Transient Deformation Excited by the 2021 M7.4 Maduo (China) Earthquake: Evidence of a Deep Shear Zone

We use Sentinel-1 and ALOS-2 Interferometric Synthetic Aperture Radar (InSAR), and Global Navigation Satellite System (GNSS) data to investigate the mechanisms of coseismic and postseismic deformation due to the 2021 M7.4 Maduo (China) earthquake. We present a refined coseismic slip model constrained by the rupture trace and precisely located aftershocks. The InSAR time series corrected for the atmospheric and decorrelation noise reveal postseismic line of sight displacements up to ∼0.1 m. The displacements are discontinuous along the fault trace, indicating shallow afterslip and velocity-strengthening friction in the top 2–3 km of the upper crust. The magnitude of shallow afterslip is however insufficient to compensate for the coseismic slip deficit, implying substantial off-fault yielding. The observed surface deformation does not exhibit obvious features that could be attributed to poroelastic effects. We developed a fully coupled model that accounts for both stress-driven creep on a deep localized shear zone and viscoelastic relaxation in the bulk of the lower crust. The mid- to near-field data can be reasonably well explained by deep afterslip and/or non-Maxwellian visco-elasticity. Our results suggest a power-law stress exponent of ∼4–4.5 assuming a power-law rheology, and transient and steady-state viscosities of 10 18 and 10 19 Pa s, respectively, assuming a bi-viscous (Burgers) rheology. However, a good fit to the GNSS data cannot be achieved assuming the bulk viscoelastic relaxation alone, and requires a contribution of deep afterlip and/or a localized shear zone extending through much of the lower crust. Plain Language Summary A large earthquake occurred in the north-east Tibetan Plateau (Qinghai Province, China) on 21 May 2021. We use data collected by orbiting satellites to measure subtle (millimeter to centimeter-scale) displacements of the Earth's surface that occurred in 1 year following the Maduo earthquake. The observed rates and spatial patterns of surface displacements are used to infer the mechanisms of stress relaxation and mechanical properties of rocks at depth. We find evidence of continued slip on the fault that produced the Maduo earthquake, both near the surface, and below the seismogenic zone. Our results suggest that major faults that cut through the Tibetan Plateau are associated with deep roots that extend into the lower crust. The observed shallow slip is insufficient to compensate for the near-surface slip deficit that accrued during the earthquake. This suggests that some amount of the near-surface coseismic shear was distributed over a wider zone around the earthquake rupture.