time due to Earth’s viscoelastic rheology, but elastic models commonly employed for estimating locking depths assume time-invariant deformation. Consequently, the apparent locking depth depends on the time when the deformation is observed. In this study, with a focus on large strike-slip faults, we propose replacing the elastic models with an existing analytical viscoelastic model that incorporates time-dependent deformation while maintaining operational simplicity. Using InSAR observations around the Altyn Tagh Fault as illustrative examples, we explain the application of this approach and discuss how to address practical complications of real faults: (1) The use of this model requires knowledge of the earthquake recurrence interval and the time since the last earthquake in terms of the viscoelastic relaxation time. We show a trade-off relation between these two parameters that can be used to assess the ambiguity of inferred locking depth due to their uncertainties. (2) A stiffness contrast across the fault causes asymmetric deformation. We provide an empirical scaling relationship to allow the application of the laterally homogeneous viscoelastic model. (3) Shallow creep of the fault causes discontinuity of surface deformation. We explain that the rate of steady shallow creep without afterslip and slow slip events is controlled by the ratio of the widths (i.e., depth ranges) of the creep zone and locked zone, and we accordingly design a procedure to incorporate shallow creep. (4) Postseismic transients from nearby earthquakes can cause underestimation of locking depth. We demonstrate that modelling the postseismic effect using the same viscoelastic model can enable an effective correction.