Studying slow earthquake activity in subduction zones provides insight into the stress buildup and rupture extent of megathrust earthquakes. Extensive slow earthquake activity occurs up-dip of the seismogenic zone of the Nankai Trough subduction zone, which may soon potentially experience a large ( \(M_w\ge\) 8) earthquake. The fault-valve theory has been used to explain the occurrence of slow earthquakes, suggesting that their triggering mechanisms are linked to temporal changes in fluid transport along faults. Trapping of these fluids by impermeable sections would lead to mass changes in the subsurface. Therefore, delineating the precise locations of mass accretion is instrumental in advancing our understanding of fluid flow mechanisms and the fundamental processes governing slow earthquakes. This study evaluates the use of time-lapse microgravimetry at the seafloor, able to detect subtle mass changes ( \(\sim \mu\) Gal), in monitoring such mass changes caused by fluid buildup. We model the gravity response from mass changes related to mass buildup from transient fluid flow in an area of the Nankai Trough accretionary prism experiencing several slow earthquake episodes. The method assumes flow through the décollement and the underthrust sediments with fluid accumulation in low-permeable regions. Our results indicate that mass changes in the subsurface leading to a 5 \(\mu\) Gal signal at the seafloor can be generated between one month and one year in the scenarios considered. These time frames are consistent with slow earthquake cycles in the Nankai Trough, indicating a possibility of mapping the spatiotemporal evolution of such mass changes through gravity monitoring.
