2-5. Long-term Observation Network in the Ocean Hemisphere

(1) Seafloor borehole seismic observatory
During the life of the OHP, four seafloor borehole seismic observation sites were installed (e.g., Araki et al., 2004): JT-1 and JT-2 offshore from the Sanriku district, northeast Japan, installed in August 1999; WP-2 installed in the Northwest Pacific Basin in August 2000; and WP-1 installed in the West Philippine Basin in April 2001. The systems at JT-1 and JT-2 include strain and tilt sensors as well as seismometers. Observations by high-quality broadband seismogram at WP-2 continued for more than 400 days, which is a record duration for such observations. This long-term record enabled us to estimate how noise levels change throughout a year and prove the high efficiency of borehole seismometry at this site, which can detect seismic signals from earthquakes of greater than M3.5 that occur at any location in the world.
(2) Onland broadband seismic observatories
There were many difficulties in maintaining broadband seismic stations installed by Japanese group prior to the OHP, as they were equipped with many different types of instruments, including varying recording system hardware, power supply, and data format. It was therefore one of the most important tasks of the OHP to unify the total observation system so that each station is of world standard performance. Finally, the OHP seismic network, with 11 broadband stations, was constructed in the western Pacific region (Fig. 2.1.1). Upon completion of the OHP, this seismic network was transferred to the IFREE of JAMSTEC for more efficient long-term operation and cooperative maintenance.
(3) Onland geomagnetic observatories
A network of geomagnetic observatories was constructed during the 6 years of the OHP (Fig. 2.1.1), consisting of nine sites equipped with a geomagnetic system (RFP-523) developed within the OHP and one site in Antarctica (Syowa Station) whose data are provided by the National Institute for Polar Research. RFP-523 provides stable observations for the study of geomagnetic secular variation with a typical baseline drift that is smaller than 5 nT/yr for three components, with only one absolute calibration every year (Shimizu and Utada, 1999). Data from the OHP geomagnetic network have been and will be used to image the 3-D distribution of mantle conductivity. The OHP geomagnetic network is also maintained in collaboration with IFREE/JAMSTEC.
(4) Submarine cable network for measurement of the geo-electric field
Seven submarine cables (one in the Japan Sea and six in the Pacific Ocean) are used for long-term monitoring of the geo-electric field after retirement from telecommunications use. Data exchange protocol exists between the OHP and US research group who run a similar experiment in the eastern Pacific region. One of the goals of this experiment is to constrain the strength and spatial distribution of the toroidal magnetic field at the core surface. Theoretical studies show that a detectable electric signal can be generated by a rapid (decadal scale) oscillation of the outer core fluid that is comparable to observed changes in voltage differences (Shimizu and Utada, 2004). Geo-electric data are also used in 3-D EM tomography and will be used to further investigate mantle conductivity together with geomagnetic data.
(5) Seafloor magnetic observatory
A seafloor instrument was developed within the OHP that enables us nearly permanent observation of four components of the geomagnetic field (three components and total intensity) and two components of the geo-electric field using measurements of the instrumentfs attitude (orientation and tilt). After 5 years of development, this instrument was deployed at site WP-2 (near the borehole seismic site in the Northwest Pacific Basin) in 2001 and recovered one year later, when an identical instrument was installed for a further year's operation. Our investigation has shown that the repetition of such deployment will provide a continuous magnetogram that is nearly equivalent to that of a standard onland geomagnetic observatory (Toh et al., 2004).
(6) GPS network in the western Pacific
More than 10 continuous GPS observation sites were deployed in the western Pacific region by the OHP, and continuous positioning data from over 40 sites were analyzed as well as campaign results in several areas. As a result, we found that the motion of oceanic plates such as the Pacific and Philippine Sea plates is generally consistent with geological models of plate motion, while the continental plates of Asia is subject to large-scale deformation, mostly due to collision with the Indian subcontinent. We precisely determined the spreading rate of a back-arc basin, the Mariana Trough, which is also consistent with the geologically estimated rate.
(7) Seafloor geodesy
A precise seafloor positioning system was developed in collaboration with the Scripps Institution of Oceanography and installed on both sides of the Japan Trench, off the Sanriku coast, as a test experiment. This system realizes a resolution of horizontal location as high as a few mm at a slant range of 14 km using a GPS-acoustic link; this enables us positioning control for the deep ocean floor at sites such as close to the trench axis. This technology is expected to measure the convergence rate of the Pacific Plate in the vicinity of the trench axis, about 300 km offshore.
(8) Super-conducting gravimetry
The OHP constructed a network of super-conducting gravimeters at Syowa in the Antarctica, Canberra in Australia, Bandong in Indonesia, Esashi in Japan, and Ny-Alesund in Norway. This network is the first in the world that extends from the polar regions, where the Earthfs rotation effect is minimum, to the equatorial region where the effect is at a maximum. Analysis of data from this network reveal the coupling between the solid and liquid Earth over a frequency range that is much wider than that considered previously. Long-term measurement in Norway is also expected to detect post-glacial rebound.