Sub-parallel fault afterslip and weak zone relaxation after the 2024 Noto earthquake

Postseismic deformation carries valuable information about the complex fault system and deep rheological structure. We analyzed ~6 months of postseismic deformation following the 2024 moment magnitude 7.5 Noto earthquake using InSAR and GNSS data. Results reveal subsidence exceeding 8 cm across the Noto Peninsula, alongside significant horizontal surface deformation extending over 400 km. Our models suggest that viscoelastic relaxation plays a more significant role than afterslip in causing this subsidence. A weak zone, characterized by viscoelastic behavior, is necessary to explain localized deformation westward of the volcanic arc. Notably, shallow afterslip overlapping with coseismic slip is also required. We propose that this afterslip occurred on a hidden fault parallel to and beneath the coseismic slip fault, and was likely driven by normal stress corresponding to fault unclamping rather than the commonly assumed shear stress increase. These findings underscore the importance of considering fault structure and rheological heterogeneity in long-term seismic hazard estimation. Space geodetic observations from the region affected by the 2024 Noto Peninsula earthquake indicate that viscoelastic relaxation of the lower crust and upper mantle likely contributed more significantly to postseismic deformations than afterslip