Constraints on Lateral Rheological Heterogeneity in Northeastern Tibet from Long-Term GNSS and InSAR Observations following the 2021 Maduo Earthquake

       The 2021 Mw 7.4 Maduo earthquake, rupturing within the Bayan Har block, provides a unique opportunity to investigate long-term postseismic deformation and lateral rheological heterogeneity of the lithosphere in northeastern Tibet. Here, we present four years (2021–2025) of postseismic deformation derived from continuous GNSS and InSAR observations to characterize the long-term deformation pattern and its underlying rheological controls. The GNSS and InSAR time series reveal a sustained growth of cumulative postseismic deformation, with both deformation amplitude and spatial extent progressively increasing with time. Maximum line-of-sight deformation inferred from InSAR reaches ~80 mm four years after the earthquake, consistent with the horizontal displacement magnitudes recorded by GNSS. The deformation exhibits a clear temporal transition, characterized by rapid growth during the first 1–2 years, followed by a substantially reduced but persistent deformation rate during years 3–4. During the later stage, localized regions exhibit additional deformation of up to ~5 mm, indicating that postseismic processes continue to operate over multi-year timescales. Modeling of the early postseismic deformation indicates that first-year displacements are jointly controlled by afterslip and viscoelastic relaxation, whereas the contribution from poroelastic rebound is negligible. Rheological inversion of the early postseismic deformation constrains optimal steady-state viscosities of 2–5 × 10¹⁹ Pa s for the lower crust and 3–10 × 10¹⁹ Pa s for the upper mantle, indicating the presence of a mechanically weak lower crust beneath the Bayan Har block. By incorporating the full four-year deformation time series, we further identify pronounced lateral variations in postseismic deformation behavior across the East Kunlun fault. South of the fault, the long-term deformation decay is broadly consistent with a weak lower crust characterized by viscosities on the order of ~10¹⁹ Pa s. In contrast, north of the fault, systematic spatial and temporal misfits between observations and homogeneous rheological models require a substantially stronger lower crust, with effective viscosities on the order of ~10²¹ Pa s. These results indicate that the East Kunlun fault represents a first-order rheological boundary separating laterally contrasting lithospheric domains in northeastern Tibet, and highlight the critical role of long-term GNSS and InSAR observations in resolving lateral rheological heterogeneity that cannot be captured by short-term postseismic data alone.