Abstract

Since 2007, 10 Integrated Ocean Drilling Program NanTroSEIZE expeditions were completed at 14 sites across the Nankai margin off Kumano, Japan. The deepest hole reached to ~3000m below seafloor in the southern part of Kumano Forearc Basin. The first borehole observatory was installed and connected to the cable network in 2013 for real-time monitoring. The shallow stress state seaward of the trough is in a normal to strike-slip faulting regime, with the horizontal maximum stress orientated nearly parallel to the plate convergence vector. Lower porosity in the hanging wall and overconsolidation near the mega-splay and frontal décollement is probably caused by surface erosion and tectonic loading. Frictional properties were determined on mega-splay fault and décollement samples for a slip rate from 10-7 to 1m/s. Both velocity-strengthening and velocity-weakening behaviors were detected, suggesting faults may host both slow slip events and earthquake rupture propagation to the seafloor. A vitrinite reflectance anomaly and higher illite content within the localized fault gouge indicate a temperature >300 °C, suggesting coseismic slip(s) in the past, but geochemical data suggest a slightly lower temperature of >250 °C. Our understanding of shallow fault slip behavior in subduction zones is much improved, but still incomplete.