Geoelectrical evidence of fluid controlling slow and regular earthquakes along a plate interface

Understanding the distribution of physical properties around shallow subducting plate interfaces, where both destructive and "slow" earthquakes occur due to rapid and slower fault slips, respectively, presents a major scientific and disaster mitigation challenge. Pore water is a key factor in understanding the different slip mechanisms and their spatial relationships; however, its distribution remains understudied. In this study, based on marine magnetotelluric survey in Hyuga-nada, southwestern Japan, we identified distinct resistive and conductive anomalies along the plate interface. These anomalies correspond to areas of scarce pore fluid and high concentration area of pore fluids sourced from subducting seamounts (Kyushu-Palau Ridge), respectively. The wet area corresponds to the slow slip area, whereas the dry and transition areas correspond to areas of fast fault slip. These findings provide clear observational evidence that pore fluid distribution correlates with fault rupture behavior.