GPS/acoustic seafloor positioning experiment in the subduction zone of the Japan Trench

Planning to make in situ observations of seismic coupling and motion of the Pacific Plate near the Japan Trench (about 200 km from the coast), we deployed three precision acoustic transponders (PXPs) in 2001 on both the Pacific plate (70 km from the trench axis at a depth of 5450 m) and the landward slope (70 km from the axis at a depth of 2800 m). The PXP system was developed for the full ocean depth in collaboration with a group at the Scripps Institution of Oceanography. We confirmed precise ranging up to a 15 km slant range with the PXPs. We obtained an accuracy of 2-3 cm for the sea-surface kinematic GPS positioning at a baseline length of 350 km. We have confirmed the anchor stability through visual inspection of PXPs deployed on thick sediment of the old Pacific Plate.