Monitoring slow earthquake activity in subduction zones can give important insight into the stress build-up and subsequent rupture extent of megathrust earthquakes. Extensive slow earthquake activity occurs up-dip of the seismogenic zone of the Nankai Trough subduction zone, an area that might be awaiting a large (M_w {greater than or equal to}8) earthquake in the near future.
Mechanisms used to explain the occurrence of slow earthquakes are often linked to temporal changes in fluid transport along faults. This study utilises this theory in evaluating the usage of 4D gravity measurements on the seafloor for monitoring changes in fluid flow, hence monitoring the slow earthquake activity and the mechanisms behind them. We model the gravity response from fluid-related density changes in an area of the Nankai Trough accretionary prism that experiences several slow earthquake episodes in the interseismic period.
The forward modelled 4D gravity response is used to estimate volumes of fluid at specific locations of the accretionary prism and plate interface corresponding to a minimum gravity signal of 5 µGal. This accuracy in the gravity signal is obtainable through technology monitoring micro-gravity effects at the seafloor. Based on the results we have formulated a hypothesis on how small fluid volume changes can be detected through a gravimetry survey at the seafloor of the Nankai Trough. The results can also be used to design a survey layout for obtaining valuable 4D gravity data at the Nankai Trough.