The Nankai trough subduction zone off southwest Japan is prone to large earthquakes, causing tsunamis and great damages to the nearby islands. Linking the sources of these earthquakes to asperities along the plate interface is of great importance in monitoring earthquake and tsunami activity at the subduction zone. However, the location of asperities near the trough axis at the Nankai trough are not well constrained. During the interseismic period between megathrust earthquakes, shallow slow earthquakes occurring along the interface are messengers of the stress distribution outside of these asperities. They occur on areas were fluid migration on the interface leads to loosely coupled regions. Through gravity measurements, small changes in subsurface density caused by this fluid migration can be detected. These changes can be observed through time-lapse gravimetric surveys at the seafloor, allowing for detailed mapping of these gravity anomalies.In addition, I wish to thank my parents, Theresa and Sigbjørn, and my sisters, Monique and Sarita, for always being supportive and for providing helpful advice. In particular, thank you for taking the time to proofread all these pages.Last, but not least, thanks to my friends for their companionship and for being there when I needed to unwind during the last year of my studies. I would also like to give warm thanks to my classmates for five and a half years of their company, in particular Vilde and Regine for making my time in Bergen "the best".