We have developed a system for observin g seafloor crustal deformation combining the two techniques, acoustic ranging and kinematic GPS (Global Positioning System) positioning. We installed seven seafloor benchmarks at Suruga Bay and Kumano Nada along the Suruga-Nank ai Trough, Japan. We have performed repeated measurements of seafloor crustal d eformation for three benchmarks from 2004. The results for last three years provide horizontal velocities alo ng the Suruga-Nankai Trough, though the velocities present here are just preliminary ones. Tem poral and spatial variations of sound speed structure may cause large errors in the determined seafloor benchmark positions. To accomplish more accurate monitoring of the seafloor crustal defor mation related to the plate convergence, we should develop a positioning algorithm and o bservation strategy that can reduce the errors caused by the variations of sound speed in seawater.
