Seafloor borehole geophysical observatories in the western Pacific

Seismological networks provide critical data for better understanding the dynamics of the Earth; however, a great limitation on existing networks is the uneven distribution of stations. In order to achieve a more uniform distribution of seismic stations, observatories must be constructed in the ocean basins. The best configuration for oceanic seismic observatories is thought to be placement of seismometers in deep boreholes drilled into oceanic basement rocks. We constructed four borehole geophysical observatories in the western Pacific area, where plate-consuming boundaries are concentrated, from 1999 to 2001. The four seafloor borehole observatories, which were installed in cooperation with Ocean Drilling Program, are situated at three sites. The geophysical observatories were designed to work as stand-alone systems. The system of these observatories is basically identical except for geophysical sensors and the power supply. From an analysis of the data from broadband seismometers, we confirm that seismic observation in deep-sea borehole gives the best environment for long-term, low-noise, high-resolution earthquake observation in marine areas. Two observatories were collecting the data from the tiltmeter which recorded the earth tides clearly. It is not so difficult to connect the borehole system to a seafloor cable system, because the borehole system works with low voltages of power supply. It is important to cooperate with Integrated Ocean Drilling Program (IODP) for establishment of standard technology of connecting a borehole observation system to a seafloor cable system.