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In the 2011 Tohoku-Oki Earthquake, large fault slips exceeding 50 meters were observed near the trench axis, an area previously thought to experience stable sliding or aseismic slip during an earthquake cycle. Various methods, including geophysical surveys, observations, ocean drilling, experiments, and simulations, have been employed to clarify the mechanisms behind these shallow, large fault slips. However, the state of fault locking before and between earthquakes - critical to understanding fault slip - remains unclear. To answer this question based on data from the Tohoku-Oki and inland earthquakes, as well as theoretical and model studies, two contrasting hypotheses have been proposed: one suggests that mechanical locking in the shallow part of the plate boundary rapidly progresses after an earthquake, while the other posits that the shallow part remains mechanically unlocked throughout an earthquake cycle. A unified view on this issue has not yet been established. This session aims to address the fundamental problem in the earthquake generation process: how fault strength recovers and stress accumulates along the source fault of a subduction seismogenic zone. We also hope to use the session as an opportunity to share new knowledge about the geophysical and geological structure of the source faults of megathrust earthquakes, and their spatial variation along and across the subduction zone. We invite research presentations from a wide range of fields, including geophysics, geodesy, geology, petrology, geochemistry, drilling science, and both model and theoretical researches, with a focus on subduction seismogenic zones worldwide.