The advent of geodetic measuxements by GPS has made it a realistic research objective to observe the process of intra-plate deformations near plate boundaries, where seismicity and volcanism are most active on the earth. Most of plate boundaries, however, are located beneath the deep ocean. Marine geodetic measurements, therefore, are of prime importance to the study of geodynamics. Precise acoustic ranging with cm-resolution per kilometers, which is the primary technique for marine geodetic measurements, has been examined in trial seafloor experiments. Based on the precise acoustic ranging, marine geodetic measurements are pursured in two ways. One is horizontal seafloor ranging between a fault zone. This experiment is simpler and easier owing to stable temperature and salinity in the deep ocean. The other is precise seafloor positioning combined with GPS geodetic measurements on the sea surface. Although this experiment is difficult, it can observe intra-plate deformations near the subduction zones.
The Hyogoken-Nanbu Earthquake occurred in the Akashi Strait on January 17, 1995, and caused severe damage in Kobe, Nishinomiya, Ashiya, Takarazuka and the northern part of Awaji Shima Island. Immediately after the earthquake, Hydrographic Department of Japan carried out the sea bottom survey in the Osaka Bay and the Akashi Strait to reveal the distribution of faults and to detect the seafloor deformation related to the event. The reflection survey revealed the existence of many faults in the Osaka Bay. Seismic profiles show that these faults are accompanied by the deformed Osaka Group correlated to Pleistocene. We compared these seismic profiles to the seismic records obtained in 1973 and 1974. Three active faults, one is located on the northern extension of the Nojima Fault and others are offing Tarumi and Suma, are considered to move at this time. Other faults in the Osaka bay are located outside of the aftershock area, then they are not directly related to this event. From our reflection data, we regard the Osaka Bay Fault as A grade active fault that is active in Holocene. Further detailed survey and study should be conducted on the Osaka Bay Fault.
The survivability of surface drifters deployed by Hydrographic Department for the period from 1988 to 1993 is estimated for two aspects of transmission life and drogue life, and is compared with the drifters deployed within WOCE SVP by other organizations such as the Global Drifter Center operated by Scripps Institution of Oceanography and AOML of NOAA. In comparison of half life which is determined as the time when 50 % drifters survive after deployment, it is concluded that our drifters have provided as robust data set as others from the view point of transmission life and not enough robust in terms of drogue life. The transmission life of our drifters has been getting longer steadily since 1988 and reached close to 300 days in 1993. Two reasons that regulate the drifter life; accidental troubles happening on drifters and the life of battery, and their effects on the characteristics of survivability are also discussed. It is suggested to be possible to describe the survivability of surface drifters by using a simplified model where only two components mentioned above are taken into account.