Mechanisms of Transient Postseismic Deformation Following the 2024 Mw 7.5 Noto Peninsula Earthquake Inferred From Horizontal and Vertical GNSS Data
Abstract
The 2024 Mw 7.5 Noto Peninsula earthquake provided an opportunity to investigate postseismic deformation in a tectonically complex back‐arc region in central Japan, where Miocene normal faults are reactivated under present‐day compression. Results show that neither viscoelastic relaxation nor afterslip alone fully explains the GNSS data. However, a simple integrated model combining viscoelastic relaxation and kinematically inferred afterslip on a single planar fault reproduces the first‐order postseismic deformation despite the region's tectonic complexity. Systematic model evaluation demonstrates that viscoelastic relaxation dominates far‐field observations and near‐field vertical deformation, whereas afterslip is essential for near‐field horizontal signals. Their relative contributions depend on the weighting between horizontal and vertical data and the assumed viscoelastic structure. Assuming a 30 km‐thick elastic top layer, the preferred model suggests that the best underlying 1D structure consists of a mantle asthenosphere with transient (Kelvin) and steady‐state (Maxwell) viscosities of ∼4.0 × 1017 and ∼4.0 × 1018 Pa · $\cdot $ s, respectively, consistent with a Burgers body. The preferred model estimates afterslip concentrated northeast of the coseismic rupture, diminishing within 6 months and releasing ∼8% additional moment. Resolution tests, together with sensitivity analyses using multi‐plane fault geometries, confirm that inferred afterslip on the southeast‐dipping fault offshore northeast of the Noto Peninsula is robust. Investigation of the spatiotemporal relationship between afterslip and on‐fault aftershocks reveals that the aftershock distribution corresponds spatially with total cumulative fault slip rather than afterslip alone, whereas aftershock activity decreases with afterslip moment decay over time, indicating that aftershock occurrence is controlled by multiple processes.