Deep fault structure and lower crust rheology beneath the northeastern Bayankara block revealed by post-seismic deformation following the 2021 Mw 7.4 Maduo Earthquake

Earthquakes with large magnitude induce massive post-seismic deformation lasting for months to years. Modeling the post-seismic deformation gains invaluable insights to understanding the physics of fault zone and the lower crustal rheology. However, the observed post-seismic deformation is originated from sources with variant mechanisms, including afterslip, poroelastic rebound, and viscoelastic relaxation, which occur at different spatial and temporal scales. Decomposing and interpreting deformation resulted from deep afterslip and viscoelastic relaxation especially remains challenging. The 2021 Mw 7.4 Maduo earthquake, which occurred on a secondary fault ~80 km south of the previously identified major block boundaries, east Kunlun fault, has generated clear afterslip signal reported by several studies. However, the interpretations regarding viscoelastic models remained debated in two aspects: 1) How can we quantify the contribution from deep afterslip and viscoelastic relaxation during the early post-seismic phase? 2) Does the lower crust exhibit the same rheological property across the ruptured Jiangcuo fault and east Kunlun fault? In this context, acquiring high-resolution and extensive coverage of post-seismic deformation data becomes critically important.

Here, we derived a high-resolution post-seismic deofrmation extending over ~1000 kilometers for 2.5 years, using 6 tracks of Sentinel-1 SAR images and 32 continuous GNSS stations. Far-field deformations showed a smooth decay, ranging from 2 cm/year at the fault to 200 kilometers away on both sides of the fault rupture, extending over 500 kilometers along the strike. Notably, no discontinuity was observed along the east Kunlun fault, indicating that the boundary fault kept silent following the Maduo earthquake. We constrained the spatial pattern of post-seismic deformation with high-resolution InSAR observations, offering significant constrains into the depth and viscoelastic structure. Additionally, we utilized GPS time-series data to accurately ascertain the viscosity magnitude. By extracting the contribution of shallow afterslip from the initial observations, we explored the trade-off between deep afterslip and viscoelastic relaxation.

We firstly used a three-layer Maxwell and Burgers model for far-field deformation (100-200 km) and then incorporated deep afterslip and viscoelastic relaxation for mid-field observations (10-100 km). Our best-fit results reveal that deep afterslip dominates in mid-field areas, while viscoelastic relaxation significantly impacts far-field deformation. The optimal model presents an upper crust depth of 20 km, with transient and steady-state viscosities in the lower crust at 10^18 and 4*10^19 Pa·s, respectively, and a steady-state upper mantle viscosity of 10^20 Pa·s. As with the preliminary results, the model did not require a strong variant viscosity to explain the data. Disregarding deep afterslip could lead to overestimating viscosity by 1-1.5 orders of magnitude. Our results imply that the ruptured secondary fault can continue to ~20 km and kept slip after earthquakes. However, for the deeper lower crust and upper mantle, the material keeps the same strength across the northeastern boundary of Bayankara block.