Sub-parallel Fault Afterslip and Weak Zone Relaxation after the 2024 Noto Earthquake

Postseismic deformation following large earthquakes provides critical insights to the stress state and rheology of seismogenic zones. Here, we use high-resolution geodetic observations to analyze the postseismic response to the 2024 moment magnitude (Mw) 7.5 Noto earthquake, highlighting complex interactions between coseismic slip and afterslip on subparallel faults. By examining approximately six months of postseismic deformation using InSAR and GNSS data, we observe dramatic subsidence exceeding 8 cm across the Noto Peninsula, alongside horizontal deformation extending over 400 km west-northwest into central Japan. Numerical models indicate that both viscoelastic relaxation and afterslip are responsible for the observed deformation, with viscoelastic relaxation playing a more significant role in the pronounced subsidence in the peninsula. A weak zone, characterized by viscoelastic behavior, is required to explain localized deformations westward of the volcanic arc. Static stress analysis suggests that shallow afterslip overlaps with coseismic slip but may occur on unknown parallel faults beneath the primary seismogenic fault, and that the afterslip is primarily driven by normal stress change rather than the commonly assumed shear stress. These findings highlight the complexity of afterslip and suggest that postseismic observations reflects both rheological heterogeneity and fault system complexity in the region.