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

Abstract

AbstractWe 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 1018 and 1019 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.