Detection of Annual and Fine-Scale Interplate Coupling Variations Using Intraplate Earthquakes and Geodetic Data: Application to the Tohoku-Oki Plate Boundary

We propose a method to infer detailed spatiotemporal variations in interplate coupling by integrating geodetic and seismic data. It examines whether focal mechanisms of intraplate earthquakes occurring near the plate interface align with the stressing rate caused by a geodetically derived slip-deficit rate model, evaluating deviations from a steady coupling state. We apply this method to the Tohoku-Oki plate boundary region over a quarter-century period, including the 2011 Mw9.0 Tohoku-Oki earthquake. The analysis successfully detects previously reported features, such as the partial decoupling of the plate interface in the large slip area of the Tohoku-Oki earthquake in the years leading up to the event, and the decadal evolution of aseismic slip off the coast of central and southern Tohoku before the Tohoku-Oki earthquake. Additionally, it reveals complex variations in the interplate coupling, including temporal fluctuations and fine-scale spatial heterogeneities not clearly captured by geodetic data alone. These findings highlight the value of integrating focal mechanism and geodetic information for a refined understanding of the evolving coupling state along subduction zones. Our method allows detection of changes on annual timescale and spatial variations at scales finer than those typically resolved by geodetic inversions. This provides a powerful new tool for monitoring interplate coupling evolution, offering potential improvements in seismic hazard assessment and early identification of anomalous coupling patterns that may precede large earthquakes.