Imaging viscosity after the 2021 Mw 7.4 Maduo earthquake: Insights on the regional rheology variation beneath the Bayan Har block

The rheology of Earth's lithosphere fundamentally governs tectonic processes and landscape evolution. Postseismic deformation following large earthquakes has been widely used to constrain rheological structures globally. However, regional-scale rheological variations remain poorly understood due to the infrequency of large earthquakes. The Bayan Har Block in the central-eastern Tibetan Plateau is a unique tectonic unit to investigate rheology regional-variations by pronounced high seismicity, heat flow anomalies, and localized lithospheric thinning. Here, we image postseismic deformation following the 2021 Maduo earthquake using InSAR and GNSS observations to probe the rheology over the block's north-east margin. We integrate long-term and short-term interferograms to separate deformation from atmospheric signals. After accounting for interseismic velocity, InSAR measurements extends out to 200 km from the rupture and its tips. Compared with prior studies, higher-quality far-field measurements allow the separation of viscoelastic relaxation from near-field afterslip effects. Our preferred model indicates a Burgers (bi-viscous) lower-crust rheology with transient and steady-state viscosities of 2.5 (+2.5/-1.3) × 10¹⁸ Pa·s and 2.5 (+0.7/-0.5) × 10¹⁹ Pa·s, respectively, beneath a 20-km-thick elastic layer. By testing laterally-variable and depth-dependent rheological structures, we identify a smaller intraplate-to-interplate viscosity contrast (factor of ∼2) than that observed after the 2008 Wenchuan (factor of ∼10) and 2001 Kokoxili (factor of ∼5) earthquakes. This result suggests a positive correlation between viscosity contrasts, fault slip rates, and topographic gradients at a regional scale. These findings highlight that the rheological contrasts within the Bayan Har block exert a fundamental control on the long-term topographic evolution of the central Tibetan Plateau.