Persistent postseismic signals: Multigeodetic insights into the 1920 Haiyuan earthquake's legacy

The 1920 M 8.5 Haiyuan earthquake in northeastern Tibet provides a unique opportunity to investigate century-scale postseismic deformation. Current geodetic observations may still retain deformation signals from this historic event, potentially biasing estimates of fault slip rates and locking depths. We integrate Sentinel-1 SAR data (2014–2020) from three tracks with GPS velocities to construct a high-resolution 3D deformation field. A 3D viscoelastic finite element model incorporating rheological heterogeneity and complex fault geometry reveals that: (1) The prominent deformation anomaly southwest of the rupture zone is dominated by lower crustal viscoelastic relaxation, with an equivalent viscosity of ∼9 × 10¹⁹ Pa·s; (2) Postseismic effects cause ∼1.0 mm/yr overestimation in far-field slip rates; (3) Slip rates decrease eastward (4.72–1.36 mm/yr), correlating positively with locking depths (15–20 km in western segments vs. <3 km in eastern segments), suggesting faster healing in high-slip regions; (4) Multi-track InSAR gradients resolve the shallow creep controversy, showing no detectable creep along the 1920 rupture. These findings demonstrate that neglecting centennial postseismic deformation introduces systematic biases in fault parameter estimation. Our study advances understanding of long-term viscoelastic processes and provides critical constraints for seismic hazard assessment in intracontinental strike-slip systems.