Japanese Seafloor Observing Systems: Present and Future

We describe in this article Japanese efforts toward building and operating long-term seafloor observing systems. Greater details are given to those systems in which the authors have been involved. The main impetus for obtaining long-term time-series from the ocean floor in Japan has been earthquake monitoring for risk assessment and hazard mitigation. Most of the earthquake energy is released near and along the oceanic trenches offshore Japan, and large inter-plate earthquakes recur at decades to 100-year intervals, which are a great threat to the society. The first cabled observatory was laid in 1978 by the Japan Meteorological Agency to monitor seismicity in an area where a M-8 earthquake has been expected to occur. Since then, national agencies and universities established more cabled observing systems (8 systems as of 2003). The very reason of seismic activity is the plate subduction, which causes numerous geophysically interesting activities including fluid vents, biological communities, and magma movements. Realizing all these processes require long-term monitoring to lead to eventual understanding of their dynamics, efforts were made particularly at JAMSTEC to establish multiple-sensor observing systems. There are now certain directions towards the future. One is establishing monitoring systems in deep ocean boreholes, which will become possible by the new Integrated Ocean Drilling Program (2003-). Another is enabling many sensors to be deployed at an appropriate spatial density so that networks realized on land can be extended over the surrounding oceans. The third is establishing observatories as components of global networks.