Early Viscoelastic Relaxation and Afterslip Inferred From the Postseismic Geodetic Observations Following the 2021 Mw7.4 Maduo Earthquake

Abstract

The 2021 Mw7.4 Maduo (China) earthquake is the first major earthquake within the Bayan Har block in recent decades. Despite this, little monitoring data has so far been undertaken to fully understand the postseismic mechanisms. This paper presents the first-year deformation within ∼350 km from the rupture area following the 2021 Maduo earthquake by Global Navigation Satellite System and Interferometric Synthetic Aperture Radar observations. We use a three-dimensional finite element model and kinematic inversion method to investigate the mechanisms of early postseismic displacement of the 2021 Maduo earthquake and constrain the lithospheric rheology in northeastern Tibet. The observations reveal that the first-year postseismic displacements reach ∼40 mm in the near-field and 1∼2 mm in the far-field. The preferred model results show that the afterslip and viscoelastic relaxation jointly play a dominant role in controlling the early postseismic displacement, while the effects of poroelastic rebound are negligible. The afterslip mainly occurs at 0–45 km depth along the main and branch faults, where the maximum slip is ∼0.1 m. The optimal steady-state viscosities of the lower crust and upper mantle are 2–5 × 10¹⁹ Pa s and 3–10 × 10¹⁹ Pa s, respectively. This implies a weak lower crust may exist beneath northeastern Tibet. Besides, our results indicate that shallow afterslip may have a mutually reinforcing relationship with aftershocks. Ignoring early viscoelastic relaxation effect could lead to a significant increase in deep afterslip. Our findings demonstrate that a wider range of observations might provide the potential capability to distinguish between afterslip and viscoelastic relaxation effects.