ODP Leg 196: Logging-while-drilling and advanced CORKS at the Nankai trough accretionary prism

The Nankai Trough marks the location where the Philippine Sea Plate begins subducting beneath the Eurasian Plate. The Nankai Trough subduction system has played an important role in causing many catastrophic earthquakes, a phenomenon observed at many other subduction zones (Ando, 1975). Recent studies have established that fluids play a major role in the physical and chemical evolution of subduction zones and, hence, their seismogenic behavior (e.g., Hyndman et al., 1995). The Philippine Sea Plate is relatively young but carries sediments ∼1 km thick into the Nankai subduction zone. These sediments, as well as underlying oceanic crust, must influence the role of fluids around the plate interface and the generation of megathrust earthquakes at depth. Leg 196 operations were based on prior Ocean Drilling Program (ODP) coring and logging at six sites along the "Muroto Transect" Drilling during Leg 131 penetrated through the décollement and into oceanic basement at Site 808 (Hill et al., 1993); coring at Sites 1173 through 1176 and 1178 occurred during Leg 190 (Moore et al., 2001). Leg 196 was designed to define the interrelationship of deformation, structure, and hydrogeology in the toe of the Nankai accretionary prism. These objectives were achieved by acquiring continuous in situ physical properties profiles with Logging-While-Drilling (LWD) intrumentation, and installing ODP's first "Advanced CORK" (ACORK) long-term hydrogeological observatories at Sites 808 and 1173 (Mikada et al., 2002).